Asymmetric hydroboration

One of the newer and more fmitful developments in this area is asymmetric hydroboration giving chiral organoboranes, which can be transformed into chiral carbon compounds of high optical purity. Other new directions focus on catalytic hydroboration, asymmetric aHylboration, cross-coupling reactions, and appHcations in biomedical research. This article gives an account of the most important aspects of the hydroboration reaction and transformations of its products. For more detail, monographs and reviews are available (1—13). 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

Monoisopinocampheylborane [64234-27-17, IpcBH2 (6) is an important asymmetric hydroborating agent. It is prepared from a-pinene (7) either directiy or better by indirect methods. The product obtained by hydroboration of a-pinene [80-56-8] is an equiUbrium mixture. Optically pure monoisopinocampheylborane is best prepared from a-pinene via diisopinocampheylborane [1091-56-1] IPC2BH (8) (64,65). Both enantiomers are readily available. 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

There have been several reviews of asymmetric synthesis via chiral organoboranes (6,8,378,382,467—472). Asymmetric induction in the hydroboration reaction may result from the chiraHty present in the olefin (asymmetric substrate), in the reagent (asymmetric hydroboration), or in the catalyst (catalytic asymmetric hydroboration). 

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. 

Asymmetric Hydroboration. Hydroboration—oxidation of (Z)-2-butene with diisopinocampheylborane was the first highly enantioselective asymmetric synthesis (496) the product was R(—)2-butanol in 87% ee. Since then several asymmetric hydroborating agents have been developed. Enantioselectivity in the hydroboration of significant classes of prochiral alkenes with representative asymmetric hydroborating agents is shown in Table 3. 

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Dilongifolyl borane [77882-24-7] M 422.6, m 169-172 . Wash with dry Et20 and dry in a vacuum under N2. It has m 160-161 in a sealed evacuated capillary. It is sparingly soluble in pentane, tetrahydrofuran, carbon tetrachloride, dichloromethane, and chloroform but the suspended material is capable of causing asymmetric hydroboration. Disappearance of solid indicates that the reaction has proceeded. [J Org Chem 46 2988 1981.]... 

Several 2-butenylboron reagents have been synthesized by the hydroboration of allenes9 36. Diisopinoeampheyl(3-methyl-2-butenyl)borane is an excellent reagent for the asymmetric iso-prenylation of aldehydes (typically 89 95% ee), and is conveniently synthesized by the hydroboration of 3-methyl-l,2-butadiene using diisopinocampheylborane9. 

The synthesis of 10 features the SN2 displacement of the allylic acetate with migration of R2 from the ate complex6. Precursors 9 are prepared by the hydroboration of 3-acetoxy-l-alkynes that are available with very high enantiomeric purity via the asymmetric reduction of the corresponding l-alkyn-3-ones, and a substantial degree of asymmetric induction occurs in the conversion of 9 to 10. Best results, based on the enantioselectivity of reactions of 10 with aldehydes, are obtained when R2 is a bulky group such as isopinocampheyl (79 85 % ee)6. The yields of reactions of 10 with aldehydes are 62-76%. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

I 7 Hydroboration, Diboration, Silylboration and Stonnylbomtion Tab. 1-6 Asymmetric Hydroboration... 

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. 

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