Abstract
The presented work describes the synthesis of new six- and seven-membered haloamidinium salts and their reaction with different metals. The isolated metal complexes were tested in a catalytic reaction. Two different synthetic routes were applied to prepare five different salts. Chloroamidinium salts were very water-sensitive in comparison to their corresponding bromoamidinium salts. Hence, the preparation of the less sensitive bromoamidinium salts was higher prioritized. The formed salts were converted with metal sources to N-heterocyclic carbene (NHC) metal complexes through an oxidative insertion into the C–X bond. This type of formation is less examined for the synthesis of extended NHC metal complexes. Pd(PPh3)4 and cobalt powder were applied as metal sources, whereby two palladium complexes were isolated, characterized, and their crystal and molecular structures determined. The palladium complexes were investigated in the Suzuki-Miyaura reaction and showed promising catalytic activity.
1 Introduction
The history of the discovery of N-heterocyclic carbenes (NHCs) and their broad application as ligands, catalysts, and reagents shows the importance of this class of compounds, which rose continuously over the last two decades [1]. Herrmann et al. [2] were the first who applied an NHC-Pd complex as catalyst for a Heck reaction. Since then, phosphane ligands were increasingly replaced by NHCs in many reactions. They have the advantage to be more stable complexes that are less sensitive to dissociation, air, and elevated temperature. In addition, NHCs offer higher electron-donating properties [3], which could even be increased through expanding the heterocyclic ring from five- to six- and seven-membered analogues [4–6]. Furthermore, complexes are formed with a variety of metals, and also high oxidation states of the metals are stabilized [7].
The most common procedure for the synthesis of NHC metal complexes is a ligand exchange, but this requires the existence of the free NHC (often formed in situ) (Scheme 1). In many cases, the complex formation occurs through the reaction of the free carbene with a metal complex (a) [8], the reaction of an imidazolium salt with a transition metal complex that possesses a basic anion or function (b) [2], with a carbene transfer reagent via transmetalation (c) [9], or with an imidazolium salt that is mixed with a transition metal salt in the presence of a base (d) [10].
Common procedures for the synthesis of NHC metal complexes.
The previously mentioned methods are less suitable for (thermally) labile and difficult to handle ligands (e.g. acyclic carbene ligands). Additionally, they require a specific acidity of the carbene precursors. Some other less common methods are the cleavage of electron-rich olefins (e) [11], an α-elimination (f) [12], and an oxidative addition (g) [13] (Scheme 2).
Further methods for the formation of NHC metal complexes.
Early research on the complex formation via oxidative addition was carried out by Lappert [14] and Stone [15]. This method enables an access to diamino- and Fischer-type carbene complexes with electron-rich transition metals (Ni, Pd, Pt) that are difficult to obtain with other procedures (Scheme 3) [16]. In comparison to other complex formation methods, this is the only case where the oxidation state of the metal is additionally increased. Fürstner et al. [16] were able to prepare several NHC-Pd complexes via this route, and Arduengo et al. [17] synthesized a bimetallic complex.
Examples of metal complexes with an Fischer carbene (top) and with an NHC (bottom).
Normally, the complex formation takes place with 2-haloimidazolium salts, whereby the halogen is often chlorine. There are only a few examples with bromine [18] or iodine [19]. 2-Chloroimidazolium salts can be easily generated from urea and thiourea compounds when they are treated with phosgene [20], oxalyl chloride [21], phosphoryl chloride [22], or phosphorous pentachloride [23] (Scheme 4). 1,3-Disubstituted-2-chloro-imidazolium salts are used as dehydrating reagents due to their strong hygroscopic properties [24]. Furthermore, haloamidinium salts can be used as organocatalysts [25].
Synthesis of 2-chloroimidazolium salts with chlorination agents.
Because of our interests in expanded NHC complexes [6, 26, 27], we report here on a synthesis of new extended NHC complexes via oxidative insertion into haloamidinium salts and their application in a Suzuki cross-coupling reaction.
2 Results and discussion
2.1 Synthesis of the haloamidinium salts
The synthesis of the desired target molecules 2 were tried first through the reaction of 1 with bromine according to a procedure for five-membered rings reported by Kunz et al. [28]. However, no product formation could be observed for the extended amidinium salts 1 (Scheme 5).
Attempts to form 2.
Synthesis routes that are based on silver (I) oxide [29, 30], NHC-CO2 adducts, or NHC-Cr(CO)5 complexes with subsequent bromination [31] are relative expensive or elaborate. For that reason, a pathway based on a thiourea derivative was chosen. Starting from 3 or 6, the corresponding diimines 4 and 7 were formed with benzaldehyde in excellent yields of 97% and 95%, respectively [32]. The following reduction with NaBH4 proceeded to 5 and 8 in excellent yields of 96% and 97%, respectively (Scheme 6) [33].
Synthesis of diamines 5 and 8.
