A Modular Approach to Atropisomeric Bisphosphines of Diversified Electronic Density on Phosphorus Atoms

The series of C2-symmetric biaryl core-based non-racemic bisphosphines possessing substituents of different electronic properties: both EDG and EWG were obtained in a short sequence of good yielding transformations, started from commercial 1,3-dimethyl-2-nitrobenzene. Several different approaches leading to the desirable ligands were practically evaluated. Notably, the synthesis of the entire series of ligands could be performed with the utilization of a single early-stage precursor DIDAB (6,6′-diiodo-2,2′,4,4′-tetramethylbiphenyl-3,3′-diamine), which could be easily obtained in enantiomerically pure form. The obtained compounds at concentrations of 50 and 200 µM showed various biological activity against normal human dermal fibroblast, ranging from inactivity through time-dependent action and ending up with high toxicity.

Herein, we describe an efficient strategy allowing access to axially chiral biaryls bearing phosphorus functionalities and diversified substituents at 5,5′-positions. Such phosphines are known to be potent bidentate ligands for transition metals to be used in different types of catalytic asymmetric transformations. The particular design of the ligands implies that the complexes derived from them will adopt the same stereometry, but the electronic properties of the transition metal will depend on the substituent introduced in ligand core [28]. The ligands were obtained in a short sequence of goodyielding transformations, which started from commercial 1,3-dimethyl-2-nitrobenzene and leading through the formation of a single universal 5,5′-diamine precursor DIDAB (6,6'-diiodo-2,2',4,4'-tetramethylbiphenyl-3,3'-diamine, 5). We also report the preparation of a new C2-symmetrical BIMOP (4a) related ligand [26] with dimethylamine group at 5,5′-positions, hereafter named BIMAP (4b), and its successful optical resolution carried out with the use of the chiral cyclopalladated derivative and/or by the resolution of the isomers using HPLC with a chiral stationary phase column. The classical approach to the new chiral non-racemic electron-deficient ligand BIClP (4c) with chlorine atoms at 5,5′positions is reported as well. Our approach was also optimised to obtain the highly known and efficient ligands TetraPHEMP (4d) [18] and BIMOP [26].

Synthesis of Racemic Precursors
An efficient route to a series of desired atropisomeric C2-symmetrical phosphines is based on the utilisation of a single chiral precursor possessing such function group which could be easily converted to several others, and that at the same time allows the enantioseparation of racemic compound. Thus, the diamine-substituted diiododiaminobiaryl DIDAB (5) (Scheme 1) was selected for this purpose. Herein, we describe an efficient strategy allowing access to axially chiral biaryls bearing phosphorus functionalities and diversified substituents at 5,5 -positions. Such phosphines are known to be potent bidentate ligands for transition metals to be used in different types of catalytic asymmetric transformations. The particular design of the ligands implies that the complexes derived from them will adopt the same stereometry, but the electronic properties of the transition metal will depend on the substituent introduced in ligand core [28]. The ligands were obtained in a short sequence of good-yielding transformations, which started from commercial 1,3-dimethyl-2-nitrobenzene and leading through the formation of a single universal 5,5 -diamine precursor DIDAB (6,6 -diiodo-2,2 ,4,4 -tetramethylbiphenyl-3,3 -diamine, 5). We also report the preparation of a new C 2 -symmetrical BIMOP (4a) related ligand [26] with dimethylamine group at 5,5 -positions, hereafter named BIMAP (4b), and its successful optical resolution carried out with the use of the chiral cyclopalladated derivative and/or by the resolution of the isomers using HPLC with a chiral stationary phase column. The classical approach to the new chiral non-racemic electron-deficient ligand BIClP (4c) with chlorine atoms at 5,5 -positions is reported as well. Our approach was also optimised to obtain the highly known and efficient ligands TetraPHEMP (4d) [18] and BIMOP [26].

Synthesis of Racemic Precursors
An efficient route to a series of desired atropisomeric C 2 -symmetrical phosphines is based on the utilisation of a single chiral precursor possessing such function group which could be easily converted to several others, and that at the same time allows the enantioseparation of racemic compound. Thus, the diamine-substituted diiododiaminobiaryl DIDAB (5) (Scheme 1) was selected for this purpose. The most direct route to DIDAB would involve iodation of a commercially available 2-nitroxylene, followed by the Ullmann coupling reaction, and then a reduction in the nitro group. The series of ligands represented by BIMOP, BIMAP, BIClP, and TetraPheMP could be obtained subsequently from the racemic or resolved as DIDAB in  The most direct route to DIDAB would involve iodation of a commercially available 2-nitroxylene, followed by the Ullmann coupling reaction, and then a reduction in the nitro group. The series of ligands represented by BIMOP, BIMAP, BIClP, and TetraPheMP could be obtained subsequently from the racemic or resolved as DIDAB in few steps.
Thus, commercially available 1,3-dimethyl-2-nitrobenzene was converted into iodoarene (7) in concentrated sulfuric acid using I 3 HSO 4 as the iodine source in a good yield of 95% (Scheme 2). Then, iodoarene was homocoupled by means of Ullmann coupling reaction. It is worth noting that in order to obtain higher yield, the prior activation of the copper surface was necessary. Hence, treating the commercially available reagent with an acidic solution of copper(II) nitrate increased the yield from 50% to 85%. The initial attempts to reduce dinitrobiphenyl 8 to the diamine derivative 9 using a mixture of iron in hydrochloric acid yielded unsatisfactory results. Utilization of LiAlH 4 for this purpose seemed to be an inconvenient and expensive approach on a larger scale. Finally, the desired diamine derivative 9 was obtained in 99% yield using palladium catalyst (10% Pd/C) under hydrogen pressure of 150 Atm at 150 • C. Next the compound 9 was converted into racemic DIDAB (5) via iodination with the benzyltrimethylammonium dichloroiodate complex (BTMA-ICl 2 ) according to the procedure described in the literature [47]. Usage of alternative iodizing reagents such as ICl or I 2 for the introduction of iodine atoms at the 2,2 -positions, resulted in much lower selectivity and/or lower conversion of the substrate. The most direct route to DIDAB would involve iodation of a commercially available 2-nitroxylene, followed by the Ullmann coupling reaction, and then a reduction in the nitro group. The series of ligands represented by BIMOP, BIMAP, BIClP, and TetraPheMP could be obtained subsequently from the racemic or resolved as DIDAB in few steps.
Thus, commercially available 1,3-dimethyl-2-nitrobenzene was converted into iodoarene (7) in concentrated sulfuric acid using I3HSO4 as the iodine source in a good yield of 95% (Scheme 2). Then, iodoarene was homocoupled by means of Ullmann coupling reaction. It is worth noting that in order to obtain higher yield, the prior activation of the copper surface was necessary. Hence, treating the commercially available reagent with an acidic solution of copper(II) nitrate increased the yield from 50% to 85%. The initial attempts to reduce dinitrobiphenyl 8 to the diamine derivative 9 using a mixture of iron in hydrochloric acid yielded unsatisfactory results. Utilization of LiAlH4 for this purpose seemed to be an inconvenient and expensive approach on a larger scale. Finally, the desired diamine derivative 9 was obtained in 99% yield using palladium catalyst (10% Pd/C) under hydrogen pressure of 150 Atm at 150 °C. Next the compound 9 was converted into racemic DIDAB (5) via iodination with the benzyltrimethylammonium dichloroiodate complex (BTMA-ICl2) according to the procedure described in the literature [47]. Usage of alternative iodizing reagents such as ICl or I2 for the introduction of iodine atoms at the 2,2′-positions, resulted in much lower selectivity and/or lower conversion of the substrate. DIDAB, as the designed universal precursor of all planned chiral bisphosphines, was subsequently transformed by classical transformation of amino groups as presented on Scheme 3, into a series of diiodobiphenyl derivatives with different substituents at 5,5′positions to be used in further phosphorylation reaction steps. DIDAB, as the designed universal precursor of all planned chiral bisphosphines, was subsequently transformed by classical transformation of amino groups as presented on Scheme 3, into a series of diiodobiphenyl derivatives with different substituents at 5,5 -positions to be used in further phosphorylation reaction steps.
To obtain diamine derivatives 10b and 10e, the amino groups of DIDAB were alkylated with formaldehyde and butyraldehyde in acidic aqueous medium under sodium borohydride reductive conditions [48,49]. The desired products were obtained in good yields 92% and 83%, respectively. DIDAB was also subjected to the bisdiazotation reaction followed by the substitution of diazonium groups with chlorine (Sandmayer reaction). That yielded compound 10c in 91%. The dimethoxy derivative 10a was obtained in the reaction of bisdiazonium salt with methanol catalysed by Pd(OAc) 2 in 68% yield, while the reductive elimination reaction of that salt with aqueous H 3 PO 2 in the presence of catalytic amounts Cu 2 O leads to product 10d in 89% yield.
Alternatively, racemic compounds 10a could be obtained from the early precursor, diamine 9, in the sequence of high-yielding reactions as presented on Scheme 4 in good 60% overall yield. The iodination reactions of other 3,3 -diamino and 3,3 -dimethoxy substituted 2,2 ,4,4 -tetramethylbiphenyls were not as efficient or selective.

