Mechanism of lithium aluminum hydride reduction of ketones. Kinetics of reduction of mesityl phenyl ketone

Pseudo-first-order kinetic studies on the reaction of LiAlH4, NaAlH4, and LiAlD4 with mesityl phenyl ketone have been carried out in tetrahydrofuran at 250C. The reactions were carried out in excess hydride and found to be first order in hydride and first order in ketone. LiAlH4 is about ten times more reactive than NaAlH4 which indicates the importance of the cation in the mechanism of the reaction. A deuterium kinetic isotope study involving the reaction of LiAlH4 and LiAlD4 with mesityl phenyl ketone gave a value of k/sub H//k/sub D/ of 1.27 which implicates the transfer of the hydride from aluminum to the carbonyl carbon in the rate-determining step of the reaction. Entropies of activation for reduction of mesityl phenyl ketone by LiAlH4 and NaAlH4 indicate that the transition state is considerably more ordered for reduction by LiAl(H4 (-26.2 vs. -5.4 cu) and that the transition state is of considerably lower energy (10.5 vs. 18.1 kcal). These data further suggest that the lithium cation is associated with the carbonyl oxygen during reduction