Selectivity in Ketenimine-Thioketone Cycloadditions. 2. Kinetic and Theoretical Studies of the Mechanism of the 1,2- and 1,4-Cycloadditions

Article abstract of DOI:10.1021/jo01307a010

The kinetics of the thermal 1,2- and 1,4-cycloadditions of thiobenzophenones to ketenimines to give four-membered adducts, 2-iminothietanes, and six-membered adducts, 4H-3,1-benzothiazines, respectively, has been studied with respect to changes of solvents and substituents as well as at different temperatures.Both reactions show the typical features of concerted processes, viz., little change of rate with the polarity of the solvent, small activation energies, and large and negative activation entropies, but have some substantial differences.The results are consistent with a scheme where the products are formed from two site-selective additions of the reactants through independent pathways, very likely by concerted mechanism.The formation of intermediates such as an open-chain zwitterion or a four-membered cycloadduct involving the C=N bond of the cumulene appears unlikely for both processes.Perturbational molecular reasonings coupled with SCF-MO computations indicate as most probable a <_?2_s + _?2_a> pericyclic process between the C=S bond of the thione and the C=C of the cumulene for the 1,2-cycloaddition and a <_?4_s + _?2_s> process between the C=S bond of the thione and the heterodiene system which consists of the C=N bond of the ketenimine and the C=C of the N-phenyl ring for the 1,4-cycloaddition.