The formation of thiourea 9 was attempted with 5 through the reaction with thiophosgene, but the isolated yield was only 8% and there were many by-products. Some of them were difficult to separate by column chromatography. The product yield could be increased to 64% using carbon disulfide [34]. In addition, the workup was simplified to a recrystallization. The cyclization of 8 with carbon disulfide was not successful for the formation of a seven-membered ring (Scheme 7).
Formation of thiourea 9 and attempts to form 10.
The formation of the new haloamidinium salts with oxalyl chloride or bromide proceeded in excellent yields (Scheme 8). The chloroamidinium chloride 11 was highly hygroscopic in comparison to 12.
Synthesis of the haloamidinium salts 11 and 12.
Trials to obtain dimesityl-substituted thiourea 14 from 13 with thiophosgene or with carbon disulfide were not successful, the starting material was recovered in both reactions. The reason might be the higher steric hindrance in comparison to 9 and due to the ambient reaction conditions (Scheme 9).
Attempts for the formation of 14.
Attempts to form 10 were also conducted with 1,3-dibenzylthiourea, with 1,4-dibromobutane, and with different solvents and bases under various conditions and were not successful. Efforts to get 16via a similar route from 15 were also not successful (Scheme 10) [35].
Attempted cyclization of 15.
Starting from 1b, the NHC was generated with KHMDS and 2 equiv. of sulfur were added (Scheme 11). The product 17b could be isolated in 44% yield after 16 h at room temperature in THF. When the reaction was carried out for 2 h in refluxing toluene, the yield was lowered to 32%. Thiourea 17a was synthesized under various reaction conditions as presented in Table 1. The best results were obtained after 18 h, when KOt-Bu was used in THF as solvent (Entry 3). A reaction with Na2S gave no product (Entry 5) [36]. The weak base and the small amount of sulfur may be a reason for the low yield in Entry 6.
Synthesis of dimesityl-substituted thiourea 17.
Reagents and reaction conditions for Scheme 11.
| Entry | Base | n(S8) (equiv.) | Solvent | Temperature (°C) | Yield (%) |
|---|---|---|---|---|---|
| 1 | KHMDS (1 equiv.) | 4 | Toluene | 20 | 66 |
| 2 | KHMDS (2 equiv.)a | 1.5 | THF | 20 | 21 |
| 3 | KOt-Bu (1 equiv.) | 1 | THF | 20 | 80 |
| 4 | NaH (1 equiv.) | 1 | THF | 20 | 39 |
| 5 | KHMDS (1 equiv.) | –b | THF | 20 | 0 |
| 6 | K2CO3 (1 equiv.) | 0.125 | MeOH | 65 | –c |
| 7 | Pyridine (6.7 equiv.) + DBU (4.5 equiv.) | 3 | MeOH | 65 | 41 |
aThe carbene was generated at –78°C, and the reaction mixture was subsequently warmed up to room temperature. bNa2S (1.85 equiv.) was added 30 min after KHMDS instead of S8; ctraces of product were found but not isolated.
The subsequent reaction of 17a with oxalyl chloride or bromide resulted in no product. A reason for this might be the higher steric hindrance due to the aryl substituents. Even an increased temperature with up to 111°C in toluene did not afford the products. All similar examples that were described in the literature are sterically less hindered.
A more simple approach was the treatment of 1 with KHMDS and bromine [37], which gave the desired product in up to 79% yield (Scheme 12).
Formation of bromoamidinium salts 18 with bromine.
2.2 Synthesis of the NHC-metal complexes
The haloamidinium salts 11 and 12 were mixed with Pd(PPh3)4 and heated in toluene at 100°C (Scheme 13). After workup, two complexes could be isolated as crystals, which were suitable for X-ray crystal structure analysis (Figs. 1 and 2). In both cases, a neutral, quadratic-planar palladium complex with one NHC, one PPh3, and two halide ligands was formed. The configuration was cis in contrast to the result of Fürstner et al. [16], who obtained trans-configured complexes with one six-membered NHC ligand incorporating smaller methyl substituents on the nitrogen atoms. Furthermore, their complex was positively charged and coordinated by a second PPh3 ligand. The reaction of 1a with Pd(PPh3)4 did not yield isolable complexes under the mentioned conditions.
Synthesis of Pd complexes.
Molecular structure of 19 in the crystal. Anisotropic displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms and solvent molecules are omitted.
Molecular structure of 20 in the crystal. Anisotropic displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms and solvent molecules are omitted.
In addition, the haloamidinium salts 18a–c were mixed with cobalt powder (20 equiv.; particle size <150 μm) and heated in different solvents. The only isolated and identified product was the cation of 1c with [CoBr4]2− and bromide anions (Scheme 14) [38]. The reaction of 12 with Co powder yielded a cobalt pyridinone complex [39].
Attempted formation of Co NHC complexes.
2.3 Catalytic reactions
The obtained complexes 19 and 20 were tested as potential catalysts in the Suzuki-Miyaura reaction [40]. The reactions with aryl halides and phenylboronic acid (1.5 equiv.) were carried out in THF with 1 mol% of the respective complex and 2.5 equiv. of a base (Scheme 15). The results are summarized in Table 2.
