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

The Journal of organic chemistry

PubMedID: 24988224

Koch R, Blanch RJ, Wentrup C. Ketene-Ketene Interconversion. 6-Carbonylcyclohexa-2,4-dienone - Hepta-1,2,4,6-tetraen-1,7-dione - 6-Oxocyclohexa-2,4-dienylidene and Wolff Rearrangement to Fulven-6-one. J Org Chem. 2014;.
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 oC results in decarbonylation of 1 and Wolff rearrangement to fulven-6-one 13 in a 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-1-13C 7a revealed a deep-seated rearrangement of the 13C-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 13C-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.