Selective Reductions. 32. Structural Effects on the reduction of Epoxides by Lithium Triethylborohydride. A Kinetic Study

Article abstract of DOI:10.1021/jo00166a032

An analytical procedure for the estimation of lithium triethylborohydride utilizing iodimetry has been developed.The rates of reduction of a number of epoxides are studied.The reaction exhibits second-order kinetics, first order with respect to each reactant.Methyl substitution decreases the reactivity of the epoxide by a factor of 8-10.In the case of aliphatic derivatives, the cis epoxide reacts 12 times faster than the trans.On the other hand, cis- and trans-β-methylstyrene oxides exhibit the opposite trend.A remarkable change in the regioselectivity of reduction of these two epoxides was observed.A possible explanation is presented.The kinetic isotope effect has been established both by rate studies and by determination, by mass spectrometry, of the deuterium incorporation in a competitive reaction between LiEt3BH, LiEt3BD, and the epoxide.A value of k_H/k_D ca. 1.4-1.5 is obtained by both methods.A mechanism is proposed to account for these results.