[2-DIAZA-1,3-BUTADIENES WITH VARIOUS MONOSUBSTITUTED AND DISUBSTITUTED KETENES AND SUPPORTING MECHANISTIC CONSIDERATIONS(2]CYCLOADDITION VERSUS [4+2]CYCLOADDITION REACTIONS OF 1,3)

Reactions of 1,3-diaza-1,3-butadienes with various mono- and disubstit uted ketenes along with the associated interesting mechanistic feature s involved are reported. Reactions of 1,3-diazabutadienes with monosub stituted ketenes e.g. cyano-, p-nitrophenyl-, succinimido-, phthalimid o- and phenoxyketenes gave [4+2] cycloadducts irrespective of the natu re of azadiene and/or ketene substituents. However, diphenylketene und erwent [2+2] cycloaddition reactions with 1,3-diazabutadienes (la) and gave [4+2] adducts with 1b, having electron donating function at 2-po sition. Interestingly, the reactions of C(N-arylamino)-1,3-diaza-1,3-b utadienes (19) with diphenylketene gave simple nucleophilic addition p roducts (22), whereas, their reaction with dimethylketene, yielded [42] cycloadducts (23). The factors influencing [2+2] versus [4+2] and c yclic versus acyclic products are also discussed.