Reactions of complex metal hydrides with borate esters

Abstract

The reaction of sodium borohydride, sodium aluminum hydride and lithium aluminum hydride with methyl borate and phenyl borate is reported. Sodium borohydride does not react with methyl borate or phenyl borate in ether solvents up to 200°; however, in tertiary amine solvent sodium borohydride does react with phenyl borate at elevated temperatures. Triethylamine-borane and dimethylaniline-borane have been prepared in this manner. In amine solvents, under the same conditions, methyl borate does not react. Lithium aluminum hydride and sodium aluminum hydride, on the other hand, react rapidly with both methyl borate and phenyl borate at room temperature in both ether and amine solvent. Diborane and lithium tetraphenoxyaluminate [LiAl(OC 6H 5 4] are formed as the major products from lithium aluminum hydride reaction with phenyl borate. If the mode of addition is reversed (borate added to hydride), no diborane is formed and lithium borohydride is the major product from normal addition. In triethylamine solvent, triethylamine-borane is formed in good yield from either methyl borate or phenyl borate reaction with lithium aluminum hydride. When an olefin is used as the solvent in conjunction with diethyl ether, the corresponding trialkyl-borane is formed. Similar reactions with sodium aluminum hydride are also discussed.