Synthetic Route to Bisphosphines
The racemic bisdimethylaminosubstituted ligand BIMAP was synthesized in the sequence of reactions leading from 10b (Scheme 5). A low temperature deiodolithiation reaction, in which combination of n-BuLi and TMEDA was found to be the most efficient, leads to the reactive intermediate, suitable for phosphorylation. The amount of the base was found to be crucial to the success of the reaction, and in particular 3.1 equivalents of TMEDA and 2.1 equivalents of n-BuLi (1.3M in hexane) were the optimum amounts of lithiation reagents. The lithiated intermediate was then exposed to Ph 2 P(O)Cl to obtain the BIMAPO in a good yield of up to 52%. The byproduct of the reaction was the monophosphine oxide (12b, 21% yield). Its formation was evidence of the completion of the lithiation process and indicates the difficulty, probably due to the steric hindrance, during the phosphorylation step. The obtained BIMAPO was efficiently reduced to BIMAP with an excess of phenylsilane at elevated up to 190 • C temperature. Such remarkable high reactivity of triaryl phosphine oxide towards the deoxygenation reaction could be understandable taking into consideration that phosphorus atom received significant injection of electronic density induced from nitrogen [28], what is known to facilitate the deoxygenation reaction [50]. To obtain diamine derivatives 10b and 10e, the amino groups of DIDAB were alkylated with formaldehyde and butyraldehyde in acidic aqueous medium under sodium borohydride reductive conditions [48,49]. The desired products were obtained in good yields 92% and 83%, respectively. DIDAB was also subjected to the bisdiazotation reaction followed by the substitution of diazonium groups with chlorine (Sandmayer reaction). That yielded compound 10c in 91%. The dimethoxy derivative 10a was obtained in the reaction of bisdiazonium salt with methanol catalysed by Pd(OAc)2 in 68% yield, while the reductive elimination reaction of that salt with aqueous H3PO2 in the presence of catalytic amounts Cu2O leads to product 10d in 89% yield.
Alternatively, racemic compounds 10a could be obtained from the early precursor, diamine 9, in the sequence of high-yielding reactions as presented on Scheme 4 in good 60% overall yield. The iodination reactions of other 3,3′-diamino and 3,3′-dimethoxy substituted 2,2′,4,4′-tetramethylbiphenyls were not as efficient or selective. To obtain diamine derivatives 10b and 10e, the amino groups of DIDAB were alkylated with formaldehyde and butyraldehyde in acidic aqueous medium under sodium borohydride reductive conditions [48,49]. The desired products were obtained in good yields 92% and 83%, respectively. DIDAB was also subjected to the bisdiazotation reaction followed by the substitution of diazonium groups with chlorine (Sandmayer reaction). That yielded compound 10c in 91%. The dimethoxy derivative 10a was obtained in the reaction of bisdiazonium salt with methanol catalysed by Pd(OAc)2 in 68% yield, while the reductive elimination reaction of that salt with aqueous H3PO2 in the presence of catalytic amounts Cu2O leads to product 10d in 89% yield.
Alternatively, racemic compounds 10a could be obtained from the early precursor, diamine 9, in the sequence of high-yielding reactions as presented on Scheme 4 in good 60% overall yield. The iodination reactions of other 3,3′-diamino and 3,3′-dimethoxy substituted 2,2′,4,4′-tetramethylbiphenyls were not as efficient or selective. lithiation process and indicates the difficulty, probably due to the steric hindrance, during the phosphorylation step. The obtained BIMAPO was efficiently reduced to BIMAP with an excess of phenylsilane at elevated up to 190 °C temperature. Such remarkable high reactivity of triaryl phosphine oxide towards the deoxygenation reaction could be understandable taking into consideration that phosphorus atom received significant injection of electronic density induced from nitrogen [28], what is known to facilitate the deoxygenation reaction [50]. The BIMAPO synthesis pathway, shown above, was applied also in the case of transformation of another substrates: 10e, which leads to the bis(di-n-buthylamino)-substituted bisphosphine dioxide 11e, and chlorosubstituted derivative 10c, which leads to BIClPO (11c) in low yields.
Unfortunately, the product 11e was unstable in oxidative conditions even as mild as air exposure. That could be rationalized by the tendency of the electron-rich amines to be oxidized in an unselective manner. The compound 10c, comparison to compounds 10b, was quite stable in oxidative media, but less reactive in reaction with n-BuLi, so the reaction conditions were modified accordingly. The temperature of the lithiation process was elevated during the reaction from −40 up to + 10 °C, and the reaction time was prolonged to 18 h. We found that the reaction of the bislithium derivative with Ph2P(O)Cl mostly furnishes product 12c in low yield. In turn, an arylphosphine moiety was introduced by reaction with more active diphenylchlorophosphine without the addition of TMEDA (Scheme 6) in much better yields. Since the chromatographic isolation of the product formed was impossible in the studied cases, the treatment of the reaction mixtures with hydrogen peroxide in basic environment was applied to obtain corresponding phosphine oxides. It turned out that oxidation step proceeded very slowly, what indicates the high stability of the electronically poor phosphine. Two monophosphine oxides bearing an unreacted iodide group 13c or a hydrogen atom at the 2-position 12c were isolated from the reaction mixture in yield of 22% and 5%, respectively. In turn, bisphospnine oxide BI- The BIMAPO synthesis pathway, shown above, was applied also in the case of transformation of another substrates: 10e, which leads to the bis(di-n-buthylamino)substituted bisphosphine dioxide 11e, and chlorosubstituted derivative 10c, which leads to BIClPO (11c) in low yields.
