Ab initio and density functional calculations on the pericyclic vs pseudopericyclic mode of conjugated nitrile ylide 1,5-electrocyclizations

Ab initio (MP2/6-311+G** and MP4(SDTQ)/6-311+G**//MP2/6-311+G**) and density functional (B3LYP/6-311+G**) calculations on the ring closure reactions of conjugated nitrile ylides 1a-e, 3, and 6 to the corresponding oxazoles 2a, 5, 7, and 8; thiazoles 2b and 4; imidazole 2c; and pyrroles 2d and 2e, respectively, are reported. Vinyl nitrile ylides Id and le cyclize with a substantially higher barrier than nitrile ylides containing a heteroatom Geometric features as well as electronic structures as obtained by NBO analysis are indicative of a pericyclic, monorotatory 1,5-electrocyclization of Id and le. For nitrile ylides where X = heteroatom, a pseudopericyclic heteroelectrocyclization pathway, characterized by in-plane attack of the heteroatom's lone pair at the nitrile ylide group, is found. For 3 and 6, where two different cyclization products are possible, the calculated barriers and reaction energies are in line with the experimentally observed direction of reaction. Vinyl nitrile ylides Id and le are characterized by an allene, acyl substituted derivatives la, Ib, 3, and 6 by a propargyl type structure. The nitrogen derivative Ic represents an intermediate case.