Suzuki-Miyaura reaction with the new Pd complexes.
Results of the catalytic Suzuki-Miyaura reactions.
| Entry | Catalyst | Aryl halide | Base | Yield (%) |
|---|---|---|---|---|
| 1 | 19 | 1-Bromo-3,5-dimethylbenzene | Cs2CO3 | 86a |
| 2 | 20 | 1-Bromo-3,5-dimethylbenzene | Cs2CO3 | 93a |
| 3 | 19 | 1-Bromonaphthalene | CsF | 96b |
| 4 | 20 | 1-Bromonaphthalene | CsF | 92b |
| 5 | 19 | 1-Bromonaphthalene | CsF | 54c,d |
| 6 | 20 | 1-Bromonaphthalene | CsF | 45c,d |
| 7 | 19 | m-Chlorotoluene | CsF | 7c,d |
| 8 | 20 | m-Chlorotoluene | CsF | 11c,d |
| 9 | 19 | m-Bromotoluene | CsF | 11c,d |
| 10 | 20 | m-Bromotoluene | CsF | 9c,d |
| 11 | 20 | 1-Bromonaphthalene | CsF | 11c, e |
a24 h at 50°C; b24 h at 20°C; c16 h at 20°C; d4-methoxyphenylboronic acid instead of phenylboronic acid; ereaction was carried out without inert gas.
With 1-bromo-3,5-dimethylbenzene, high yields of 86% and 93% were obtained, respectively (Entries 1 and 2). The change of the aryl halide and the base enhanced the yields (Entries 3 and 4). The use of 4-methoxyphenylboronic acid and a shorter reaction time decreased the yields of entries 5 and 6 in comparison to entries 3 and 4. Low yields were always obtained, when m-chloro- or m-bromotoluene was used (entries 7–10). Table 2 also shows that the influence of the applied base was low [41, 42]. The steric demand seems to be more influential because the yield was always lower when 1-bromo-3,5-dimethylbenzene, m-chloro- and m-bromotoluene were used (entries 7–10). The low steric shielding of the metal core by the freely rotating benzyl side arms and the relatively low steric demand of the aryl halides probably causes a slow reductive elimination because the steric repulsion between the ligands is small. The decelerated reductive elimination lowers the turnover frequency, so that only low yields were obtained. Increased temperature and longer reaction time could improve the results (entries 1 and 2). The use of 1-bromonaphthalene led to high yields at room temperature (entries 3 and 4).
Furthermore, 4-methoxyphenylboronic acid was applied as coupling partner (entries 5–10). The reactivity differences between m-chloro- and m-bromotoluene are low (entries 9–12). Based on the results, both complexes showed a similar promising activity.
To investigate the stability of the complex in air, the catalyst 20 was also applied in a reaction without inert gas (entry 11), but the conversion was low.
The five-membered analogue in Scheme 3 gave for an electron-deficient bromoarene a 79% yield in a Suzuki-Miyaura reaction [16].
3 Conclusion
Three new haloamidinium salts were successfully synthesized and characterized. Thereby an efficient route was applied for extended haloamidinium salts. The benzyl-substituted chloroamidinium salt 11 was obtained after treatment of the thiourea derivate with oxalyl chloride. Moreover, it was shown for the first time that bromoamidinium salts can be produced from oxalyl bromide and the corresponding thiourea. Two new mixed Pd-extended-NHC phosphane complexes could be isolated and successfully applied in the Suzuki reaction cross-coupling reaction.
4 Experimental section
Reactions were carried out under inert gas using standard Schlenk techniques, unless mentioned otherwise. All commercially available chemicals were used without further purification. Solvents were dried prior to use by standard procedures. Thin layer chromatography was performed on plates from Merck (Silica gel 60, F254), substances were detected under UV light at 254 nm.
NMR spectra were recorded at 30°C on a Bruker Avance 500 (1H, 500 MHz; 13C, 125 MHz; 15N, 51 MHz; 31P, 202 MHz) referenced to the residual proton or carbon signals of the deuterated solvent (1H and 13C NMR). The NMR signals are reported in ppm relative to TMS. Liquid ammonia was used as an external reference for 15N NMR. Phosphoric acid (85%) was used as an external reference for 31P NMR. Mass spectrometry was carried out on Waters Quadrupole-ToF Synapt 2G using electrospray ionization (ESI). IR spectra were recorded on a Bruker Vertex 70 spectrometer. Elemental analyses were performed on an Elementar vario MicroCube analyzer. Melting points were measured with a Büchi Melting Point B-545 apparatus.
1a [43], 1b [29], 1c [44], 13 [43], and 15 [45] were synthesized according to literature procedures, and spectral data were consistent.
4.1 General procedure for the synthesis of diimines
Diamine (0.05 mol), benzaldehyde (10.1 mL, 0.1 mol), and water (30 mL) were stirred vigorously for 16 h without inert gas. Afterward, the emulsion was extracted with dichloromethane (3 × 100 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure to yield the product.
