Kinetics and Activation Parameters for the Reduction of Alkylcyclohexanones by Lithium Tri-tert-butoxyaluminohydride

Article abstract of DOI:10.1021/jo01292a021

The kinetics of reduction of 15 cyclohexanones by lithium tri-tert-butoxyaluminohydride in tetrahydrofuran solvent are reported.The data confirm that the reaction is well represented by a simple second-order kinetic process.Second-order rate constants determined at various temperatures are recorded and the activation parameters determined.Rate constants vary from 5.0E-3 to 4.4 l mol^-1 s^-1; these rate constants exceed those of reduction by NaBH4 by factors varying from 50 (unhindered cyclohexanones) to 450 (hindered cyclohexanones).The reductions appear to be nearly isoenthalpic, all but three of the values of ΔH(excit.) being in the range 6.8 +/- 0.8 kcal mol^-1.Variations in rates between ketones are caused by changes in ΔS(excit.), and entropy is also more significant than enthalpy in the free-energy barrier to reaction.Mechanistic aspects of the reduction are discussed.