Chemoselective reducing agents

Midland, M. M., Tramontane, A. B-Alkyl-9-borabicyclo[3.3.1]nonanes as mild, chemoselective reducing agents for aldehydes. J. Org. Chem. 1978,43, 1470-1471. 

Carbonyl reduction. The complexes of BH2CN with propylamine or buty-lamine are very efficient and chemoselective reducing agents. 

Reductions using Boron Compounds.—The advantages of lithium triethylboro-hydride as a very powerful reducing agent, and of amine—boranes derived from primary and secondary amines as mild, chemoselective reducing agents for aldehydes and ketones, have been extolled. Several reagents are recom-... 

Reduction.—/8-(3 -Methyl-2 -butyl)-9-borabicyclo[3,3,l]nonane and the 9-borabicyclo[3,3,l]nonane-pyridine complex are mild chemoselective reducing agents for aldehydes. Tri-n-butyltin hydride on dried silica gel reduces aldehydes rapidly and unhindered ketones relatively slowly yielding the corresponding alcohols in high yields. Sodium borohydride adsorbed on to alumina and the zirconium borohydride (9) reduce aldehydes and ketones to alcohols in aprotic... 

The use of B-alkyl-9-BBN compounds, especially (15), as mild and very chemoselective reducing agents for aldehydes (Scheme 8), even in the presence of ketones, has been explored and the variation in reducing ability with different S-alkyl groups examined. The cyclic transition state (16) is consistent with an increase in rate both with increase in substitution /3- to boron and when the B—C C—H unit can adopt a planar arrangement. This system has also been... 

Sodium cyanoborohydride has become important in biochemical appHcations that require hydrolytic stabiHty of the reducing agent and chemoselectivity, in sensitive molecules. It is also a preferred reagent for oxime reductions. 

In synthesis, the principal factors that affect the choice of a reducing agent are selectivity among functional groups (chemoselectivity) and stereoselectivity. Chemo-selectivity can involve two issues. One may wish to effect a partial reduction of a particular functional group or it may be necessary to reduce one group in preference to another,78 In the sections that follow, we consider some synthetically useful partial and selective reductions. 

Chemoselectivity between aldehydes and ketones is demonstrated by this method in the competitive reduction of a mixture of pentanal and cyclohexanone. The ratios of primary and secondary alcohols are 75 25 when catechol is used at 0° and 79 21 when 2,2/-dihydroxybiphenyl is used at room temperature. These regents are not as chemoselective as other reducing agents such as LiAlH(OBu-i)3 (87 13) and LiAlH(OCEt3)3 (94 6) at 0°.93... 

The preferential chemical reaction of a reagent with one of two or more different functional groups. For example, sodium borohydride exhibits greater chemoselectivity as a reducing agent than does lithium aluminum hydride, because the latter reacts with a wider spectrum of substances. 

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