4.1.1 N1,N3-Dibenzylidenepropane-1,3-diamine (4)
Propane-1,3-diamine (4.17 mL), benzaldehyde, and water were reacted at room temperature. After workup, the product was isolated as a colorless oil (12.22 g, 0.0487 mol; 97%). The spectroscopic data were consistent with the literature [46]. – 1H NMR (CDCl3): δ = 8.31 (s (broad), 2 H, N=CH), 7.76–7.72 (m, 4 H, ArH), 7.43–7.40 (m, 6 H, ArH), 3.73 (dt, J = 7.0 Hz, 4 H, N–CH2), 2.13 (quint, J = 7.0 Hz, 2 H, CH2–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 161.3 (C=N), 136.4 (ArC), 130.5 (ArC), 128.6 (ArC), 128.1 (ArC), 59.2 (N–CH2), 32.0 (CH2–CH2–CH2) ppm.
4.1.2 N1,N4-Dibenzylidenebutane-1,4-diamine (7)
Butane-1,4-diamine (5 mL), benzaldehyde, and water were reacted at room temperature. After workup, the product was isolated as a colorless oil (12.60 g, 0.0476 mol; 95%). The spectroscopic data were consistent with the literature [47]. – 1H NMR (CDCl3): δ = 8.29 (s (broad), 2 H, N=CH), 7.74–7.71 (m, 4 H, ArH), 7.42–7.38 (m, 6 H, ArH), 3.70–3.65 (m, 4 H, N–CH2), 1.84–1.78 (m, 4 H, N–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 160.9 (C=N), 136.4 (ArC), 130.5 (ArC), 128.6 (ArC), 128.1 (ArC), 61.5 (N–CH2), 28.7 (N–CH2–CH2) ppm.
4.2 General procedure for the synthesis of diamines
The diimine (1 equiv.) was dissolved in methanol (50 mL), after that NaBH4 (2 equiv.) was added in small portions to the reaction vessel. The mixture was refluxed for 15 min; after cooling, water (70 mL) was added carefully. The aqueous phase was extracted with dichloromethane (3 × 50 mL). The combined organic phases were dried over K2CO3, and the solvent was removed under reduced pressure to yield the product.
4.2.1 N1,N3-Dibenzylpropane-1,3-diamine (5)
NaBH4 (3.69 g, 0.098 mol) was added to a solution of 4 (12.22 g, 0.048 mol) in methanol. After workup, the product was isolated as a pale yellow oil (11.87 g, 0.047 mol; 96%). The spectroscopic data were consistent with the literature [46]. – 1H NMR (CDCl3): δ = 7.35–7.31 (m, 8 H, ArH), 7.27–7.24 (m, 2 H, ArH), 3.79 (s, 4 H, N–CH2-Ph), 2.72 (t, J = 6.7 Hz, 4 H, CH2–CH2–CH2), 1.74 (quint, J = 6.7 Hz, 2 H, CH2–CH2–CH2), 1.49 (s (broad), 2 H, NH) ppm. – 13C NMR (CDCl3): δ = 140.6 (ArC), 128.4 (ArC), 128.1 (ArC), 126.9 (ArC), 54.1 (N–CH2-Ph), 48.0 (CH2–CH2–CH2), 30.3 (CH2–CH2–CH2) ppm.
4.2.2 N1,N3-Dibenzylpropane-1,3-diamine (8)
NaBH4 (3.31 g, 0.087 mol) was added to a solution of 7 (11.56 g, 0.044 mol) in methanol. After workup, the product was isolated as a pale yellow oil (11.43 g, 0.043 mol; 97%). The spectroscopic data were consistent with the literature [47]. – 1H NMR (CDCl3): δ = 7.40–7.22 (m, 10 H, ArH), 3.79 (s, 4 H, N–CH2-Ph), 2.67–2.62 (m, 4 H, N–CH2–CH2), 1.59–1.54 (m, 4 H, N–CH2–CH2), 1.35 (s (broad), 2 H, NH) ppm. – 13C NMR (CDCl3): δ = 140.6 (ArC), 128.4 (ArC), 128.1 (ArC), 126.9 (ArC), 54.1 (N–CH2-Ph), 49.3 (N–CH2–CH2), 27.9 (N–CH2–CH2) ppm.
4.2.3 1,3-Dibenzyltetrahydropyrimidine-2(1H)-thione (9)
Compound 5 (6.018 g, 0.024 mol), carbon disulfide (2.11 mL, 0.035 mol) and pyridine (20 mL) were refluxed for 24 h. After the volatile components were removed under reduced pressure, the residue was recrystallized two times from ethanol and washed afterward with cold ethanol. Colorless crystals (4.534 g, 0.015 mol; 64%) were obtained after vacuum drying; m.p. 135°C. IR (KBr): v = 3437, 2906, 1509, 1352, 704 cm−1. – 1H NMR (CDCl3): δ = 7.41–7.32 (m, 8 H, ArH), 7.30–7.25 (m, 2 H, ArH), 5.37 (s, 4 H, N–CH2-Ph), 3.28 (t, J = 6.1 Hz, 4 H, CH2–CH2–CH2), 1.88 (quint, J = 6.1 Hz, 2 H, CH2–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 180.6 (N–C–N), 137.4 (ArC), 128.6 (ArC), 127.7 (ArC), 127.4 (ArC), 58.7 (N–CH2-Ph), 46.2 (CH2–CH2–CH2), 21.2 (CH2–CH2–CH2) ppm. – 15N NMR (CDCl3): δ = 115.6 (N–C–N) ppm. – HRMS ((+)-ESI): m/z = 319.1232 (calc. 319.1240 for C18H20N2SNa, [M+Na]+).