Unfortunately, the product 11e was unstable in oxidative conditions even as mild as air exposure. That could be rationalized by the tendency of the electron-rich amines to be oxidized in an unselective manner. The compound 10c, comparison to compounds 10b, was quite stable in oxidative media, but less reactive in reaction with n-BuLi, so the reaction conditions were modified accordingly. The temperature of the lithiation process was elevated during the reaction from −40 up to + 10 • C, and the reaction time was prolonged to 18 h. We found that the reaction of the bislithium derivative with Ph 2 P(O)Cl mostly furnishes product 12c in low yield. In turn, an arylphosphine moiety was introduced by reaction with more active diphenylchlorophosphine without the addition of TMEDA (Scheme 6) in much better yields. Since the chromatographic isolation of the product formed was impossible in the studied cases, the treatment of the reaction mixtures with hydrogen peroxide in basic environment was applied to obtain corresponding phosphine oxides. It turned out that oxidation step proceeded very slowly, what indicates the high stability of the electronically poor phosphine. Two monophosphine oxides bearing an unreacted iodide group 13c or a hydrogen atom at the 2-position 12c were isolated from the reaction mixture in yield of 22% and 5%, respectively. In turn, bisphospnine oxide BICLPO (11c) was isolated in 30% yield. These observations indicate that the lithiation process is a limiting factor for the efficiency of the phosphorylation reaction but some better results could be obtained if Ph 2 PCl was used instead of less reactive Ph 2 P(O)Cl derivative.
Molecules 2022, 27, x FOR PEER REVIEW 6 of 28 CLPO (11c) was isolated in 30% yield. These observations indicate that the lithiation process is a limiting factor for the efficiency of the phosphorylation reaction but some better results could be obtained if Ph2PCl was used instead of less reactive Ph2P(O)Cl derivative. Scheme 6. Synthesis of racemic bisphosphine dioxides.
In the synthesis of bisphosphine oxide BIMOPO (11a) bearing the MeO-substituents at 5,5′-positions of biaryl skeleton, the precursor 10a was used. The phosphorylation reaction proceeded smoothly in 72% overall yield of 11a, formation of the monophosphine oxide 12a was observed as a byproduct in 15% yield.
At the same time, we developed an alternative route of synthesis of BIMOPO, wherein diphenylphosphine oxide acted as a donor of the phosphorus moieties (Scheme 7). The synthesis based on the modified Hirao method was carried out in two steps. In the first step, the mixture of monophosphine oxided 12a and 13a was obtained under mild Scheme 6. Synthesis of racemic bisphosphine dioxides.
In the synthesis of bisphosphine oxide BIMOPO (11a) bearing the MeO-substituents at 5,5 -positions of biaryl skeleton, the precursor 10a was used. The phosphorylation reaction proceeded smoothly in 72% overall yield of 11a, formation of the monophosphine oxide 12a was observed as a byproduct in 15% yield. At the same time, we developed an alternative route of synthesis of BIMOPO, wherein diphenylphosphine oxide acted as a donor of the phosphorus moieties (Scheme 7). The synthesis based on the modified Hirao method was carried out in two steps. In the first step, the mixture of monophosphine oxided 12a and 13a was obtained under mild conditions, and then intermediates were subjected to the complete phosphorylation to BIMOPO in the second step. The BIMOPO yield (31%) was lower than that obtained by the classical iodide phosphorylation; however, it provided access to a number of valuable hard-to-reach products such as 12a and 13a and products of reactions with other readily available secondary phosphine oxides. In turn, with substrate 10d, the reaction proceeded with full conversion and with moderate isolated yield of TetraPHEMPO (11d) (51%) and monophosphine oxide 12d (42%). Scheme 6. Synthesis of racemic bisphosphine dioxides.
In the synthesis of bisphosphine oxide BIMOPO (11a) bearing the MeO-substituents at 5,5′-positions of biaryl skeleton, the precursor 10a was used. The phosphorylation reaction proceeded smoothly in 72% overall yield of 11a, formation of the monophosphine oxide 12a was observed as a byproduct in 15% yield.
At the same time, we developed an alternative route of synthesis of BIMOPO, wherein diphenylphosphine oxide acted as a donor of the phosphorus moieties (Scheme 7). The synthesis based on the modified Hirao method was carried out in two steps. In the first step, the mixture of monophosphine oxided 12a and 13a was obtained under mild conditions, and then intermediates were subjected to the complete phosphorylation to BI-MOPO in the second step. The BIMOPO yield (31%) was lower than that obtained by the classical iodide phosphorylation; however, it provided access to a number of valuable hard-to-reach products such as 12a and 13a and products of reactions with other readily available secondary phosphine oxides. In turn, with substrate 10d, the reaction proceeded with full conversion and with moderate isolated yield of TetraPHEMPO (11d) (51%) and monophosphine oxide 12d (42%). Finally, the desired bisphosphines BIMAP, BIMOP, BICLP and TetraPHEMP, with full conversion were obtained by reduction of corresponding dioxides by trichlorosilane in the presence of tributylamine or by phenylsilane (BIMAP).

