Lithium borohydride Lewis acids, esters

Lewis acid strength and hardness of the lithium cation. Both LiBH4 and Ca(BH4)2 are more reactive than sodium borohydride. This enhanced reactivity is due to the greater Lewis acid strength of Li+ and Ca2+, compared with Na+. Both of these reagents can reduce esters and lactones efficiently. 

The cation is a stronger Lewis acid than the Na cation. LL coordination with the carbonyl group enhances the electrophilicity of the carbonyl carbon, thereby facilitating hydride transfer. Lithium borohydride is a more powerful reducing agent than sodium borohydride it reduces esters to primary alcohols but is unreactive towards amides. 

By Reduction of Carbonyl Compounds. Use of high (10 kbar) pressures has been shown to effect trialkylstannane reductions of ketones in the absence of radical initiators or Lewis acids.10 Zinc borohydride has been demonstrated to be a mild reducing agent for the conversion of benzenethiol esters into alcohols in good yield. Use of mixed solvents containing methanol has been found to confer some chemoselectivity upon reductions with lithium borohydride and permits enhanced rates of reduction of esters, lactones, and... 

High enantioselectivities are obtained using tartaric acid-derived boronate ester 31 in combination with lithium borohydride or sodium borohydride for asymmetric reduction of alkyl or aryl ketones. The chiral Lewis add is easUy prepared in one hour, and the resulting alcohols are obtained in enantiomeric excesses of 88-99% (Equation 46) [44]. 

Reduction of Carbonyl Groups. Rapid reduction of salts of carboxylic acids to alcohols with 2 molar equivalents of borane in tetrahydrofuran (THF) has been reported. Full details have appeared of the improved procedure for reduction of esters to primary alcohols, using borane dimethyl sulphide (6,157) in THF at reflux, which allows the dimethyl sulphide that is liberated to distil off during the reaction. Further suggested procedures for the reduction of esters to primary alcohols use lithium borohydride in diethyl ether or THF, calcium borohydride in THF, and sodium borohydride in a t-butyl alcohol-methanol mixed solvent. The reactions using lithium borohydride-diethyl ether can be catalysed by lithium 9-boratobicyclo[3.3.1]nonane (1), lithium triethylboro-hydride, or the Lewis acid 9-methoxy-9-borabicyclo[3.3.1]nonane (2). ... 

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