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

Elimination reactions of (E)- and (Z)-benzaldehyde O-benzoyloximes 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 36000 fold faster than that from 1. For reactions of 1 with DBU in MeCN, k_H/k_D = 3.3 ± 0.2, Hammett ρ value of 2.19 ± 0.05, β_1g = -0.49 ± 0.02, ΔH‡ = 10.4 ± 0.6 kcal/mol, and ΔS‡ = -34.3 ± 2.6 eu have been determined. The corresponding values for 2 are k_H/k_D = 7.3 ± 0.2, ρ = 1.21 ± 0.05, β_1g = -0.40 ± 0.01, ΔH‡ = 6.8 ± 0.5 kcal/mol, and ΔS‡ = -25.8 ± 1.9 eu, respectively. The results indicate that the anti-eliminations from 2 proceed via more symmetrical transition states with smaller degrees of proton transfer and N_α-OC (O)Ar bond cleavage, less negative charge development at the β-carbon, and a greater extent of triple bond formation than that for the syn-elimination.