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 (Δ E ST  =  E CS  −  E T  = 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 ‐BrC 6 H 4 ) 3 N •+ SbCl 6 − 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.