Synthetic Approaches to Triarylboranes from 1885 to 2020

Abstract In recent years, research in the fields of optoelectronics, anion sensors and bioimaging agents have been greatly influenced by novel compounds containing triarylborane motifs. Such compounds possess an empty p‐orbital at boron which results in useful optical and electronic properties. Such a diversity of applications was not expected when the first triarylborane was reported in 1885. Synthetic approaches to triarylboranes underwent various changes over the following century, some of which are still used in the present day, such as the generally applicable routes developed by Krause et al. in 1922, or by Grisdale et al. in 1972 at Eastman Kodak. Some other developments were not pursued further after their initial reports, such as the synthesis of two triarylboranes bearing three different aromatic groups by Mikhailov et al. in 1958. This review summarizes the development of synthetic approaches to triarylboranes from their first report nearly 135 years ago to the present.

This review is divided into sections based on symmetrically (BAr 3 ,B Ar 2 Ar') andu nsymmetrically substituted (BArAr'Ar'')t riarylboranes depending on the startingm aterial used as the boron source. In this context,t he term unsymmetrically substi-tuted triarylboranesm eans that the boron centeri sb ound to three different aromatic systems. Symmetrically substituted triarylboranes bear one or two different types of aromatic groups as indicated in parentheses (vide infra) and, thus, have either (exact or approximate) 3-fold or 2-fold symmetry,respectively.

Boron trichloride, tribromide and boronic esters as boron sources
In 1880, Michaelis and co-workers began to investigate arylboranes to determine the valency of boron which was, at the time, debated to be three or five. [37] They reacted gaseous BCl 3 with diphenylmercury at elevated temperatures in as ealed tube and observed the formation of dichlorophenylborane 1 and HgCl 2 (Scheme 1). Compound 1 was isolated via distillation and wasc haracterized by elementala nalysisa nd conclusive follow-up chemistry.
After addition of different aqueous solutions, they obtained phenylboronic acid, the respective ethyl ester and the sodium, calcium and silver salts of the acid, as well as their p-tolyl analogues. [38] On an interesting side note, phenylboronic acid, as well as its sodiums alt,w ere also investigated for their antiseptic behavior,a nd were consumed by humans on ag ram scale without causing any considerable complaints. [38] In 2015, [39] a series of boronic acids and esters weret ested using the Ames assay, [40,41] and mosto ft hem were found to be mutagenic. Thus, this class of compounds should be treated with appropriate care and due testing shouldb ep erformed prior to use in humans,a lthough several boronic acids or relatedc ompounds have been approved for use as drugs. [39] In ad ifferent approach, Michaelis andc o-workers developed ap rocedure to generate triphenyl derivativeso fv ariousm ain group elements (M), namely phosphorus, arsenic, antimony, and boron. [42,43] The respective MCl 3 compound wasr eacted with ap henylhalide and elemental sodium at low temperature to generate the corresponding triphenyl compound according to Scheme2.
In 1885, Michaelis and co-workers mentioned that, via this general route, as mall amount of triphenylborane 2 was obtained, but it was not discussed further. [26] To the best of our knowledge,t his was the first literature report of the synthesis of at riarylborane. Four years later,t he synthesis of 2 wasi mprovedb yr eacting dichlorophenylborane 1 with chlorobenzene and sodium. [44] Thist ime, compound 2 was characterized by elemental analysis and the appearance of ag reen flame characteristic of boron [45] when burning the compound.
In 1889, Gattermanna nd co-workers reported ac onvenient methodf or the synthesis of BCl 3 . [46] Thus, access to the starting materialw as facilitated. Therefore, Michaelis et al. synthesized more dichloroarylboranes and their respective boronic acids, namely the o-tolyl, a-naphthyl, b-naphthyl, p-methoxyphenyl, o-methoxyphenyla nd p-ethoxyphenyl derivatives. [47] For the latter three compounds, the reactionp roceeded smoothly at room temperature. Furthermore, chlorodiphenylborane 3 and its borinic acid derivativew ere reported and characterized. Compound 3 was formed by reacting dichlorophenylborane with diphenylmercury at ca. 300 8Ci nasealed tube (Scheme 3). It was noted that triphenylborane 2 was not obtained this way.
