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Weighing Different Mechanistic Proposals for the Dötz Reaction:  A Density Functional Study

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journal contribution
posted on 28.01.1999, 00:00 by Maricel Torrent, Miquel Duran, Miquel Solà
Three different routes have been theoretically explored for the benzannulation of heteroatom-stabilized chromium carbene complexes with ethyne (Dötz reaction). The most widely accepted mechanistic proposals assume that the central part of the reaction proceeds through either the vinylketene route (Dötz's hypothesis) or the chromacycloheptadienone route (Casey's suggestion). Our calculations reveal that, from a thermodynamic viewpoint, the latter proposal is surpassed by the former, because conversion of vinylcarbenes into chromacycloheptadienones is computed to be a notably endothermic process. Moreover, a recently postulated new third route is found to be an even better proposal than the one from Dötz's hypothesis since it involves lower energy barriers and more stable intermediates than the vinylketene route postulated by Dötz. The novel findings query revision of the classically assumed paths and are a doorway to new formulisms for the mechanism of the title reaction.