Demethylation of an Allene Bearing Two Dimethoxythioxanthene Groups by Oxidation via a Vinyl Cation Intermediate

Abstract

With the objective of preparing an isolable triplet carbene, we have carried out the oxidation of an allenic compound bearing two thioxanthene moieties (5). Relatively weak oxidants such as Ph3C+BF4 – gave 8, which is the conjugate acid of 5, as a result of a one-electron oxidation followed by hydrogen abstraction, whereas relatively strong oxidants such as SbCl5 furnished a dicationic ketal (9) as a consequence of oxidation and demethylation. Computations on the supposed dicationic intermediate suggest that the singlet state is more stable than the triplet state by 6.7 kcal mol–1 and that the reason for this peculiarity is because the singlet state is essentially a vinyl cation stabilized by a coordinating methoxy group.

Generation of triplet carbenes by oxidation of an allene compound

“…Oxidation waves were observed at E pa = 1.23 and 1.53 V (in 100 mM TBAPF 6 /CH 2 Cl 2 , Glass Carbon (GC), versus Saturated Calomel Electrode (SCE), room temperature) for 7a and E pa = 0.74 and 1.08 V (in 100 mM TBAPF 6 /CH 2 Cl 2 , GC, versus SCE, room temperature) for 7b . These results suggested that oxidation processes would proceed in two steps like the allenic compound previously examined by us . Also implied was that 7a might not be suitable as a precursor because its high second oxidation potential would be troublesome for mild oxidation and the product carbene was expected to be unstable.…”

“…Thus, we envisaged that this feature would also be favorable for the introduction of other rigid steric protecting groups around the anticipated carbenic center. To this end, we examined the oxidation reaction of allenic precursor 5 . Unfortunately, this resulted in the generation of dicationic ketal 6 .…”

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

“…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.…”

“…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. [7] According to theoretical calculations, this decomposition resulted from the vinyl cation character of the generated dicationic species.…”

5th Heron Island Conference on Reactive Intermediates and Unusual Molecules

“…J. Chem. captures the flavour of the meeting by publishing 15 papers covering gold catalysis in organic synthesis, [2] corannulene-based curved p-radicals, [3] formation of 1,3-diradicals from bicyclic azo compunds, [4] electron spin resonance spectroscopy of a,n-didehydrotoluene diradicals, [5] allene-carbene conversion in the thioxanthene series, [6] nitrene and free radical intermediates from azidomethylacetophenones, [7] s-hole binding, [8] synthesis and reactions of cyclobutenimines, [9] development of methodology for natural product synthesis, [10] organic-inorganic composite nanoscale materials, [11] matrix isolation of nitrile sulfides, [12] unusual thioketenes and extended cumulenes, [13] mixed cyclophanes, [14] photoelectron spectroscopy of short-lived molecules containing Ge and As, [15] and the HERON reaction (Heteroatom rearrangements on nitrogen) of N-acyloxy-N-alkoxyamides. [16] We are pleased to acknowledge Dr Brett Schwartz for designing the cover art.…”