The Enantiomerically Pure Bisphosphines
Access to enantiomerically pure ligands could be provided by the separation of the racemic mixtures at several different stages of their synthesis. Thus, the resolution of racemic DIDAB would lead from one enantiomerically pure precursor to the entire series of optically pure or enantiomerically enriched ligands. Otherwise, the enantioseparation Scheme 7. Alternative route to BIMOPO and TetraPHEMPO.
Finally, the desired bisphosphines BIMAP, BIMOP, BICLP and TetraPHEMP, with full conversion were obtained by reduction of corresponding dioxides by trichlorosilane in the presence of tributylamine or by phenylsilane (BIMAP).

The Enantiomerically Pure Bisphosphines
Access to enantiomerically pure ligands could be provided by the separation of the racemic mixtures at several different stages of their synthesis. Thus, the resolution of racemic DIDAB would lead from one enantiomerically pure precursor to the entire series of optically pure or enantiomerically enriched ligands. Otherwise, the enantioseparation has to be applied individualy for each ligand. The enantioseparation of diphosphine oxides could be performed by crystallizating their diastereomeric salts with chiral non-racemic acids such as DBTA, naproxen [51]. The enantioseparation of diphosphines may go through the synthesis and crystallization of diastereomeric palladium complexes steps [40].
The crucial intermediate DIDAB (5) was efficiently separated by crystallization using (−)-O,O -dibenzoyl-L-tartaric acid ((-)-DBTA) as the resolving reagent. The optical resolution was carried out in hot chloroform to give 64% of 70% de salt DIDAB*DBTA, which was subjected to crystallization from methanol to yield pure (+)-DIDAB (in basic form) in 26% yield and enantiomeric excess above 99%, [α] D 20 = +8.9 (c = 1, CH 2 Cl 2 ). Interestingly, that the recrystallization of enantio enriched DIDAB from non-polar solvents, for example toluene or toluene/hexane, did not furnish the product with an optical purity greater than 96% ee. The absolute configuration of (R)-DIDAB was determined using X-ray analysis of crystals of basic (R)-DIDAB, which grew from acetonitrile solution ( Figure 2). It was additionally confirmed by circular dichroism [52]. The enantiomeric composition of DIDAB was determined by means of 1 H NMR spectroscopy: the spectrum of mixture 3.5 mg of DIDAB and 25 mg Eu(hfc) 3 [53] was recorded and signals of corresponding to methyl groups and aromatic protons were integrated to calculate an enantiomeric excess according to the equation ee, % = A−A1 A+A1 100%, where A and A1 are values of integrals of corresponding signals on spectra. The proper selection of the signals was confirmed in the experiment with racemic compound. The crystallographic analysis of monocrystalline (R)-DIDAB indicates that the aromatic rings are nearly perpendicular to each over with torsion between the phenyl rings being of 82.3(3) • which indicates significant repulsion of the bulky iodine atoms and methyl groups. (S)-DIDAB was isolated from the above crystallization residue after the enantio separation with (+)-DBTA in 31% yield and 95% ee, [α] D 20 = −8.6 (c = 1, CH 2 Cl 2 ).
was additionally confirmed by circular dichroism [52]. The enantiomeric composition of DIDAB was determined by means of 1 H NMR spectroscopy: the spectrum of mixture 3.5 mg of DIDAB and 25 mg Eu(hfc)3 [53] was recorded and signals of corresponding to methyl groups and aromatic protons were integrated to calculate an enantiomeric excess according to the equation ee, % 100%, where A and A1 are values of integrals of corresponding signals on spectra. The proper selection of the signals was confirmed in the experiment with racemic compound. The crystallographic analysis of monocrystalline (R)-DIDAB indicates that the aromatic rings are nearly perpendicular to each over with torsion between the phenyl rings being of 82.3(3)° which indicates significant repulsion of the bulky iodine atoms and methyl groups. (S)-DIDAB was isolated from the above crystallization residue after the enantio separation with (+)-DBTA in 31% yield and 95% ee, [α]D 20 = −8.6 (c = 1, CH2Cl2). From the enantiomerically enriched up to about 95% ee (S)-DIDAB the enantiomerically enriched diiodobiphenyl (S)-10d was obtained according to the Scheme 3. The (S)-TetraPHEMPO was obtained as described above (Scheme 7). The optical purity 90% ee of the resulting bisoxide was assessed by 31 P NMR [52], In order to reach the complete optical purity, obtained (S)-TetraPHEMPO was recrystallized from methylcyclohexane. Subsequently, the enantiomerically pure (S)-TetraPHEMP was obtained by reduction of the corresponding bisoxide by trichlorosilane in toluene and tributylamine without racemization thereof. Similarly, enantiomerically pure (R)-BIMOP was obtained in phosphorylation reaction of (R)-10a, followed by deoxygenation of the bisphosphine dioxide formed.
In turn, BIClLPO (11c) was an excellent example, when racemic bisoxide could be separated to enantiomerically pure forms using fractional crystallization of its salts with DBTA. From the solution of (rac)-BICLPO and (-)-DBTA in mixture of methylene chloride/carbon tetrachloride after the evaporation of a portion of CH2Cl2, the diastereomerically pure complex (S)-BICLPO•(-)-DBTA in 40% yield of one enantiomer (ee >99%, Figure  3) was crystalized. (S)-BICLPO was separated from its salt by extraction with methylene chloride from sodium carbonate aqueous solution and crystalized from a mixture of hexane/acetone. The enantiomeric purity of obtained bisphosphine dioxide was determined From the enantiomerically enriched up to about 95% ee (S)-DIDAB the enantiomerically enriched diiodobiphenyl (S)-10d was obtained according to the Scheme 3. The (S)-TetraPHEMPO was obtained as described above (Scheme 7). The optical purity 90% ee of the resulting bisoxide was assessed by 31 P NMR [52], In order to reach the complete optical purity, obtained (S)-TetraPHEMPO was recrystallized from methylcyclohexane. Subsequently, the enantiomerically pure (S)-TetraPHEMP was obtained by reduction of the corresponding bisoxide by trichlorosilane in toluene and tributylamine without racemization thereof. Similarly, enantiomerically pure (R)-BIMOP was obtained in phosphorylation reaction of (R)-10a, followed by deoxygenation of the bisphosphine dioxide formed.
In turn, BIClLPO (11c) was an excellent example, when racemic bisoxide could be separated to enantiomerically pure forms using fractional crystallization of its salts with DBTA. From the solution of (rac)-BICLPO and (-)-DBTA in mixture of methylene chloride/carbon tetrachloride after the evaporation of a portion of CH 2 Cl 2 , the diastereomerically pure complex (S)-BICLPO·(-)-DBTA in 40% yield of one enantiomer (ee > 99%, Figure 3) was crystalized. (S)-BICLPO was separated from its salt by extraction with methylene chloride from sodium carbonate aqueous solution and crystalized from a mixture of hexane/acetone. The enantiomeric purity of obtained bisphosphine dioxide was determined by NMR technique [52,54]. The 1 H and 31 P spectra of solution of mixture of BIClPO and mandelic acid in CDCl 3 were recorded and the signals which correspond to aromatic hydrogen and phosphorus atoms were integrated to calculate an enantiomeric excess. The proper selection of the signals was confirmed in the experiment with racemic compound. The single crystal x-ray analysis confirmed the stereochemistry of this compound. by NMR technique [52,54]. The 1 H and 31 P spectra of solution of mixture of BIClPO and mandelic acid in CDCl3 were recorded and the signals which correspond to aromatic hydrogen and phosphorus atoms were integrated to calculate an enantiomeric excess. The proper selection of the signals was confirmed in the experiment with racemic compound. The single crystal x-ray analysis confirmed the stereochemistry of this compound. The additional comparison of CD spectra of (S)-(-)-BICLPO and (R)-MeO-BIPhEPO, obtained from commercial (R)-MeO-BIPhEP ligand [23] shows that these compounds adopt the opposite absolute configurations ( Figure 4). The additional comparison of CD spectra of (S)-(-)-BICLPO and (R)-MeO-BIPhEPO, obtained from commercial (R)-MeO-BIPhEP ligand [23] shows that these compounds adopt the opposite absolute configurations ( Figure 4). The additional comparison of CD spectra of (S)-(-)-BICLPO and (R)-MeO-BIPhEPO, obtained from commercial (R)-MeO-BIPhEP ligand [23] shows that these compounds adopt the opposite absolute configurations (Figure 4). Other than the cases described in Achiwa's work [55,56], our attempts to separate BIMOPO (9c) enantiomers by fractional crystallization with addition of DBTA did not lead to satisfactory results. This was the case with further efforts to use chiral acids such as 2,3-di(phenylaminocarbonyl) tartaric acid [57], monodimethylamide DBTA, naproxen, mandelic acid or similar tested in a wide range of organic solvents. Surprisingly, resolution of racemic BIMAPO (11b), which contains amino groups at 5,5′-positions, also failed. Our further attempts focused on the application of chiral C,N-palladacycle complex which binds the bisphophine ligands. For this purpose, the palladium complexes 14 and 15 were used, which have been synthesized according to the literature data [58] ( Figure 5).  