In 1901, Michaelis andc o-workersr eporteda ni mproved methodf or the synthesis of mono-and diarylboranes in which they replaced gaseous BCl 3 with liquid and, thus, easier to handle BBr 3 as the boron source. [48] For this purpose, they developedaconvenient and large-scale synthesis of BBr 3 from elemental boron and bromine. BBr 3 wast hen reacted with diphenylmercuryi nd ry benzene. The reaction was performed in af lask with ar eflux condenser at 80 8C. Depending on the stoichiometry,d ibromophenylborane 4 and bromodiphenylborane 5 were synthesized and isolated via distillation( Scheme 4).
Another difficulty of arylborane syntheses were the often laborious and multistep syntheses of the required diarylmercury compounds. With the discoveryo ft he Grignardr eagent in 1900, ap owerful tool for the transfer of aryl groups became available. [49] The first to utilize this in arylborane chemistry, were Khotinsky and Melamed. [50] They treated various alkylborate esters with an aryl Grignardr eagent in ac old Et 2 Os olution. The best results were obtained for the iso-butylborate ester.F urthermore, Khotinsky and Melamed characterized the phenylboronic iso-butyl ester and the m-tolylboronic iso-butyl ester,a sw ella st he respective boronic acids after saponification. In an attempt to attach two arenes to the boron using Grignardr eagents, Strecker reacted an excess of phenylm agnesium bromide with BCl 3 ,b ut obtained only phenylboronic acid after aqueous work up. [51] Am ore extensive study of the reactions of aryl Grignardr eagents with the iso-butylborate ester was carried out by Kçnig and Scharrnbeck in 1915. The results were reportedi n1 930. [52] They characterized several novel arylboronic acids and diarylborinic acids which weres ynthesized accordingt oS cheme 5a nd isolated after aqueous work-up indicating that the organometallic reagentu sed was too unreactivet of orm the corresponding triarylborane. More than 70 years later,itw as demonstratedb ys everalg roups that triarylboranes can also be synthesized using boronic esters as startingm aterials and more reactive organometallic reagents (vide infra). [39,[53][54][55][56][57][58]

Boron trifluoride as the boron source
Despite reports of the synthesis of triphenylborane, [26,44] reproducible synthetic access was only availablef or mono-and diarylboranes in the beginning of the 20th century.T his changed with the studies of Krause, who made use of the synthesis of gaseous boron trifluoride (BF 3 )f rom boric anhydride (B 2 O 3 ), sulfuric acid (H 2 SO 4 )a nd potassium tetrafluoroborate (KBF 4 ), reported by Schiff et al. [59] In 1921, Krause and co-workers used gaseous BF 3 in combinationw ith Grignard reagents to yield trialkylboranes as well as alkylboronic acids. [60] Subsequently, Krause et al. applied this method for the synthesiso ft riphenylborane 2 (Scheme 6). [61] They isolatedB Ph 3 2 by distillation of the crude reaction mixture in ca. 50 %y ield. The product crystallized easily,b ut it was also mentioned that 2 decomposesi na ir.F urthermore, Krausea nd co-workers observed the formation of phenyldifluoroborane 6 as well as diphenylfluoroborane 7,b ut isolation of these two compounds was not possible by distillation. This indicates that the reactivity of the Grignard reagent is insufficient to generateo nly BPh 3 ,a sb yproducts 6 and 7 wereo bserved. However,w ith BPh 3 2 in hand, the group investigated its reactivity with neat sodium [62] and the other alkali metals potassium, lithium,r ubidium and cesium. [63] Krause and coworkers observed the formation of intensely colored solutions as well as the formation of, mostly,y ellow crystals. Both solutions and solids were reported to be highly air sensitive, as the solutionst urned colorless when exposed to air.T he colorless solution was converted into the colored solution again if neat metal was still present in the solution. After Krause and coworkers had isolatedt he reaction product of BPh 3 2 with neat sodium, [63] they titrated the reaction product under an itrogen atmosphere with elemental iodine whichr egeneratedB Ph 3 and sodium iodide. In the same study,the synthesis of tri-p-tolylborane 8 was mentioned. Its final synthesis and full characterization were reported two years later. [64] Again, the reactivity of 8 with sodium and potassium was investigated as well as its reactionw ith nitrogenous bases such as ammonia,p yridine, and piperidine. The reactiono f8 with neat sodium was described to be the same as for 2.D uring the reactions of 8 with nitrogenous bases, the group observed at emperature increase of the reactionm ixture as well as the formation of crystalline and more air-stable products which were assigned to be addition products of the nitrogenous bases with 8 (Scheme 7). This assumption was confirmed by elemental analysis of the reaction products. In 1930, Krause and co-workersa lso reported the synthesis of tri-p-xylylborane 9 and tri-a-naphthylborane 10,w hich were investigated similarly to the previousc ompounds 2 and 8. [65] For the isolation of 9 and 10,t he work up was slightly modified. Thus, to quench the remaining Grignard reagent, water was added and the resulting crude mixture was distilledw ith exclusion of air,a sn oneo ft he previously synthesized triarylboranes are stable to air.
