Generation of triplet carbenes by oxidation of an allene compound

Computational analysis on the dicationic species generated upon two-electron oxidation of the corresponding allenic compounds bearing two thioxanthene (7a) or two acridene (7b) moieties indicated that the ground-state multiplicity would be of the triplet state (ΔEST = ECS − ET = 12.6 and 14 kcal/mol, respectively). Under this premise, oxidation reactions of 7a and 7b were carried out. In the case of 7b, oxidation with (p-BrC6H4)3N•+SbCl6− gave a dimer (13), which could be presumed to have formed by dimerization of the corresponding carbene. Oxidation of 7b in the presence of tetramethylpiperidine N-oxide afforded the corresponding ketone derivative (14), thus implying the likelihood of a triplet carbene being the reactive intermediate. These results suggest that double oxidation of heterocyclic allene compounds is viable as a new approach for generating triplet carbenes. Copyright © 2011 John Wiley & Sons, Ltd.