Other than the cases described in Achiwa's work [55,56], our attempts to separate BIMOPO (9c) enantiomers by fractional crystallization with addition of DBTA did not lead to satisfactory results. This was the case with further efforts to use chiral acids such as 2,3-di(phenylaminocarbonyl) tartaric acid [57], monodimethylamide DBTA, naproxen, mandelic acid or similar tested in a wide range of organic solvents. Surprisingly, resolution of racemic BIMAPO (11b), which contains amino groups at 5,5 -positions, also failed. Our further attempts focused on the application of chiral C,N-palladacycle complex which binds the bisphophine ligands. For this purpose, the palladium complexes 14 and 15 were used, which have been synthesized according to the literature data [58] ( Figure 5).
The additional comparison of CD spectra of (S)-(-)-BICLPO and (R)-MeO-BIPhEPO, obtained from commercial (R)-MeO-BIPhEP ligand [23] shows that these compounds adopt the opposite absolute configurations (Figure 4). Other than the cases described in Achiwa's work [55,56], our attempts to separate BIMOPO (9c) enantiomers by fractional crystallization with addition of DBTA did not lead to satisfactory results. This was the case with further efforts to use chiral acids such as 2,3-di(phenylaminocarbonyl) tartaric acid [57], monodimethylamide DBTA, naproxen, mandelic acid or similar tested in a wide range of organic solvents. Surprisingly, resolution of racemic BIMAPO (11b), which contains amino groups at 5,5′-positions, also failed. Our further attempts focused on the application of chiral C,N-palladacycle complex which binds the bisphophine ligands. For this purpose, the palladium complexes 14 and 15 were used, which have been synthesized according to the literature data [58] ( Figure 5).  Racemic BIMAP ligand reacted quantitatively in mild conditions with both palladium complexes 14 and 15 giving mixtures of the corresponding diastereomeric products 16b, 17b, respectively. It was suspected that such mixtures would crystallize and provide an access to diastereomerically pure products. The formation of diastereomeric adducts 16b and 17b was detected by NMR and mass spectroscopy. However, crystallization of the mixture of diastereomers 16b did not allow to achieve the complete separation, even after several crystallizations the de was about 60%. In contrast, resolution of (S,R a )-17b and (S,S a )-17b complexes succeeded. The less soluble diastereoisomer (S,R a )-17b was isolated after several crystallizations from the mixture of ethanol/water and next hexane/ethyl acetate with de >98% (with respect to the total Pd) and with yield of 11% (Scheme 8).
The enantiopure diphosphine (R a )-BIMAP was released from the (S,R a )-17b complex by replacing the ligand with dppe in methylene chloride as presented on Scheme 9. The optical purity was verified by the 31 P NMR spectrum analysis of their corresponding complexes with the chiral ortho-palladium complex 14 as described in the literature [58,59].
An alternative route to enantiomerically pure ligands include the application of (semi)preparative chiral column chromatography. The racemic bisphosphine oxide BIMAPO (11b), was separated using Daicel Chiralpak AD column (250 × 10 × 10 um), which was eluted with hexane 88%, Et 2 NH (10-3% in hexane) 10%, i-Pr 2% with a rate of 4 mL/min. Obtained enantiomers were then reduced with phenylsilane to the desired bisphosphine without any racemization in 87% yield. The enatiomers of TetraPHEMPO (11d), and BIClPO (11c) were separated similarly. The enantiomeric composition of all bisphosphine dioxides could be determined by means of chiral HPLC analysis or NMR spectroscopy [52,54]. The Figure 6 presents X-ray structure of enantiomerically pure (S)-(-)-BIMAPO which crystallizes as a solvate with benzene molecules, and the CD spectra of (S)-(-)-BIMAPO and (R)-MeO-BIPhEPO used as a reference compound with known opposite to BIMAPO absolute configuration of biaryl core. dium complexes 14 and 15 giving mixtures of the corresponding diastereomeric products 16b, 17b, respectively. It was suspected that such mixtures would crystallize and provide an access to diastereomerically pure products. The formation of diastereomeric adducts 16b and 17b was detected by NMR and mass spectroscopy. However, crystallization of the mixture of diastereomers 16b did not allow to achieve the complete separation, even after several crystallizations the de was about 60%. In contrast, resolution of (S,Ra)-17b and (S,Sa)-17b complexes succeeded. The less soluble diastereoisomer (S,Ra)-17b was isolated after several crystallizations from the mixture of ethanol/water and next hexane/ethyl acetate with de >98% (with respect to the total Pd) and with yield of 11% (Scheme 8). The enantiopure diphosphine (Ra)-BIMAP was released from the (S,Ra)-17b complex by replacing the ligand with dppe in methylene chloride as presented on Scheme 9. The optical purity was verified by the 31 P NMR spectrum analysis of their corresponding complexes with the chiral ortho-palladium complex 14 as described in the literature [58,59]. the mixture of diastereomers 16b did not allow to achieve the complete separation, even after several crystallizations the de was about 60%. In contrast, resolution of (S,Ra)-17b and (S,Sa)-17b complexes succeeded. The less soluble diastereoisomer (S,Ra)-17b was isolated after several crystallizations from the mixture of ethanol/water and next hexane/ethyl acetate with de >98% (with respect to the total Pd) and with yield of 11% (Scheme 8). The enantiopure diphosphine (Ra)-BIMAP was released from the (S,Ra)-17b complex by replacing the ligand with dppe in methylene chloride as presented on Scheme 9. The optical purity was verified by the 31 P NMR spectrum analysis of their corresponding complexes with the chiral ortho-palladium complex 14 as described in the literature [58,59]. An alternative route to enantiomerically pure ligands include the application of (semi)preparative chiral column chromatography. The racemic bisphosphine oxide BIMAPO (11b), was separated using Daicel Chiralpak AD column (250 × 10 × 10 um), which was eluted with hexane 88%, Et2NH (10--3% in hexane) 10%, i-Pr 2% with a rate of 4 mL/min. Obtained enantiomers were then reduced with phenylsilane to the desired bisphosphine without any racemization in 87% yield. The enatiomers of TetraPHEMPO (11d), and BIClPO (11c) were separated similarly. The enantiomeric composition of all bisphosphine dioxides could be determined by means of chiral HPLC analysis or NMR spectroscopy [52,54]. The Figure 6 presents X-ray structure of enantiomerically pure (S)-(-)-BIMAPO which crystallizes as a solvate with benzene molecules, and the CD spectra of (S)-(-)-BIMAPO and (R)-MeO-BIPhEPO used as a reference compound with known opposite to BIMAPO absolute configuration of biaryl core. All the ligands obtained in chromatographic approach exhibited the enantiomeric purity over 99% ee. Nevertheless, from the practical point of view this method is not perfect since expensive chiral columns have to be used.