Nevertheless, what they refer to as an "oxidation process" of compound 10 starts only after two weeks in air.F urthermore, Krause et al. reported that solutions of 10 in benzene, chloroform, tetrachloromethane, carbon disulfide, and diethyl ether show al ight blueish fluorescencet hat was more clearly visible with aq uartz lamp. They did not provide any furtheri nformation regarding whatk ind of lamp or which wavelength they used for excitation.F urthermore, the observed fluorescence was not investigated in detail. In 1931, the same group reported another triarylborane, namely tri-p-anisylborane 11,w hich was found to be as air-sensitive as triphenylborane 2. [66] In addition, Krause et al. had to change their work up once again, as they could not isolate 11 in pure form from the crude reaction mixture. Therefore, they reactedacrude mixture of 11 with gaseous ammonia to form the corresponding tetra-coordinate Lewis acid-basea dduct whichw as then purified and subsequently reacted with sulfuric acid with exclusion of air to yield compound 11.S imilarly,2 0years later,t he same group described the formationo fB Ph 3 upon heating different tetraarylborates alts to at least 200 8C. [67] Tetraarylborates are used on rare occasions to this day as valuable, alternative precursors to triarylboranes. [68][69][70] Based on this work, Brown et al. re-synthesized tri-a-naphthylborane 10 as ar eference Lewis acid to estimate the Lewis base strength of primary,s econdary,a nd tertiary amines, [71] having slightly modified the synthesiso ft he triarylborane. To make the synthesis safer,B rown and co-workersu sed boron trifluoride etherate( BF 3 ·OEt 2 )i nstead of gaseous boron trifluoride. Furthermore, they found that the triarylborane they synthesized was stable to air for more than one year.A st his finding was in contrast with the reports of Krause et al., [65] Brown et al. had ac loser look into the geometry of the compound. They assigned the discrepancy between their and the earlier results to the existence of two possible rotational conformers, i.e.,s teric hindrance resulted in restricted rotationa roundt he BÀCb onds.
In 1947, Wittig et al. investigated the possible application of triphenylborane 2 as ac atalystf or the lithiation of hydrocarbons. [72] Instead of successful catalysis of the reaction, they foundt he formation of as table complexw hich was later identified as lithium tetraphenylborate 12-Li. [73] Further investigations of such compounds, especially the reactiono fs odium tetraphenylborate 12-Na with variousm ono-cationic elements in aqueous solution,l ed to the discovery of an almosti nsoluble complex 12-K formed after addition of potassium salts. Later on, compound 12-Na became commercially available as Kalignost for the quantitative analysiso fp otassium in aqueous solution( Scheme8). [74] Wittig and co-workersf ound that triarylboranes such as 2 can also be synthesized from the corresponding, more reactive lithiated species instead of the Grignard reagent. [73] As long as it wasp ossible to synthesize the desired compounds from Grignardr eagents and BF 3 etherate,t hey did so. However,f or tri(o-diphenylyl)-13 and tri(4-(N,N-dimethylamino)phenyl)borane 14,W ittig et al. used the corresponding aryllithium reagent. [74] Nevertheless,the synthesis of 14 was still challenging, as the amine formed complexes with excessB F 3 .F urthermore, this group reported ay ellowish fluorescence from 14 in the solid state as well as ab lue fluorescencei na cetoneu pon irradiationw ith UV light. Compounds 15 and 16 were described as having ay ellowish-white fluorescence upon UV-irradiation. None of these observations were further explained or investigated by Wittig et al. Very recently,M arder and co-workersr eportedt hat as ample of pure 16 showed only blue fluorescence, with no phosphorescence being observeda tr oom temperature. [75] In 1956, Lappert summarized the preparation,c hemical and physicalp roperties, reactivities, etc. of almost all organoboranes that had been synthesized up to that date. [24] In this summary,s everalm ethods to synthesize monohaloboranes as well as unsymmetrically substituted diaryl borinice sters were described. However,a lmostn oneo ft hese synthesesw ere utilized for the formation of triarylboranes, especially not for the formationo fb oranes bearing three different aromatic systems.
