Sodium acyloxyborohydrides

The applications of sodium acyloxyborohydrides, formed from sodium borohydrides in carboxylic acid media, are reviewed. ° Useful reviews of the stereoselective reduction of endocyclic C=N compounds and of the enantioselective reduction of ketones have appeared. ... 

Organocopper reagents, 207 Palladium catalysts, 230 Potassium-Graphite, 252 Raney nickel, 265 Sodium acyloxyborohydrides, 277 Sodium borohydride, 278 Sodium borohydride-Nickel chloride, 279... 

Reduction of nitriles. This sodium acyloxyborohydride reduces nitriles (aliphatic and aromatic) to primary amines at room temperature in high yield. The reaction appears to be sekclive —COOC2H5, —NOa, and —Cl groups are not reduced. ... 

Reduction of ketones with sodium borohydride in the presence of a carboxylic acid and 1,2 5,6-di-0-cyclohexylidene-a -D-glucofuranose gave 35-50% enantiomeric enhancement values.Another group has reported a similar reaction with the corresponding di-O-isopropylidene-glucose derivative and prochiral aromatic ketones. Optical yields of up to 64% were claimed. The chiral reagents appear to be sodium acyloxyborohydrides, which complex with the carbohydrate before reduction takes place. 

Sodium acyloxyborohydride has been found to effect the reduction of aliphatic and aromatic nitriles to the corresponding amines any chloro, nitro, or aromatic substituents remain unaffected under the reaction conditions. Full details have appeared of the [3,3]sigmatropic rearrangement of trichloroacetamidic esters of allylic alcohols, which provides a superior method for the 1,3-transposition of amino (and hydroxy) functions (Scheme 2). 

Unusual reducing properties can be obtained with borohydride derivatives formed in situ. A variety of reductions have been reported, including hydrogenolysis of carbonyls and alkylation of amines with sodium borohydride in carboxyHc acids such as acetic and trifluoroacetic (38), in which the acyloxyborohydride is the reducing agent. 

High enantioselectivities (up to 94%) are obtained in the sodium borohydride reduction of aliphatic ketones using a tartaric acid-derived boronic ester (TarB-N02) as a chiral catalyst. A mechanism (Scheme 14) involving an acyloxyborohydride intermediate has been postulated.319... 

A new chiral acyloxyborohydride has been prepared by combining sodium borohy-dride with a tartaric acid-based reagent. This reagent effects the reduction of aromatic ketones to provide the product alcohols in ees of 93-98%. Several aliphatic ketones were also reduced with moderate to excellent enantioselectivity. A mechanism has been provided with supporting calculations for the proposed active species and tran- sition state.262... 

The utility of sodium borohydride (and related metal hydrides) in carboxylic acid media in the reduction of indoles has been reviewed recently and several additional examples can be found cited in this review.There is increasing evidence to suggest that acyloxyborohydride species are the actual reducing agents that form under these conditions. ... 

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