SUBSTITUENT EFFECTS OF RATE CONSTANTS FOR THERMAL [4-PI-PI] CYCLOREVERSION OF SPIRO-FUSED BETA-LACTAM OXADIAZOLINES(2)

Twelve DELTA3-1,3,4-oxadiazolines in which C-2 is also C-4 of a beta-l actam moiety (spiro-fused beta-lactam oxadiazoline system) were thermo lyzed as solutions in benzene. Substituents in the beta-lactam portion affect the rate constant for thermal decomposition of the oxadiazolin es to N2, acetone, and a beta-lactam-4-ylidene. The total spread of fi rst-order rate constants at 100-degrees-C was 47-fold and the average value was 6.7 x 10(-4) s-1. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and Cl were also rate enhancing, relative to methyl. The data are interpreted in t erms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of corresponding transition states for concerted, suprafacial, [4pi + 2pi] cycloreversion. The fi rst products, presumably formed irreversibly, are N2 and a carbonyl yl ide. The latter subsequently fragments to form acetone (quantitative) and a beta-lactam-4-ylidene.