One year later,B rowne tal. reported the synthesis of the sterically demandingt rimesitylborane 17 from the corresponding Grignardr eagent andB F 3 etherate. [76] The group heated the reagents in tolueneu nder reflux for 4h whicht hey described as forcing conditions. If the reaction was stopped after 2h,o nly fluorodimesitylborane 18 was isolated showing once again that the formation of triarylboranes from Grignard reagents is possible, but requires heat to achievec ompletion due to the lower reactivity of arylmagnesium reagents compared to, e.g.,a ryllithium reagents (Scheme 9). Furthermore, Brown and co-workers examined the reactivity of 17 with amines as well as its decompositionw ith water and oxygen.I t was found that 17 was less reactivet han tri-a-naphthyl-10 or triphenylborane 2 due to itsgreater steric hindrance.
Subsequently,t he syntheses of compounds 17 and 18 were furtheri mprovedb yH awkins et al. [77] who changed the solvent for the formation of the Grignardr eagent from diethyle ther to THF according to ag eneral procedure reported by Ramsden et al. [78] This change resulted in as horter reactiont ime for the formation, as well as an increased yield, of the Grignardr eagent. In addition, this led to the isolation of fluorodimesitylborane 18 in 96 %y ield. Due to its steric hinderance,t he reaction of excess mesityl Grignard reagent with BF 3 etherate at 55 8Cs tops at the fluorodimesitylborane stage as long as the reactiont ime is shorter than 2h.F urthermore, Hawkins and co-workerswere able to nitrate 17 to yield compound 19.
In 1967, at Eastman Kodak, Grisdale and co-workers began to investigatet he photophysical reactions of tetraarylborates and triarylboranes in solution. [79] They again found trimesitylborane 17 to be more stable than triphenylborane 2.T oi nvestigate furthert he influence of different substituents on the sta-bility of triarylboranes,G risdale et al. had ac loser look at the influence of the para-substituent in various dimesitylphenylboranes. [80] To synthesize av ariety of these new triarylboranes 20,this group was the first to combine the methods previously developed by different groups. First, Grisdalea nd co-workers isolatedf luorodimesitylborane 18 as reported by Brown. [76] This fluoroborane was then added to al ithiateds peciesp repared from the corresponding halogenated aromatics yielding 20 a-e in 40-90 %, as Wittig et al. had found lithium reagents to be suitable to react with boronhalides. [73] This reaction sequencer eflects the differentr eactivities of Grignard and organolithium reagents.G risdale et al. also conducted one of the first systematic investigationso ft he photophysical properties of the new triarylboranes in various solvents, observing emission solvatochromism, suggesting the stabilization of charge transfer excited states in polar solvents.

Metal-boron exchange reactions for the synthesis of triarylboranes
To date, the most widely used method for the synthesis of triarylboranes is the procedure developed by Grisdalea nd coworkers (Scheme 10) [80] i.e.,r eactionofB F 3 with either Grignard reagents or lithium reagents as discovered by Krause et al. [61] and Wittig et al., [73] respectively. However,m ercury,z inc, copper, silicon, and tin reagents have also been employed in the synthesis of triarylboranes with different reactivities, solvent compatibilities, stabilities, and accessibilities of these organometallic reagents. Furthermore, while mercury and tin reagents are not widely used currentlyd ue to their toxicities, other safetya spects may dominate the choiceo fo rganometallic reagent, depending on the organic group to be transferred.
