Elimination Reactions of (E)- and (Z)-Benzaldehyde O-Pivaloyloximes. Transition-State Differences for the Syn and Anti Eliminations Forming Nitriles

Elimination reactions of (E)- and (Z)-benzaldehyde O-pivaloyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and k_H/k_D values, and an E2 mechanism is evident. The rate of elimination from 2 is approximately 20 000-fold faster than that from 1. For reactions of 1 with DBU in MeCN, a Hammett ρ value of 2.4 ± 0.1, k_H/k_D = 2.7 ± 0.3, ΔH‡ = 12.5 ± 0.2 kcal/mol, and ΔS‡ = -31.0 ± 0.6 eu have been determined. The corresponding values for 2 are ρ = 1.4 ± 0.1, k_H/k_D = 7.8 ± 0.3, ΔH‡ = 8.8 ± 0.1 kcal/mol, and ΔS‡ = -23.6 ± 0.4 eu, respectively. The results indicate that the nitrile-forming anti eliminations from 2 proceed via a more symmetrical transition state with a smaller degree of proton transfer, less negative charge development at the β-carbon, and greater extent of triple-bond formation than that for the syn elimination.