4.2.4 1,3-Dimesityltetrahydropyrimidine-2(1H)-thione (17a)
Compound 1a (50 mg, 0.125 mmol) was mixed with KOt-Bu (14 mg, 0.125 mmol) and sulfur (32 mg, 0.125 mmol) in THF (3.1 mL). The mixture was stirred for 18 h at room temperature and quenched with water (10 mL). The aqueous phase was extracted with CH2Cl2 (3 × 10 mL). The combined organic phases were dried over Na2SO4, and the solvent was removed under reduced pressure. After column chromatography of the residue (petroleum ether/EtOAc = 8:2; Rf = 0.41) the product was obtained as colorless crystalline solid (35 mg, 0.099 mmol; 80%). Spectral data were consistent with literature values [48]. IR (KBr): v = 3433, 2949, 2855, 1607, 1485, 1299, 1196, 851 cm−1. – 1H NMR (CDCl3): δ = 6.92 (s, 4 H, ArH), 3.60 (t, J = 5.8 Hz, 4 H, N–CH2), 2.35 (quint, J = 5.8 Hz, 2 H, CH2–CH2–CH2), 2.30 (s, 12 H, Ar-o-CH3), 2.29 (s, 6 H, Ar-p-CH3) ppm. – 13C NMR (CDCl3): δ = 177.9 (C=S), 141.8 (ArC), 136.9 (ArC), 134.7 (ArC), 129.5 (ArC), 48.5 (N–CH2), 22.3 (CH2–CH2–CH2), 21.1 (Ar-p-CH3), 17.9 (Ar-o-CH3) ppm. – 15N NMR (CDCl3): δ = 115.7 (N–C–N) ppm. HRMS ((+)-ESI): m/z = 353.2043 (calcd. 353.2051 for C22H29N2S, [M+H]+).
4.2.5 1,3-Dimesityl-1,3-diazepane-2-thione (17b)
KHMDS (1.06 mL, c = 0.5 mol L−1 in toluene, 0.529 mmol) was added to a solution of 1b (200 mg, 0.481 mmol) in THF (7 mL) and stirred for 30 min at room temperature. Then sulfur (0.247 g, 0.963 mmol) was added, and the mixture was stirred for 16 h. After filtration, the solvent was removed under reduced pressure. After two separations by column chromatography (n-hexane/EtOAc = 8.5:1.5; Rf = 0.33), the product was obtained as an colorless crystalline solid (77 mg, 0.212 mmol; 44%). IR (KBr): v = 3437, 2916, 2359, 1461, 1293 cm−1. – 1H NMR (CDCl3): δ = 6.92 (s, 4 H, ArH), 3.88 (s (broad), 4 H, N–CH2), 2.34 (s, 12 H, Ar-o-CH3), 2.29 (s, 6 H, Ar-p-CH3), 2.06 (quint, J = 2.9 Hz, 4 H, CH2–CH2–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 186.7 (C=S), 144.2 (ArC), 136.4 (ArC), 134.6 (ArC), 129.7 (ArC), 54.7 (N–CH2), 25.8 (CH2–CH2–CH2–CH2), 21.0 (Ar-p-CH3), 18.9 (Ar-o-CH3) ppm. – 15N NMR (CDCl3): δ = 121.6 (N–C–N) ppm. – HRMS ((+)-ESI): m/z = 367.2208 (calcd. 367.2203 for C23H31N2S, [M+H]+).
4.2.6 General procedure for the synthesis of benzyl substituted haloamidinium salts
Pyrimidinethione 9 (1 equiv.) was stirred with oxalyl halide (1.2 equiv.) in toluene at 60°C for 16 h. The solvent was decanted subsequently, and the residue was washed three times with Et2O. After vacuum-drying, the product was obtained as a beige powder.