Arylmercuryc ompounds were the first ArM reagents to be used for the synthesis of arylboranes (vide supra), but apart from af ew reports on arylboronic acids yntheses, [81,82] they have generally been replaced by Grignard or organolithium reagents. However,i n2 001, Piers and co-workerso btained the diborylated ferrocene compound 21 by reacting 1,1'-Fc(HgCl) 2 with ClB(C 6 F 5 ) 2 (Scheme 11 A). [83] The same group made use of Hg-B exchange to generatet he diborylated compound 22, which was then converted into at riarylborane via Zn-B exchange (Scheme 11 B). [84] Ay ear before,t hey reported Zn(C 6 F 5 ) 2 as ap otential C 6 F 5 transfer agent, whichr eacted with BCl 3 to generate inseparable mixtures of mono-, di-and triarylboranes. [85] In 2003, Jäkle et al. demonstrated the applicabilityo fa rylcopperr eagents in Cu-B exchange reactions. [86] Using mesitylcopper, am aximum of two arenes were attached even when the reaction with BX 3 (X = Cl,B r) was heated to 100 8C, or when dichlorophenylborane 1 was used as the startingm aterial to decrease the stericd emanda round the boron.R eaction of C 6 F 5 Cu with BX 3 at room temperature gave B(C 6 F 5 ) 3 23 irrespectiveo fs toichiometry (Scheme 12 A). Pentafluorophenylcopperw as also employed by Ashley,O 'Hare and co-workers as an aryl transfer reagent for the synthesis of triarylboranes 24 and 25 (Scheme 12 B, C) and, in one case, they made use of aZ n-B exchange to form the dibromoarylborane precursor (Scheme12B). [87] Later,J äkle and co-workers demonstrated that 2,4,6-tri-isopropylphenylcopper (CuTip) could also be employed in Cu-B exchange reactions. Thus, CuTip wasr eactedw ith sterically unhindered bromodiarylboranes to add the third arene to the boron center, [88,89] and theset riarylboranep recursorsw ere used in the formation of organoborane macrocycles and borazine oligomers.
Apart from Grignarda nd lithium reagents, the most widely used substrates for exchange reactionsw ith boron are organosilanes and organotin reagents. Aryltin reagents were used in Sn-B exchange reactions in the 1960s. In af irst approach by Burch et al.,t he phenylg roups of SnPh 4 were transferred to BCl 3 to give compound 1. [90] Reactiono fS nPh 4 with BCl 3 in CH 2 Cl 2 transferred one of the four phenylr ings from tin to boron.Withoutthe use of solvent, and under reflux conditions, all four rings were transferred (Scheme 13 A). In 1970, am ore selectivem ethodw as reported by Chivers, who synthesized ortho-substitutedm onoarylboranes from the corresponding monoaryltrimethylsilanes according to Scheme 13 B. [91] Halogen exchange between BCl 3 and the ortho-trifluoromethyl group was observed.
Furthers tudies as well as potentiala pplications were reported one year later by Kaufmann et al. [96,97] and Snieckus et al. [98] Jäkle and co-workersr eporteda ne fficient methodf or the introduction of at riarylborane moiety into the side chain of polystyrene. [99,100] The first step involved Si-B exchange and, in the next step, the triarylborane was formed via Sn-B and Cu-B exchanges, respectively (Scheme 16).
More recently,H elten and co-workersi mproved the Si-B exchange reactions ignificantly by employing ac atalytic amount of Me 3 SiNTf, [101] synthesizing three triarylboranes via Si-B ex-change and subsequentL i-B exchange reactions (Scheme 17). Helten and co-workerse mployed this method for the synthesis of triarylborane-containing macromolecules and polymers. In each case, the third arene was attached to the boron using an aryl lithium reagent. [102,103]

Potassiumaryltrifluoroboratesa sb oron sources
Potassium aryltrifluoroborates (ArBF 3 Ks alts) have been known since 1960. [104] Chambers et al. reported the synthesis of potassium (trifluoromethyl)trifluoroboratef rom ab oiling, aqueous solution of trimethyltin( trifluoromethyl)trifluoroborate and potassium fluoride (Scheme 18 A). In 1963, Stafford reported the synthesis of ap otassium vinyltrifluoroborate that was isolated in as imilarw ay to that previously described by Chambers. [105] Twoyears later,Chambers reported the synthesis of an aromatic potassium trifluoroborate 26 whichw as obtained from reaction of (pentafluorophenyl)difluoroborane and potassium fluoride (Scheme 18 B). [106] In 1967, Thierig and Umland reported the synthesis of potassium phenyltrifluoroborate 27 from Flavognost and potassium bifluoride (Scheme 18 C). [107] About 20 years later,K aufmann and co-workers made use of the solubility of potassium fluoride in acetonitrile to convert RBBr 2 compounds into their RBF 2 analogues or the corresponding potassium trifluoroborates (Scheme 19 A). [108] They also found BF 3 ·OEt 2 to be as uitable reagent to convertt he latter salts in situ into RBF 2 compounds.
