Imidoylketene: An ab Initio Study of Its Conformations and Reactions.

MP4(SDQ)/6-31G//MP2/6-31G calculations on the conformations of imidoylketene (1) as well as transition structures for several of its reactions are reported. Anti-Z-1 and anti-E-1 are of equal energy, while syn-Z-1 and syn-E-1 are respectively 2.0 and 0.5 kcal/mol higher. As the nitrogen analog of formylketene (2), the reactivity of 1 is modified from that of 2. While the nitrogen in 1 is more basic and nucleophilic than the oxygen in 2, greater resonance donation from the nitrogen decreases the electrophilicity of the ketene carbon in 1 relative to 2. Thus the barriers for the addition of water (6.3 kcal/mol) and of formaldehyde (10.6 kcal/mol) to 1 are very similar to those previously calculated for their respective additions to 2. Transition structures for the 1,3-hydrogen shifts of 3-amino-1,2-propadien-1-one (5a) and formylketene imine (6a) to give 1 were also located. The barriers to these reactions, (41.0 and 48.6 kcal/mol, respectively) are higher than those of the analogous reactions to form 2; reasons for these trends are discussed. The transition structures for these reactions are all planar or nearly so, reflecting the pseudopericyclic orbital topology which makes all the reactions allowed.