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 rho 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 rho value of 2.19 +/- 0.05, beta(1g) = -0.49 +/- 0.02, Delta H-double dagger = 10.4 +/- 0.6 kcal/mol, and Delta S-double dagger = -34.3 +/- 2.6 eu have been determined. The corresponding values for 2 are k(H)/k(D) = 7.3 +/- 0.2, rho = 1.21 +/- 0.05, beta(1g) = -0.40 +/- 0.01, Delta H-double dagger = 6.8 +/- 0.5 kcal/mol, and Delta S-double dagger = -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-alpha-OC(O)Ar bond cleavage, less negative charge development at the beta-carbon, and a greater extent of triple bond formation than that for the syn-elimination.