[2+2] versus [4+2] cycloaddition reactions of 1,3-diaza-1,3-butadienes with various mono- and disubstituted ketenes and supporting mechanistic considerations

Reactions of 1,3-diaza-1,3-butadienes with various mono- and disubstituted ketenes alongwith the associated interesting mechanistic features involved are reported. Reactions of 1,3-diazabutadienes with monosubstituted ketenes e.g. cyano-, p-nitrophenyl-, succinimido-, phthalimido- and phenoxyketenes gave [4+2] cycloadducts irrespective of the nature of azadiene and/or ketene substituents. However, diphenylketene underwent [2+2] cycloaddition reactions with 1,3-diazabutadienes (la) and gave [4+2] adducts with 1b, having electron donating function at 2-position. Interestingly, the reactions of 4-(N-arylamino)-1,3-diaza-1,3-butadienes (19) with diphenylketene gave simple nucleophilic addition products (22), whereas, their reaction with dimethylketene, yielded [4+2] cycloadducts (23). The factors influencing [2+2] versus [4+2] and cyclic versus acyclic products are also discussed.