SEMIEMPIRICAL CALCULATIONS ON HETEROCUMULENE-HETERODIENE CYCLOADDITION REACTIONS - KETENE PLUS 1-AZABUTADIENE

Semi-empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene-1-azabutadiene [4+2] and [2+2] cycloaddition reactions as a model system for the reaction of ketenes with 4-iminobenzylfuran-2,3-diones are presented. A [4+2] type of reaction of the ketene C=C double bond leading to a six-membered Lactam compound is found to be highly favoured both thermodynamically and kinetically. [2+2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon-nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two-step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, same predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa-1,3-dienes are discussed.