Understanding the kinetics and molecular mechanism of the Curtius rearrangement of 3-oxocyclobutane-1-carbonyl azide

Abstract

Abstract The approach presented here is an unprecedented insight into the understanding of kinetics and molecular mechanism of thermal Curtius rearrangement of 3-oxocyclobutane-1-carbonyl azide. Curtius rearrangement can proceed via concerted and stepwise mechanisms. The CBS-QB3 and CBS-APNO composite methods indicated that concerted pathway is dominant and 104–105 times faster than stepwise path. The bonding evolution theory analysis at the B3LYP/6-311G(d,p) revealed that the reaction via concerted pathway can be described with catastrophe sequence 9-CF†C†TSFC†FC†C-0 by the following chemical events: (a) change of topological signature of N2 N3 bond; (b) increasing the number of non-bonding monosynaptic attractor on N1 atom; (c) breaking of N1 N2 bond and extrusion of nitrogen molecule; (d) decreasing the number of non-bonding monosynaptic attractors on N1 atom; (e) breaking of C4 C5 bond and formation of pseudoradical centers on the C4 and C5 atoms; (f) annihilation of pseudoradical center on the C5 atom; (g) change of topological signature of N1 C5 bond; and (h) formation of N1 C4 bond. Along the reaction course electron flow redistribution is asynchronous and bond breaking/forming do not takes place simultaneously demonstrating that the reaction is concerted yet highly asynchronous process.