Another way to activate potassium aryltrifluoroborates was reported by Vedejs in 1995, [109] who showedt hat potassium aryltrifluoroborates can be activated in situ to form aryldifluoro-boranes by addition of trimethylsilyl chloride (TMSCl). Furthermore, they provided ac onvenient route to potassium aryltrifluoroborates from the corresponding boronic acids and potassium bifluoride, KHF 2 (Scheme19B).
To date, BF 3 Ks alts are mostly employed in reactions in which the boron motif is lost, for example, in coupling reactions. [110] However,s uch compoundsc an also be used as the boron source for the syntheses of triarylboranes.
In 2004, Morrisone tal. were the first to synthesize triarylboranes 28 from potassium aryltrifluoroborate reagents which were activated with BF 3 etherate and then reactedw ith the Grignardr eagent C 6 F 5 MgBr (Scheme 20). [27] Since then, af ew other groups reportedt he synthesis of triarylboranes from these bench-stable boron precursors. Especially for applications in frustrated Lewis pairs, this approach was used for the synthesis of triarylboranes bearing aromatic systemsi nw hich multiple fluoro-and chloro-substituents are desired. Soósa nd co-workers [28,111,112] and Hoshimoto et al. [113] synthesized triarylboranes 29-35 from potassium aryltrifluoroborates and Grignard reagents without prior activation of the BF 3 Ks alt. The Grignard reagents in these cases were each prepared from the corresponding brominated precursor in combination with the so called "Turbo-Grignard" iso-propyl magnesium chloride lithium chloride (iPrMgCl·LiCl) as summarized in Scheme21A.Avery similar strategy,w ithout the use of the Turbo-Grignard, was used by Marder and co-workerstosynthesize ap ush-pull system with ap yrene core 36 (Scheme 22 A). [30] In contrast, Wagner and co-workerss ynthesized triarylboranes as precursors to polycyclic aromatic hydrocarbons [29] or quadruplya nnulated borepins. [114] In both cases, the required triarylboranes were synthesized from potassium aryltrifluoroborates which were reacted with various aryl lithium reagents yieldingc ompounds 37-40 (Scheme 22 B, C).

Direct dimesitylborylation
Ito and co-workers reportedt he direct dimesitylborylation of variousa ryl halides [115] by reaction of (diphenylmethylsilyl)dimesitylborane with aryl halidesi nt he presence of ab ase (Scheme 23 A). The halide was replaced by boron or silicon in a ratio of ca. 9t o1 .F urthermore, the reaction was tolerantt o several functional groups,a nd the resulting triarylboranes were isolated in moderate to good yields. In 2019, the same group reported an iridium-catalyzed CÀHd imesitylborylation of benzofuran using as ilyldimesitylborane reagent (Scheme23B), [116] preparing several derivatives and isolating the triarylboranes in moderate to good yields. Under optimized conditions, they reportedt he formation of the silylated side product in ca. 29 %y ield.

Synthesiso fU nsymmetrically Substituted Triarylboranes
Thus far,w eh ave summarized the syntheses of symmetrically substituted BAr 3 and BAr 2 Ar't riarylboranes. The synthesis of unsymmetrically substituted BArAr'Ar'' triarylboranes bearing three different aromatic rings bound to the boron centerc an be achieved by different routes, mosto fw hich use the same approaches used for the syntheses of symmetrically substituted triarylboranes.H owever,o ther routes employed symmetrically substituted triarylboranes as precursors.

Boronic esters as boron sources
In 1955, Letsinger and co-workers reported the synthesis of the first unsymmetrically substituted borinic acid starting from ab oronic ester. [118] They reacted phenylboronic acid butyl ester with a-naphthylmagnesium bromide andi solated the borinic acid as its b-aminoethyl ester,w hich was readily hydrolyzed to the borinic acid (Scheme 24). They also demonstrated that the synthesis works when the aryl starting materials are switched to a-naphthylboronic acid butyl ester and phenylmagnesium bromide, respectively,b ut the product was not used for the synthesis of aB ArAr'Ar'' triarylborane.