The Other Derivatives
It is important that the functional groups which are present in the ligands' structures All the ligands obtained in chromatographic approach exhibited the enantiomeric purity over 99% ee. Nevertheless, from the practical point of view this method is not perfect since expensive chiral columns have to be used.

The Other Derivatives
It is important that the functional groups which are present in the ligands' structures may allow for the introduction of different modifications providing the ligands with special properties such as solubility in water or nonpolar solvents or affinity to the solid supports without the influence on the ligand stereometry. At the same time, the electronic properties of the new ligands will be the same as those in the original ligand. This opportunity was presented on an example of modification of BIMOPO on methoxy groups (see Scheme 10).  Figure 6. X-ray structure of (S)-(-)-BIMAPO (solvent molecule and hydrogen atoms were omitted for clarity), its CD spectrum (black) as well the CD spectrum of (R)-MeO-BIPhEPO (red).
All the ligands obtained in chromatographic approach exhibited the enantiomeric purity over 99% ee. Nevertheless, from the practical point of view this method is not perfect since expensive chiral columns have to be used.

The Other Derivatives
It is important that the functional groups which are present in the ligands' structures may allow for the introduction of different modifications providing the ligands with special properties such as solubility in water or nonpolar solvents or affinity to the solid supports without the influence on the ligand stereometry. At the same time, the electronic properties of the new ligands will be the same as those in the original ligand. This opportunity was presented on an example of modification of BIMOPO on methoxy groups (see Scheme 10). For example, the oxygen atoms were deprotected in reaction with hydrogen bromide solution and the substituted biphenol 11f obtained in quantitative yields was subjected to the alkylation reaction to obtain bisbenzylic derivative 11g in excellent yield. The same protocol could be applied to introduce long-chain aliphatic substituents, polyether-chain substituents and some other substituents bearing basic and acidic functions. For example, the oxygen atoms were deprotected in reaction with hydrogen bromide solution and the substituted biphenol 11f obtained in quantitative yields was subjected to the alkylation reaction to obtain bisbenzylic derivative 11g in excellent yield. The same protocol could be applied to introduce long-chain aliphatic substituents, polyether-chain substituents and some other substituents bearing basic and acidic functions.
The utilization of obtained ligands in asymmetric catalytic reactions as well as their special modifications, will be reported in a due time.

Cytotoxicity Assay
Some chiral biaryls (e.g., colchicine, allocolchicine, steganacin, rhazinilam) are known because of their biological activity, but in the majority of cases of biaryl compounds only those that are natural or synthetic (with the structures inspired by nature) are expected to be active and are therefore carefully assessed. On the other hand, the ligand, used in the chemical synthesis to form catalysts, could contaminate the products of the reactions and industrial or laboratory places. Surprisingly, this important issue is only rarely discussed, but must be taken into consideration during the designing of the synthesis of biologically active compounds e.g., medicines. Access to a small library of unnatural compounds based on generally common structural biaryl motif makes it possible to verify whether simple biaryls and triaryl phosphine oxides, commonly considered as biologically neutral, are safe.
The biological activity of the compounds was determined at the highest possible concentration achieved in the cellular test conditions. The studied compounds showed various effects on human dermal fibroblasts. In the group of eight more soluble compounds tested at a concentration of 200 µM (see Figure 7), there were those that showed no cytotoxic effect (the maximum decrease in viability was about 3%) after 72 h of incubation (9a and 5), others whose cytotoxic effect increased depending on the incubation time (12a, 10f, 12c, 8,  9f) while for compound 10a, the effect appeared after 24 h and remained at a constant level for the next 48 h of exposure. The strongest cytotoxic effect within this group was shown by compound 12c, leading to a decrease in cell viability up to 15.46% after 72 h of incubation.
tested at a concentration of 200 μM (see Figure 7), there were those that showed no cytotoxic effect (the maximum decrease in viability was about 3%) after 72 h of incubation (9a and 5), others whose cytotoxic effect increased depending on the incubation time (12a,  10f, 12c, 8, 9f) while for compound 10a, the effect appeared after 24 h and remained at a constant level for the next 48 h of exposure. The strongest cytotoxic effect within this group was shown by compound 12c, leading to a decrease in cell viability up to 15.46% after 72 h of incubation. In the second set of poorly soluble compounds, tested at a concentration of 50 μM (see Figure 8), one compound (12b) showed a slight cytotoxic effect on human cells (5% decrease in cell viability), some, despite initially demonstrated effectiveness, weakened with increasing exposure time (11b, 10b, 11f, 10c), and some compounds' activity increased in a time dependent manner (11a, 13c, 11c, 11e, 9a, 9). Compound 11d exhibited the highest toxicity, which after 24 h of incubation led to a decrease in cell viability by In the second set of poorly soluble compounds, tested at a concentration of 50 µM (see Figure 8), one compound (12b) showed a slight cytotoxic effect on human cells (5% decrease in cell viability), some, despite initially demonstrated effectiveness, weakened with increasing exposure time (11b, 10b, 11f, 10c), and some compounds' activity increased in a time dependent manner (11a, 13c, 11c, 11e, 9a, 9). Compound 11d exhibited the highest toxicity, which after 24 h of incubation led to a decrease in cell viability by about 95%, and such effect remained constant up to 72 h of exposure, therefore it was found to be the most cytotoxic among all of the tested agents.

General Information
The reagents were purchased from commercial suppliers and used without further purification. Solvents were dried and distilled under argon before use. All of the reactions involving formation and further conversions of phosphines were carried out under argon atmosphere. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV300 ( 1 H 300 MHz, 31 P 121.5 MHz, 13 C NMR 75 MHz) and Bruker AV500 ( 1 H 500 MHz, 31 P 202 MHz, 13 C NMR 126 MHz) spectrometers (Bruker; Billerica, MA, USA). All spectra were recorded in CDCl3 solutions, unless mentioned otherwise, and the chemical shifts

General Information
The reagents were purchased from commercial suppliers and used without further purification. Solvents were dried and distilled under argon before use. All of the reactions involving formation and further conversions of phosphines were carried out under argon atmosphere. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV300 ( 1 H 300 MHz, 31 P 121.5 MHz, 13 C NMR 75 MHz) and Bruker AV500 ( 1 H 500 MHz, 31 P 202 MHz, 13 C NMR 126 MHz) spectrometers (Bruker; Billerica, MA, USA). All spectra were recorded in CDCl 3 solutions, unless mentioned otherwise, and the chemical shifts (δ) are expressed in ppm using internal reference to TMS and external reference to 85% H 3 PO 4 in D 2 O for 31 P. Coupling constants (J) are given in Hz. The abbreviations of signal patterns are as follows: s-singlet, d-doublet, t-triplet, q-quartet, m-multiplet, b-broad, and i-intensive. The IR spectra were recorded in KBr pallets and with ATR module on the Nicolet 8700A FTIR-ATR spectrometer: wave numbers are in cm-1. All separations and purifications by column chromatography were conducted by using Merck Silica gel 60 (230-400 mesh), unless noted otherwise. The X-ray data were collected at Nonius Kappa-CCD diffractometer using the MoKα = 0.71073 Å wavelength at 150 K for DIDAB and at room temperature for all other compounds. The structures were solved by direct methods (SHELXS) and refined by the full-matrix least-squares method based on F 2 [60]. Hydrogen atoms were placed at calculated positions. The water molecule in BIClPO occupies a special position at 2-fold axis. Benzene molecule in BIMAPO was refined isotropically because of positional disorder. The quality of the X-ray measurement for (S,R a )-17b was not satisfactory for a full structure refinement. Only the symmetry, unit cell parameters and initial model of the molecule was obtained to confirm the molecular structure. All the details from data collecting and structure refinement are presented in Supplementary Materials in Tables S1-S4. The HDF1 (human dermal fibroblasts) cell line was obtained from ATCC. Cells were cultured in DMEM (Dulbecco's Modified Eagle Medium, high glucose)+ GlutaMAX supplemented with penicillin (100 U/mL), streptomycin (100 U/mL) and 10% heat-inactivated FBS. Cells were maintained in a humidified atmosphere at 37 • C and 5% CO 2 and passaged twice before performing an experiment.

