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 (<b>1</b>) has been generated by flash vacuum thermolysis (FVT) with Ar-matrix isolation of methyl salicylate (<b>7</b>), 2-phenylbenzo-1,3-dioxan-4-one (<b>8</b>), phthalic peranhydride (<b>9</b>), and benzofuran-2,3-dione (<b>11</b>) and also by matrix photolysis of <b>9</b>, <b>11</b>, and 2-diazocyclohepta-4,6-dien-1,3-dione (<b>12</b>). In each case, FVT above 600 °C results in decarbonylation of <b>1</b> and Wolff rearrangement to fulven-6-one (<b>13</b>) either concertedly or via open-shell singlet 6-oxocyclohexa-2,4-dienylidene (<b>18</b>). Ketenes <b>1</b> and <b>13</b> were characterized by IR spectroscopy. Photolysis of matrix-isolated <b>1</b> at 254 nm also results in the slow formation of <b>13</b>. The sequential formation of ketenes <b>1</b> and <b>13</b> from <b>7</b> has also been monitored by FVT-mass spectrometry, and <b>13</b> has been trapped with MeOH to afford methyl 1,3-cyclopentadiene-1- and -2-carboxylates <b>15</b> and <b>16</b>. FVT of methyl salicylate-1-¹³C <b>7a</b> revealed a deep-seated rearrangement of the ¹³C-labeled <b>1a</b> to hepta-1,2,4,6-tetraen-1,7-dione (<b>17a</b>) 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 <b>13</b>. 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 <b>7a/7b</b> and methyl cyclopentadienecarboxylates <b>20</b> and <b>21</b> resulting from trapping of <b>13</b> with MeOH