Generation of triplet carbenes by oxidation of an allene compound

Abstract

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.

Oxidation of allenes bearing 1,8‐diphenoxy or diaryloxyacridene moieties

“…The two-electron oxidation of 5a resulted in the formation of dimer 7 (Scheme 4). [8] This result indicated that the intermediate was carbene 6a. Furthermore, trapping experiments with tetramethylpiperidine N-oxide (TEMPO) to afford the carbonyl compound 8a, and theoretical calculations suggested that intermediate 6a had a triplet ground state.…”

“…The nitrogen‐bridged allenes were expected to oxidize more easily than the analogous sulfur‐bridged system. The two‐electron oxidation of 5a resulted in the formation of dimer 7 (Scheme ) . This result indicated that the intermediate was carbene 6a .…”

“…[6] We designed a new strategy to generate triplet carbenes, based on 2-electron oxidation of allenic precursors bearing heterocyclic moieties at their terminal positions (Scheme 2). [7][8][9][10] Because this atomic arrangement does not change before and after oxidation, it is possible to introduce protecting groups X around the central carbon atom.…”

Oxidation of an allene compound bearing 1,8‐dichloroacridene moieties and photolysis of the halogenated allene compound for the generation of triplet carbenes

“…[8] Two-electron oxidation of 5a afforded dimer 7 as the product, which could be assumed to have arisen via 6a. Successful trapping of the reactive intermediate by tetramethylpiperidine N-oxide (TEMPO) suggested the intermediacy of triplet carbene 6a.…”

“…Our method was based on the two-electron oxidation of allenic precursors bearing two thioxanthene or acridene moieties. [7][8][9] Thus, we attempted the oxidation of allenic compound 3 bearing two dimethoxythioxanthene moieties [10] that was previously reported by our group (Scheme 2). However, the oxidation of 3 did not furnish a stable triplet carbenic species but instead provided ketal 4, likely via a singlet ground-state carbene.…”