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 1d and 1e 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 1d and 1e. 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 1d and 1e are characterized by an allene, acyl substituted derivatives 1a, 1b, 3, and 6 by a propargyl type structure. The nitrogen derivative 1c represents an intermediate case.
Fabian
WM
Institute of Organic Chemistry, Karl-Franzens University, Graz, Heinrichstr. 28, A-8010 Graz, Austria, and Department of Technology of Organic Synthesis, Urals State Technical University, Mira 19, 620002 Ekaterinburg, Russia.
Kappe
CO
Bakulev
VA