In 1958, Mikhailov et al.r eportedt he sequential synthesiso f unsymmetrically substituted triarylboranes [119] (Scheme 25) startingf rom an iso-butyl borinice ster wherein the boron atom is additionally bound to one phenyl and one chlorine atom, respectively.I nt he first step, the chlorine atom was substitutedb ya no-tolyl group introduced from aG rignardr eagent. In the second step, the iso-butyl substituent was converted to ac hloride via reaction with PCl 5 .T he chlorine atom was subsequently substituted by other arenes introduced from Grignard reagents yielding two different unsymmetrically substitutedt riaryboranes (41 and 42). Mikhailov andc o-workers reported the synthesis of two other borinic acids (43 and 44), but their conversion to unsymmetrically substituted triarylboranes was not described.
Yamaguchi and co-workers reported the synthesis of as eries of unsymmetricallys ubstituted triarylboranes 48 a-d from a boronic ester precursor( Scheme 26 B). [32] This was then converted with TipMgBr to ad imerici ntermediate 47,w hich was cleavedb yt he addition of al ithiated speciesy ieldingc ompounds 48 a-d (Scheme26B). In the same paper,Y amaguchi and co-workersr eported the synthesis of ad erivative of compound 48 bearing tert-butyl groups insteado ft he iso-propyl groups,b ut it was not possible to synthesize 48 e accordingt o Scheme 26 B. Therefore, they used adifferent approachstarting from boron tribromide (vide infra, Scheme 30 B).

Borane dimethyl sulfide as the boron source
In 2016, Blagg et al. reported the synthesis of the "first 1:1:1 hetero-tri(aryl)borane",b yt heir own account. [122] In terms of investigating the Lewis acidity of such "hetero-tri(aryl)boranes", they substituted the hydrogen atoms of aborane dimethyl sulfide complex stepwise with arenes (Scheme 27). The first aromatic groups were introduced using aryl lithium reagents. The resultingi ntermediate was converted to ab orinic ester with methanol and was then activated with BBr 3 for reaction with an organozinc reagent yieldingt he unsymmetrically substituted triarylborane 49.

Boron tribromidea st he boron source
In 2005, Jäkle and co-workers reported the synthesis of different unsymmetrically substituted triarylboranes as reference compounds for their polymers. [92] Both monomeric and polymeric boron-containing systemsw ere synthesized from aryldibromoboranes and organotin reagents (vide supra, Scheme14; vide infra, Scheme 29). They subsequently used this strategy for similar applications with slight modifications of the synthetic procedure, the third aryl group being added via at in, [92] a copper [124][125][126] or aG rignardr eagent [127] (Scheme 29).
The same group then reported the synthesis of an unsymmetrically substituted triarylborane 53 (Scheme 29 B) via stochiometric Stille coupling of as ymmetric precursor [128] which had been obtainedf rom boron tribromide and an excess of a tin reagent. [129] In 2014, Kelly et al. reported the synthesis of af errocenecontaining triarylborane bearingt hree different aromatic systems by stepwise reaction of dibromoferrocenylborane with two different aryl lithium reagents (Scheme 30 A). [130] As shown in Scheme 26 B, Yamaguchi and co-workers reported ar oute to unsymmetrically substituted triarylboranesf rom boronic esters, [32] but for 48 e,t he route was not successful as the incorporated arene was sterically too demanding. Therefore, they used ar oute established by Jäkle and co-workers: after ab oron-silicon exchange at the thiophene, the ArBBr 2 system was reacted with at in reagent followed by 2,4,6-tritert-butylphenyllithium to give 48 e (Scheme 30 B).

Summary and Outlook
Over the years, the synthetic approaches to triarylboranes presentedh erein has led to the generation of countless compounds containing triarylborane motifs. Initially, examination of their properties was limited to their reactivity with other metalso ra sL ewis acids. Some of the early reaction sequences, such as those developed by Krausee tal. and Grisdale et al., are still used. To day, the applicationso ft hese compoundsa re no longer limited to their reactivity.T he photophysical and electronic properties of triarylboranes and compounds containing this structural motif remain under increasingly active investigation as such properties lead to numerous applications,f or example, in OLEDS, [2] optoelectronics, [1,3,23] sensors for anions [4][5][6] or small molecules, [7,8] as catalysts, fore xample, for hydrogenation or amination of carbonyls, [28,111,112] or bioimaging agents. [9,10,12,14,15] With the further exploration of more gen-eral routes to unsymmetrically substituted triarylboranes, the applicability of these compounds can be expected to continue to increase as this structuralm otif providest he possibility for fine tuningo ft he photophysical and electronical properties of the resulting smallm olecules and, therefore, also of potential macromolecules and polymers.