4.2.7 1,3-Dibenzyl-2-chloro-3,4,5,6-tetrahydropyrimidin-1-ium chloride (11)
Compound 9 (400 mg, 1.35 mmol) reacted with oxalyl chloride (0.138 mL, 1.62 mmol) in toluene (15 mL). The product (413 mg, 1.23 mmol; 91%) was highly hygroscopic and had to be protected from humidity; m.p. 101°C. – IR (KBr): v = 3429, 3387, 1627, 1325, 757, 704 cm−1. – 1H NMR (CDCl3): δ = 7.43–7.34 (m, 6 H, ArH), 7.33–7.28 (m, 4 H, ArH), 5.11 (s, 4 H, N–CH2-Ph), 3.94 (t, J = 5.8 Hz, 4 H, CH2–CH2–CH2), 2.17 (quint, J = 5.8 Hz, 2 H, CH2–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 152.9 (N–C–N), 132.6 (ArC), 129.5 (ArC), 129.0 (ArC), 127.6 (ArC), 59.4 (N–CH2-Ph), 49.2 (CH2–CH2–CH2), 19.7 (CH2–CH2–CH2) ppm. – 15N NMR (CDCl3): δ = 127.5 (N–C–N) ppm. – HRMS ((+)-ESI): m/z = 299.1301 (calcd. 299.1310 for C18H20ClN2, [cation]+).
4.2.8 1,3-Dibenzyl-2-bromo-3,4,5,6-tetrahydropyrimidin-1-ium bromide (12)
Compound 9 (500 mg, 1.69 mmol) reacted with oxalyl bromide (0.288 mL, 2.02 mmol) in toluene (20 mL) to yield the product (707 mg, 1.67 mmol; 99%); m.p. 221°C (decomposition). – IR (KBr): v = 3027, 2922, 1615, 1325, 753, 704 cm−1. – 1H NMR (CDCl3): δ = 7.40–7.29 (m, 10 H, ArH), 5.14 (s, 4 H, N–CH2-Ph), 3.85 (t, J = 5.8 Hz, 4 H, CH2–CH2–CH2), 2.15 (quint, J = 5.8 Hz, 2 H, CH2–CH2–CH2) ppm. – 13C NMR (CDCl3): δ = 149.0 (N–C–N), 132.7 (ArC), 129.4 (ArC), 129.0 (ArC), 127.6 (ArC), 62.0 (N–CH2-Ph), 49.3 (CH2–CH2–CH2), 19.9 (CH2–CH2–CH2) ppm. – 15N NMR (CDCl3): δ = 131.0 (N–C–N) ppm. – HRMS ((+)-ESI): m/z = 345.0789 (calcd. 345.0784 for C18H20BrN2, [cation]+).
4.3 General procedure for mesityl-substituted bromoamidinium salts
The carbene precursor 1 (1 equiv.) was suspended in benzene and treated with KHMDS solution (1.5 equiv.). After 1 h, bromine (2 equiv.) was added under the exclusion of light; afterward, the mixture was stirred for 16 h at room temperature. The solid was separated with a centrifuge, washed with benzene (2 × 5 mL) and with Et2O (5 mL). The residue was dissolved in chloroform (50 mL) and washed with water (3 × 20 mL). The organic phase was dried over Na2SO4, and the solvent was removed under reduced pressure. The residue was dissolved in a small amount of chloroform, dropwise addition of Et2O under continuous stirring led to precipitation of a solid. The solid was separated with a centrifuge and washed with Et2O (5 mL). After vacuum drying, the product could be obtained as a powder.
4.3.1 2-Bromo-1,3-dimesityl-4,5-dihydro-1H-imidazol-3-ium bromide (18c)
Compound 1c (400 mg, 1.032 mmol) in benzene (5.3 mL) reacted with KHMDS (2.2 mL, c = 0.7 mol L−1 in toluene) and bromine (106 μL, 2.064 mmol). The product was obtained as a white solid (380 mg, 0.815 mmol; 79%). The spectroscopic data were consistent with the literature [37]. 1H NMR (CDCl3): δ = 6.97 (s, 4 H, ArH), 4.83 (s, 4 H, CH2), 2.31 (s, 12 H, Ar-o-CH3), 2.30 (s, 6 H, Ar-p-CH3) ppm. – 13C NMR (CDCl3): δ = 151.6 (N–C–N), 141.3 (ArC), 135.2 (ArC), 130.5 (ArC), 130.3 (ArC), 52.6 (N–CH2), 21.1 (Ar-p-CH3), 17.8 (Ar-o-CH3) ppm. – HRMS ((+)-ESI): m/z = 385.1295 (calcd. 385.1274 for C21H26BrN2, [cation]+).
4.3.2 2-Bromo-1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-1-ium bromide (18a)
Compound 1a (414 mg, 1.032 mmol) in benzene (10 mL) reacted with KHMDS (2.2 mL, c = 0.7 mol L−1 in toluene) and bromine (106 μL, 2.064 mmol). The product was obtained as a yellow solid (356 mg, 0.741 mmol; 72%). The spectroscopic data were consistent with the literature [37]. 1H NMR (CDCl3): δ = 6.98 (s, 4 H, ArH), 4.39 (t, J = 5.6 Hz, 4 H, N–CH2), 2.68 (quint, J = 5.6 Hz, 2 H, CH2–CH2–CH2), 2.33 (s, 12 H, Ar-o-CH3), 2.30 (s, 6 H, Ar-p-CH3) ppm. – 13C NMR (CDCl3): δ = 148.2 (N–C–N), 140.7 (ArC), 139.4 (ArC), 133.6 (ArC), 130.4 (ArC), 51.9 (N–CH2), 21.1 (Ar-p-CH3), 20.5 (CH2–CH2–CH2), 17.8 (Ar-o-CH3) ppm. – HRMS ((+)-ESI): m/z = 399.1429 (calcd. 399.1431 for C22H28BrN2, [cation]+).
