Chiral organoboranes, asymmetric

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). 

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). 

Asymmetric substrate, 13 664-665 Asymmetric synthesis, via chiral organoboranes, 13 664-671 ATI receptor, 5 158 Atacama Desert, sodium nitrate in, 22 844-845 Atacamite, 7 769... 

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

H. C. Brown and P. V. Ramachandra, Asymmetric Syntheses via Chiral Organoboranes Based on a-Pinenein Advances in Asymmetric Synthesis (A. Hassner, Ed.), 1995,1, JAI, Greenwich, CT. 

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... 

P. V. Ramachandran and H. C. Brown, Asymmetric Syntheses of Fluoroorganic Compounds via Chiral Organoboranes, in Asymmetric Fluoroorganic Chemistry , ACS Symposium Series 746, eds. P. V. Ramachandran, and H. C. Brown, American Chemical Society, Washington, DC, 2000, p. 22. 

Asymmetric Hydroboration. Dilongifolylborane (Lgf2BH) is a chiral dialkylborane intermediate in steric requirement between the two widely investigated chiral organoboranes derived from a-pinene Monoisopinocampheylborane (IpcBH2) and Di-isopinocampheylborane (Ipc2BH). ... 

P. V. Ramachandran and H. C. Brown, Asymmetric Syntheses of Fluoroorganic Compounds via Chiral Organoboranes, in... 

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. 

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. ... 

Asymmetric Syntheses of Fluoroorganic Compounds via Chiral Organoboranes... 

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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