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 densit y functional (B3LYP/6-311+G**) calculations on the ring closure reactions o f 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, re spectively, are reported. Vinyl nitrile ylides Id and le cyclize with a sub stantially higher barrier than nitrile ylides containing a heteroatom Geome tric features as well as electronic structures as obtained by NBO analysis are indicative of a pericyclic, monorotatory 1,5-electrocyclization of Id a nd le. For nitrile ylides where X = heteroatom, a pseudopericyclic heteroel ectrocyclization pathway, characterized by in-plane attack of the heteroato m'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 re action energies are in line with the experimentally observed direction of r eaction. Vinyl nitrile ylides Id and le are characterized by an allene, acy l substituted derivatives la, Ib, 3, and 6 by a propargyl type structure. T he nitrogen derivative Ic represents an intermediate case.