Synthesis of 1-Iodo-2,4-dimethyl-3-nitrobenzene (7)
To prepare iodine reagent solution, solid KIO 3 (16 g, 0.07 mol) was added in small portions over 45 min to the stirred solution of powdered iodine (120 g, 0.47 mol) in 95% H 2 SO 4 (400 mL). The mixture was stirred for another 3 h at room temperature to fully dissolve the iodine. 1,3-dimethyl-1,2-nitrobenzene (6, 50 g, 0.33 mol) was dissolved and cooled to 0 • C concentrated H 2 SO 4 (600 mL) in a two-necked round bottom flask equipped with a stirring bar and cooled into an ice bath. Next, the iodine reagent solution was added dropwise over the period of 2 h to the solution of 1,3-dimethyl-1,2-nitrobenzene. The reaction temperature was kept between 10-15 • C. After 1 h of continuous stirring the precipitated iodine was filtered off and the dark brown solution poured onto crushed ice. The resulting precipitate was filtered off, washed with water (20 × 200 mL), 1 M aqueous NaHCO 3 (20 × 200 mL) and dissolved in warm chloroform (550 mL). Chloroform solutions were combined and a 3% solution of NaHCO 3 in saturated aqueous solution of Na 2 S 2 O 3 was added portion wise to complete discoloration of both phases. The organic phase was separated, washed with 1% aqueous NaHCO 3 (100 mL), H 2 O (100 mL) and dried over MgSO 4 . The solvent was removed, and the resulting crude product purified by fractional distillation under reduced pressure. The collected fraction of 110-130 • C at 0.5 Torr contained a product with purity greater than 98%. Yield 90 g (95%

Separation of DIDAB Enantiomers
To the boiling solution of (-)-DBTA (8.2 g, 0.023 mol) in CHCl 3 (500 mL), a solution of racemic DIDAB (4.5 g, 0.049 mol) in CHCl 3 (30 mL) was added dropwise. After addition of whole amount of diamine, the mixture was heated for 1 h and then the solution was left without stirring at room temperature. Salt (DBTA·DIDAB = 1:1, 5 g (64%), 70% ee) slowly precipitated in duration of 72 h. Further proceedings were carried out in two variants.

Variant 1
The precipitate was filtered off, dissolved in hot MeOH and slowly cooled to room temperature. Pure

Synthesis of 4,4 ,6,6 -Tetramethyl-2,2 -diiodobiphenyl (10d) and (S)-10d
To prepared solution of DIDAB (0.6 g, 1.3 mmol) in THF (25 mL), 3 M aqueous H 2 SO 4 (2.2 mL) was added and the mixture was cooled down to −10 • C followed by addition of NaNO 2 (0.2 g, 2.9 mmol) in H 2 O (1 mL) and stirred for 45 min. Next, a solution of NH 2 SO 3 H (0.1 g, 1 mmol) in water (2 mL) was added in three portions and mixture was stirred 15 min more at 0 • C. Then 50% aqueous H 3 PO 2 (5 mL) and Cu 2 O (50 mg) were added sequentially. The mixture thus obtained was stirred for 18 h at room temperature and then for 5 h at 60 • C. Next, THF was evaporated and water (50 mL) was added, the product was extracted with benzene (50 mL), dried and purified by column chromatography (hexane/ethyl acetate: 160/1). Yield 500 mg (89%). Colorless crystals, mp = 116-118 • C (crystallized from hexane). Into a stirred suspension of DIDAB powder (3 g, 6 mmol) in concentrated HCl (15 mL) water (10 mL) was added and the whole was cooled to −15 • C. Next, a solution of NaNO 2 (1.14 g, 16 mmol) in 2 mL of water was added dropwise over the period of 30 min. After that time, a solution of NH 2 SO 3 H (0.7 g, 7.5 mmol) was added in several portions and the reaction mixture was stirred for 20 min. A catalyst solution was prepared as follows: copper(I) oxide (2 g, 14 mmol) mixed with concentrated HCl (5 mL) for 30 min and acetone (30 mL) and CuCl 2 (50 mg) were added to the reaction with bisdiazonium salt. The reaction mixture was stirred for several hours at 0 • C and overnight at ambient temperature.

Determination of Enantiomeric Composition BIMAPO (11b)
The enantiomeric purity of obtained bisphosphine dioxide was determined by NMR technique: [52,54]  . Due to its instability in diluted solution of the compound, the specific rotation measurement was not performed.

Determination of Enantiomeric Composition BIMAP (4b)
The solution of 4 mg of BIMAP in 0.4 mL of benzene was added to the solution of (S)-15 (3 mg in 1 mL of EtOH). The solution was stirred overnight at ambient temperature. The solvents were evaporated off under the reduced pressure and residual was dissolved in CDCl 3 , and 31 P NMR spectrum was recorded. The ratio of the signals corresponding to the phosphorus atoms of the diateriomeric complexes 17c, correspond to the ratio of the enantiomers in the bisphosphine 4b assessed. 31  The separation of enantiomers of BIMAP (4b) with utilization of chiral palladium complex (S)-15. (S,R a )-17b to the slurry of (S)-15 (15 mg, 3.3 mmol) in 10 mL of dry degassed methanol the racemic BIMAP (200 mg, 3.0 mmol) was added. After 24 h of stirring at ambient temperature under the argon atmosphere, the insoluble precipitate was filtered off, the solvent was evaporated under the reduced pressure and the residual was crystalized twice from the mixture of ethanol/water (about 40 volume-%) and one additional time from a mixture hexane/ethyl acetate. The obtained in 11% (with respect to the total Pd used) yield yellow crystalline powder of (S,R a )-17b has a purity of >98% de. 31  -] cation was in excellent agreement with the calculated one for the ion [C 58 H 63 N 3 P 2 Pd] + . The crystallographic analysis of the obtained complex allowed to assign the absolute configuration of the phosphine. To liberate the (R a )-4b from the obtained complex, 7 mg of DPPE in 1 mL of DCM was added to the 20 mg of (S,R a )-17b placed under the argon atmosphere into the NMR tube. The progress of the reaction was monitored by 31 P NMR spectroscopy. After 14 days of the reaction in ambient temperature, the phosphine was chromatographically separated on small SiO 2 -filled column eluted with degassed mixture of hexane/ethyl acetate = 160/1 to yield 8 mg (67%) of enantiomerically pure the (R a )-4b. 31   In the sealed reactor were placed: 4,4 ,6,6 -tetramethyl-5,5 -dimethoxy-2,2 -diiodobiphenyl (10a) (0.16 g, 0,3 mmol), diphenylphosphine oxide (0.2 g, 1 mmol), DABCO (0.25 g, 2.2 mmol), dppb (6 mg, 0.014 mmol), Pd(OAc) 2 (3 mg, 0.014 mmol) and CH 3 CN (10 mL). The reactor was sealed and heated to 80-85 • C and the reaction mixture was intensively stirred for 3 days. After cooling, the solvents were evaporated under the reduced pressure and the residue was mixed with CH 2 Cl 2 (200 mL). The resulting solution was washed with 1 M HCl (2 × 100 mL), treated with 3 portions of 10% H 2 O 2 in 1 M aqueous NaOH (3 × 100 mL) for 2, 4 and 4 h, respectively, then washed with 1 M aqueous NaOH (100 mL) and dried over MgSO 4 . After solvent evaporation, the mixture composition was determined by HPLC (RP-18 column (250 × 4.5 mm), Mobile phase: MeOH 70%, H 2 O 30%, flow rate: 1.5 mL/min, inj. vol.: 20 µL. 10a (17 min, 26%); BIMOPO (28 min, 3%); 12a (30 min, 72%), 13a (55 min, 4%). The resulting mixture was placed again in the reactor, diphenylphosphine oxide (0.2 g, 1 mmol), DABCO (0.25 g, 2.2 mmol), triphenylphosphine (8 mg, 0.014 mmol), Pd(OAc) 2 (3 mg, 0.014 mmol) and CH 3 CN (10 mL) were added. The reaction mixture was heated to 95 • C and intensively stirred for two days, then the temperature was elevated to 125 • C and stirring was continued for two more days. After the reaction mixture was proceeded as described above, BIMOPO was isolated by column chromatography (hexane/ethyl acetate/methanol 5/3/0.25); BIMOPO yield 86 mg (60%).

