Organoboranes, asymmetric reductions

Brown, H. C. and Ramachandran, P. V. Asymmetric Reduction with Chiral Organoboranes Based on a-Pinene. Acc. Chem. Res. 1992, 25, 16-24. 

M. M. Midland, Asymmetric Reduction with Organoborane Reagents, Chem. Rev. 1989, 89, 1553. 

E. Block, Olefin synthesis via deoxygenation of vicinal diols, Org. React. (N.Y.) 1984, 30, 457. M. M. Midland, Asymmetric reduction with organoborane reagents, Chem. Rev. 1989,89, 1553. H. C. Brown and P. V. Ramachandran, Asymmetric reduction with chiral organoboranes based... 

Brown, H. C., Srebnik, M., Ramachandran, P. V. Chiral synthesis via organoboranes. 22. Selective reductions. 44. The effect of the steric requirements of the alkyl substituent in isopinocampheylalkylchloroboranes for the asymmetric reduction of representative ketones. J. Org. Chem. 1989,54, 1577-1583. 

Asymmetric reduction. This organoborane reduces aldehydes within minutes, but reduces ketones rather slowly. Of greater interest, the chiral organoborane from ( + )-cc-pinene reduces benzaldehyde-a-d quantitatively to (S)-( + )-benzyl-a-d-alcohol in 81.6%, chemical yield (equation 1). In principle the opposite enantiomer could be prepared when (- )-ct-pinene is used. 

It is postulated that the reduction of benzaldehydes proceeds predominately through a cyclic process [9]. This conclusion is based on the second-order kinetic data, the change in rate with structural and electronic changes in the aldehydes, and the results of asymmetric reduction. However, greater steric effects, both in the organoboranes and substrate, lead predominantly to the dehydrobo-ration process. 

In addition to asymmetric hydroboration of C=C, the hydroborating reagents also exhibit remarkable asymmetric induction in the reduction of the carbonyl moiety. Chiral organoboranes derived via hydroboration with 9-BBN are proven to be highly useful for the asymmetric reduction. The characteristics of asymmetric induction in the reduction of prochicral carbonyl compounds of individual chiral organoboranes are discussed in this chapter. 

Asymmetric Reductions. In a review of asymmetric syntheses via chiral organoborane intermediates. Brown has surveyed ketone-to-alcohol reductions with chiral trialkylboranes, chiral borohydrides, and mono- or di-isopinocam-pheylborane. ... 

A review of the current status of the applications of chiral organoboranes derived from a-pinene for the synthesis of fluoroorganic molecules has been made. Our three-pronged approach for the synthesis of asymmetric fluoro-organics involve (i) asymmetric reduction of fluoiinated ketones, (ii) asymmetric allylboration of fluoiinated aldehydes, and (iii) asymmetric enolboration-aldolization of fiuoro-ketones and aldehydes. It appears that the presem e of fluorite atom(s) in the molecule influences the stereoctemi< outcome in asymmetric reduction and asymmetric enolboration-aldolization using cMorodiisopinocampheylborane. 

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