Ketene–Ketene Interconversion. 6-Carbonylcyclohexa-2,4-dienone–Hepta-1,2,4,6-tetraene-1,7-dione–6-Oxocyclohexa-2,4-dienylidene and Wolff Rearrangement to Fulven-6-one

Abstract

6-Carbonylcyclohexa-2,4-dienone (1) has been generated by flash vacuum thermolysis (FVT) with Ar-matrix isolation of methyl salicylate (7), 2-phenylbenzo-1,3-dioxan-4-one (8), phthalic peranhydride (9), and benzofuran-2,3-dione (11) and also by matrix photolysis of 9, 11, and 2-diazocyclohepta-4,6-dien-1,3-dione (12). In each case, FVT above 600 °C results in decarbonylation of 1 and Wolff rearrangement to fulven-6-one (13) either concertedly or via open-shell singlet 6-oxocyclohexa-2,4-dienylidene (18). Ketenes 1 and 13 were characterized by IR spectroscopy. Photolysis of matrix-isolated 1 at 254 nm also results in the slow formation of 13. The sequential formation of ketenes 1 and 13 from 7 has also been monitored by FVT-mass spectrometry, and 13 has been trapped with MeOH to afford methyl 1,3-cyclopentadiene-1- and -2-carboxylates 15 and 16. FVT of methyl salicylate-1-(13)C 7a revealed a deep-seated rearrangement of the (13)C-labeled 1a to hepta-1,2,4,6-tetraen-1,7-dione (17a) by means of electrocyclic ring opening followed by a facile 1,5-H shift and recyclization prior to CO-elimination and ring contraction to (13)C-labeled 13. The rearrangement mechanism is supported by M06-2X/6-311++G(d,p) calculations, which predict feasible barriers for the FVT rearrangements and confirm the observed labeling pattern in the isolated methyl salicylate 7a/7b and methyl cyclopentadienecarboxylates 20 and 21 resulting from trapping of 13 with MeOH.

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry

“…In contrast to benzazirine (14), 13 C-labeling clearly demonstrated the absence of benzoxirene (19;S cheme 5) in the FVP of methyl salicylate.I na greement with this,t he computed energy barrier for the Wolff rearrangement of oxocarbene 17 to 18 is lower than the barrier for interconversion of benzoxirene. [20]…”

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

“…[6a, 12] Recently the reversible rearrangement of 6 to the ketene 7 was shown at high temperatures. [13] Irradiation of 4 with UV light (λ > 305 nm) does not result in loss of CO 2 . Rather, 4 undergoes a ring opening via a retro [2+2] cycloaddition to give 8 that is a carbon analog of 7 (Scheme 1).…”

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

“…Prolonged irradiation of 5 leads to ortho ‐benzyne 6a. b Recently the reversible rearrangement of 6 into the ketene 7 was reported to proceed at high temperatures 6d. Irradiation of 4 with UV light (λ>305 nm) did not result in loss of CO 2 .…”