Determination of Enantiomeric Composition BClPO (11c)
The enantiomeric purity of obtained bisphosphine dioxide was determined by NMR technique: [52,54] the 1 H and 31 P spectra of solution of 2 mg of compounds and 3 mg of mandelic acid in 1 mL of CDCL 3 were recorded and the signals which correspond to aromatic hydrogen and phosphorus atoms were integrated to calculate an enantiomeric excess. The proper selection of the signals was confirmed in the experiment with racemic compound.
3.3.29. Synthesis of (S)-(5,5 -Dichloro-4,4 ,6,6 -tetramethylbiphenyl-2,2 -diyl)bis(diphenylphosphane) ((S)-BIClP, (S)-4c) Into a glass reactor (S)-BIClPO (60 mg) and toluene (20 mL) were placed and during vigorous stirring, tributylamine (2 mL) and trichlorosilane (0.36 mL) were sequentially added. The reactor was sealed and heated to 120 • C for 24 h. Upon cooling, the reaction mixture was poured into 30% aqueous NaOH (30 mL) and stirred vigorously for 2 h. The organic phase was separated, washed with water (10 mL), and dried over MgSO 4 . The solvent was evaporated and the product was isolated by column chromatography (argon flashed column, degassed hexane/ethyl acetate: 160/1). Yield 47 mg (79%). 31  The solution of 2.6 mg of BIClP in 0.3 mL of benzene was added to the solution of (S)-15 (2 mg in 1 mL of EtOH). The solution was stirred overnight at ambient temperature. The solvents were evaporated under the reduced pressure and residual was dissolved in CDCl 3 , and 31 P NMR spectrum was recorded. The ratio of the signals corresponding to the phosphorus atoms of the diateriomeric complexes 17c, correspond to the ratio of the enantiomers in the bisphosphine 4c assessed. 31  The slurry of BIMOPO (0.5 g, 0.75 mmol) in HBr (40% in CH 3 CO 2 H, 15 mL) had been stirred at ambient temperature for 14 days. Next, the solvents were evaporated under the reduced pressure and residual was dissolved in 15 mL of dry ethanol which had contained 1.5 g NaOH. After the 3 h of stirring under the argon atmosphere at ambient temperature, the solution was cooled down to 0 • C and acidified with 1 M H 2 SO 4 to obtain pH = 2. Formed white precipitate was filtered off, washed with 50 mL of water and 10 mL of methanol and dried under the reduced pressure to furnish 0. 48  3.3.32. Synthesis of [5,5 -Bis(benzyloxy)-4,4 ,6,6 -tetramethylbiphenyl-2,2 -diyl]bis(diphenylphosphane) Dioxide (11g) 0.1 g, (0.16 mmol) of 11f was added to stirred mixture of 10 g K 2 CO 3 in 50 mL of dry DMF. After a 5 min 10 mg (0.03 mmol) of TBA·Br was added, followed by addition of 1 mL (8 mmol) of benzyl bromide. The reaction mixture was argonated and stirred at 45 • C for 24 h, at 50 • C for 96 h and at 70 • C for 10 days. Unreacted K 2 CO 3 was filtered off and solvent evaporated under the reduced pressure. The residual was dissolved in 50 mL of CHCl 3 , washed with 1 M aqueous NaOH (2 × 30 mL), 1 M aqueous HCl (2 × 30 mL) and dried with MgSO 4 . The product was purified on SiO 2 column chromatography using as eluent mixture hexane/ethyl acetate/methanol = 5/3/0. 25 13

Cytotoxicity Assay
For cytotoxicity assay cells were seeded in 96-well microplates at a density of 2.5 × 10 4 cells/mL in 100 µL DMEM + GlutaMAX supplemented with 10% heat-inactivated FBS in three sets for different periods of tested compound exposure. After 24 h of cell attachment, plates were washed with 100 µL/well of Dulbecco's phosphate buffered saline (DPBS) and treated with specific concentration of each compound prepared in fresh FBS-free medium for 24, 48 and 72 h. Due to differences in solubility, the compounds were divided into two groups in order to obtain the maximum possible concentration while maintaining complete solubility and obtaining a homogeneous solution for a given compound. The group of twelve compounds that showed relatively low solubility were tested at a final concentration of 50 µM, while the second group of eight compounds at a final concentration of 200 µM. All compounds were dissolved in DMSO, in order to prepare a stock solutions. During experiments, stock solution was diluted in cell culture medium to reach a maximum 0.01% w/v DMSO in final solution. Each concentration was tested in triplicate. All sets included wells containing 0,01% DMSO as a negative control and 1% of Triton X-100 as a positive control. The cytotoxicity of compounds was assessed using MTT assay as described below. Following 24, 48 and 72 h of compound exposure, control medium or test exposures medium were removed, the cells were rinsed with DPBS and 100 µL of fresh medium (without FBS or antibiotics) containing 0.5 mg/mL of MTT was added to each well and the plates were incubated for 3 h at 37 • C in a 5% CO 2 humidified incubator. After incubation period the medium was discarded, the cells were washed with 100 µL of DPBS and 100 µL of DMSO was added to each well to extract the dye. The plate was shaken for 10 min and the absorbance was measured at 570 nm. Viability was calculated as the ratio of the mean of OD obtained for each condition to the control condition.

Conclusions
In summary, we have designed and synthesized atropisomer 4,4 ,6,6 -tetramethyl biaryls bearing phosphorus functionalities at 2,2 -positions and different substituents at 5,5 -positions from one universal non-racemic 5,5 -diamine substituted precursor DIDAB. The other approaches leading to the C2-symmetric enantiomerically pure bisphosphine ligands were practically assessed. We demonstrated that optical resolution of racemic mixtures could be carried out in different stages of ligand synthesis and in various ways such as: fractional crystallization of phosphine oxide complex with the chiral acid, with the use of chiral palladium complexes, with application of the chiral high performance liquid chromatography for individual ligands or their precursors, otherwise a single early stage precursor could be prepared in an enantiomerically pure form and used to give access to the entire series of chiral non-racemic ligands. The new atropisomeric ligands with enantiomeric purity over 99% ee were obtained in reasonable yields. The compounds at tested concentrations of 50 and 200 µM showed various biological activity against normal human dermal fibroblast, ranging from inactivity for non-phosphorus contained