Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines

Abstract

Twelve A3-1 ,3,4-oxadiazolines in which C-2 is also C-4 of a p-lactam moiety (spiro-fused p-lactam oxadiazoline system) were thermolyzed as solutions in benzene. Substituents in the p-lactam portion affect the rate constant for thermal decomposition of the oxadiazolines to N2, acetone, and a P-lactam-4-ylidene. The total spread of first-order rate constants at 100°C was 47-fold and the average value was 6.7 X s-'. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and C1 were also rate enhancing, relative to methyl. The data are interpreted in terms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of [Traduit par la rkdaction]