Conformations and Reactions of Bicyclo[3.2.1]oct-6-en-8-ylidene

Bicyclo­[3.2.1]­oct-6-en-8-ylidene (<b>1</b>) can assume either the conformation of “classical” carbene <b>1a</b> or that of foiled carbene <b>1b</b> in which the divalent carbon bends toward the double bond. Oxadiazoline precursors for the generation of <b>1</b> were prepared, followed by photochemical and thermal decomposition as well as flash vacuum pyrolysis (FVP) of a tosyl hydrazone sodium salt precursor, to give a number of rearrangement products. Matrix isolation experiments demonstrate the presence of a diazo intermediate and methyl acetate in all photochemical and thermal precursor reactions. The major product from rearrangements of “classical” bridged carbene <b>1a</b> is bicyclo[3.3.0]­octa-1,3-diene as a result of an alkyl shift, while dihydrosemibullvalene formed from a 1,3-C–H insertion. In contrast, thus far unknown strained bicyclo[4.2.0]­octa-1,7-diene formed by a vinyl shift in foiled carbene <b>1b</b>. The experimental results are corroborated by density functional theory (DFT), MP2, and G4 computations