Alcohols organoborane reduction

General information on the hydroboration reaction to form a C—C bond is given in Section D.1.5.7. and for a C —H bond by reduction in Section D.2.5.2. of this volume, whereas the oxidation of organoboranes is described in Vol. 13/3 a, pp 489-853. This section deals with the formation of C—O bonds, e.g., the synthesis of alcohols by the oxidation of organoboranes bearing the boron atom at the stereogenic center (see Vol. 6/1 a, pp 494-553). 

The reduction is bimolecular and thus the rate is dependent on concentration. Running the reaction neat provides the fastest rates. Usually an excess of Alpine-Borane is used to insure that the reaction does not become excessively slow at the end of the reduction. The excess organoborane may be destroyed by addition of an aldehyde such as Acetaldehyde. The resulting alkoxy-9-BBN may be treated with Ethanolamine to liberate the alcohol and precipitate the majority of the 9-BBN. Any remaining borane impurities may be removed by oxidation with basic Hydrogen Peroxide. 

The Overall process of reduction of an alkene to an alkane, or of reduction of an alkyne to either an al-kene or an alkane, via hydroboration requires that the intermediate organoborane be protonolyzed (Section 3.10.1.1). However, simple trialkylboranes are remarkably resistant to hydrolysis. For example, trimethylborane gives only 69% of hydroxydimethylborane after 7 h at 180 °C with 1 equiv. of water. Similar resistance is shown to alcohols, phenols and amines, and even mineral acids do not completely protonolyze trialkylboranes with any ease. Thus, aqueous or anhydrous hydrogen bromide removes only one alkyl group from tributylborane after reflux for 1 h. ... 

Yields of aldehydes, as indicated by the yields of alcohols obtained on reduction, were 87-98% in five cases. One disadvantage is that only one of the three alkyl groups of the organoborane is converted into the aldehyde. 

KETONES AND ALCOHOLS FROM ORGANOBORANES 1. PHENYL HEPTYL KETONE 2. HEX-ANOL 3. 1-OCTANOL,53, 77 Ketones, Clemmensen reduction of, 53, 89... 

B-l-Alkenyl-9-borabicyclo[3.3.1]nonanes (1). These substances are prepared by hydroboration of alkynes with 9-BBN (2, 31 3, 24-29 4, 41 5, 46-47 6, 62-64). -Allylic alcohols. Usually reaction of organoboranes with carbonyl groups results in reduction. However, B-l-alkenyl-9-borabicyclo[3.3.1.]nonanes (1) add to simple aldehydes to give, after oxidation, allylic alcohols, with retention of configuration (equation I). The reaction is a Grignard-like synthesis and should be a useful route to allylic alcohols, even to those containing functional groups. 

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. 

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. ... 

Srebnik, M., Ramachandran, P. V., Brown, H. C. 1988. Chiral synthesis via organoboranes. 18. Selective reductions. 43. Diisopinocampheylchloroborane as an excellent chiral reducing reagent for the synthesis of halo alcohols of high enantiomeric purity. A highly enantioselective synthesis of both optical isomers of Tomoxetine, Fluoxetine, and Nisoxetine. J. Org. Chem. 53 2916-2920. 

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