4.3.3 2-Bromo-1,3-dimesityl-4,5,6,7-tetrahydro-1H-1, 3-diazepin-3-ium bromide (18b)
Compound 1b (350 mg, 0.843 mmol) in benzene (10 mL) reacted with KHMDS (2.5 mL, c = 0.5 mol L−1 in toluene) and bromine (87 μL, 1.685 mmol). The product was obtained as a beige solid (206 mg, 0.417 mmol; 49%). IR (KBr): v = 3433, 2953, 2920, 1607, 1567 cm−1. – 1H NMR (CDCl3): δ = 6.96 (s, 4 H, ArH), 4.76 (s (broad), 4 H, N–CH2), 2.49 (quint, J = 3.1 Hz, 4 H, CH2–CH2–CH2–CH2), 2.36 (s, 12 H, Ar-o-CH3), 2.28 (s, 6 H, Ar-p-CH3) ppm. – 13C NMR (CDCl3): δ = 152.0 (N–C–N), 141.9 (ArC), 140.4 (ArC), 133.1 (ArC), 130.6 (ArC), 58.0 (N–CH2), 23.3 (CH2–CH2–CH2–CH2), 21.0 (Ar-o-CH3), 18.2 (Ar-p-CH3) ppm. – 15N NMR (CDCl3): δ = 141.4 (N–C–N) ppm. – HRMS ((+)-ESI): m/z = 413.1584 (calcd. 413.1587 for C23H30BrN2, [cation]+).
4.4 General procedure for the synthesis of Pd complexes
The haloamidinium salt (0.354 mmol) was mixed with Pd(PPh3)4 (0.408 g, 0.354 mmol) in toluene (20 mL) and heated for 2 h at 100°C. After cooling, the solvent was removed via condensation. The residue was stirred for 30 min with n-pentane (3 × 20 mL). Then the residue was dissolved in CH2Cl2, and the insoluble components were removed with a syringe filter. The CH2Cl2 solution was layered with n-pentane to yield crystals after 2 days.
4.4.1 (1,3-Dibenzylhexahydropyrimidin-2-yl)(triphenylphosphane)palladium(II)chloride (19)
Compound 11 (0.119 g) yielded pale yellow prism-shaped crystals (0.158 g, 0.224 mmol; 63%) after workup; m.p. 270°C (decomposition). – IR (KBr): v = 3433, 3057, 1539, 704 cm−1. – 1H NMR (CD2Cl2): δ = 7.91–7.78 (m, 6 H, ArH), 7.62–7.56 (m, 3 H, ArH), 7.55–7.48 (m, 10 H, ArH), 7.41–7.32 (m, 6 H, ArH), 6.59 (d, J = 14.1 Hz, 2 H, N–CH2-Ph), 4.30 (d, J = 14.1 Hz, 2 H, N–CH2-Ph), 2.85–2.77 (m, 2 H, N–CH2–CH2), 2.45–2.36 (m, 2 H, N–CH2–CH2), 1.56–1.46 (m, 1 H, CH2–HCH–CH2), 1.39–1.26 (m, 1 H, CH2–HCH–CH2) ppm. – 13C NMR (CD2Cl2): δ = 187.0 (d, J = 6.9 Hz, N–C–N), 134.8 (ArC), 134.6 (d, J = 11.0 Hz, ArC), 131.3 (d, J = 2.7 Hz, ArC), 129.2 (ArC), 128.6 (d, J = 11.0 Hz, ArC), 128.5 (ArC), 128.1 (ArC), 62.6 (d, J = 3.2 Hz, N–CH2-Ph), 43.8 (CH2–CH2–CH2), 19.5 (CH2–CH2–CH2) ppm. – 15N NMR (CD2Cl2): δ = 127.4 (N–C–N) ppm. – 31P NMR (CD2Cl2): δ = 26.24 (PPh3) ppm. – HRMS ((+)-ESI): m/z = 725.0704 (calcd. 725.0842 for C36H35Cl2N2PPdNa, [M+Na]+).
