Solvent and Temperature Effects on the Reduction and Amination Reactions of Electrophiles by Lithium Dialkylaminoborohydrides
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The influence of temperature and solvent effects on the reduction and amination mechanisms ofiodomethane by lithium N,N-diisopropylaminoborohydride (iPr-LAB) was examined in varying concentrations of THF and dioxane. The reactions of benzyl chloride and trimethylsilyl chloride with iPr-LAB inTHF were also studied. The amination of iodomethane is favored over reduction at low and roomtemperatures in pure THF and with increasing the amount of dioxane in THF. At higher temperatures,the reduction reaction appears to compete with the amination. In dioxane solvent, however, iodomethaneyields exclusively the amination product regardless of temperature. On the other hand, reduction byiPr-LAB to the aminoborane is the only product observed in THF when benzyl chloride and trimethylsilylchloride are used. To understand the solvent effects on the product distribution, ab initio and densityfunctional theory (DFT) calculations were used to examine the mechanisms of reduction and aminationof chloromethane and bromomethane by lithium dimethylaminoborohydride (LAB) in THF and dioxane.The results of these calculations show that the relative reaction barrier heights are significantly affectedby the nature of the coordinated solvent molecule and thus lend support to the experimental observations.

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