4.4.2 (1,3-Dibenzylhexahydropyrimidin-2-yl)(triphenylphosphane)palladium(II) bromide (20)
Compound 12 (0.150 g) yielded pale yellow needle-shaped crystals (0.208 g, 0.262 mmol; 74%) after workup; m.p. 286°C (decomposition). – IR (KBr): v = 3433, 3053, 1543, 700 cm−1. – 1H NMR (CD2Cl2): δ = 7.90–7.75 (m, 6 H, ArH), 7.62–7.56 (m, 3 H, ArH), 7.54–7.44 (m, 10 H, ArH), 7.40–7.32 (m, 6 H, ArH), 6.49 (d, J = 14.1 Hz, 2 H, N–CH2-Ph), 4.34 (d, J = 14.1 Hz, 2 H, N–CH2-Ph), 2.88–2.78 (m, 2 H, N–CH2–CH2), 2.46–2.36 (m, 2 H, N–CH2–CH2), 1.63–1.52 (m, 1 H, CH2–HCH–CH2), 1.38–1.28 (m, 1 H, CH2–HCH–CH2) ppm. – 13C NMR (CD2Cl2): δ = 188.4 (d, J = 9.1 Hz, N–C–N), 134.8 (d, J = 11.0 Hz, ArC), 134.6 (ArC), 131.3 (d, J = 2.7 Hz, ArC), 129.1 (ArC), 128.5 (d, J = 11.0 Hz, ArC), 128.5 (ArC), 128.1 (ArC), 62.4 (d, J = 2.7 Hz, N–CH2-Ph), 44.0 (CH2–CH2–CH2), 19.5 (CH2–CH2–CH2) ppm. – 15N NMR (CD2Cl2): δ = 127.6 (N–C–N) ppm. – 31P NMR (CD2Cl2): δ = 25.71 (PPh3) ppm. – HRMS ((+)-ESI): m/z = 814.9745 (calcd. 814.9812 for C36H35Br2N2PPdNa, [M+Na]+).
4.5 General procedure for the Suzuki cross-coupling reaction
Aryl halide (0.71 mmol), arylboronic acid (1.065 mmol), base (1.776 mmol), and the catalyst (1 mol%) were stirred in THF (2 mL). After the reaction time given in Table 2, the suspension was chromatographed to yield the desired coupling product. Spectral data of the coupling products were consistent with the literature [49–52].
4.6 X-ray crystal structure determinations
Data collections were done with a Bruker AXS SMART APEX CCD diffractometer using graphite- monochromatized MoKα radiation. Data reduction and absorption corrections were performed with Saint and Sadabs [53, 54]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were clearly identified in difference syntheses, refined at idealized positions riding on the carbon atoms with isotropic displacement parameters Uiso(H) = 1.2 × Ueq(C) and C–H 0.95–0.99 Å. For 19, 0.85 CH2Cl2 solvent molecule per asymmetric unit was treated with the Squeeze facility of Platon [55] due to severe disorder. In 20, 1.5 CH2Cl2 are present per asymmetric unit. Table 3 contains the crystal data as well as numbers pertinent to data collection and structure refinement. See also below for additional crystallographic data given as Supporting Information.
Crystal structure data for 19 and 20.
| 19 | 20 | |
|---|---|---|
| Formula | C39.40H41.80Cl8.80N2PPd | C37.50H38Br2Cl3N2PPd |
| Mr | 992.68 | 920.24 |
| Cryst. size, mm3 | 0.37 × 0.20 × 0.08 | 0.47 × 0.03 × 0.02 |
| Crystal system | Monoclinic | Monoclinic |
| Space group | P21/n | P21/c |
| a, Å | 9.6544(7) | 7.9014(5) |
| b, Å | 15.9129(11) | 23.2167(14) |
| c, Å | 22.7511(16) | 20.1717(12) |
| β, deg | 94.444(2) | 99.488(2) |
| V, Å3 | 3484.7(4) | 3649.8(4) |
| Z | 4 | 4 |
| Dcalcd, g cm−3 | 1.892 | 1.675 |
| μ(MoKα), cm−1 | 1.292 | 2.992 |
| F(000), e | 2011 | 1836 |
| hkl range | –12 ≤ h ≤ 11 | –10 ≤ h ≤ 10 |
| –20 ≤ k ≤ 20 | –30 ≤ k ≤ 30 | |
| –29 ≤ l ≤ 29 | –21 ≤ l ≤ 26 | |
| ((sinθ)/λ)max, Å−1 | 0.66 | 0.66 |
| Refl. measured | 32811 | 34613 |
| Refl. unique | 8316 | 8722 |
| Rint | 0.0448 | 0.0790 |
| Param. refined | 379 | 424 |
| R(F)/wR(F2)a (all reflexions) | 0.034/0.079 | 0.044/0.095 |
| GoF (F2)b | 1.029 | 0.993 |
| Δρfin (max/min), e Å−3 | 0.63/−0.33 | 1.16/−1.30 |
aR1 = ||Fo| – |Fc||/∑|Fo|, wR2 = [∑w(Fo2 – Fc2)2/∑w(Fo2)2]1/2, w = [σ2(Fo2) + (AP)2 + BP]−1, where P = (max(Fo2, 0) + 2Fc2)/3; bGoF = [∑w(Fo2 – Fc2)2/(nobs – nparam)]1/2.
CCDC 1445899 (19) and 1445900 (20) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.
5 Supporting information
The 1H and 13C NMR spectra of 4, 5, 7, 8, 9, 11, 12, 17a, 17b, 18a, 18b, 18c, 19, and 20 as well as crystallographic data for 19 and 20 including the atom coordinates are given as Supporting Information available online (DOI: 10.1515/znb-2016-0011).
Further materials for the crystal structure determination can be found in Table 3 or in the supporting information.
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