Conjugatively Stabilized Bridgehead Olefins:  Formation and Reaction of Remarkably Stable Homoadamant-3-enes Substituted with Phenyl and Methoxycarbonyl Groups

Conjugatively stabilized double bonds were formed at the bridgehead of homoadamantane by way of the 1,2-carbon shift of adamantylcarbene (-carbenoid) intermediates generated from decomposition of the diazo precursors (1-adamantyl)diazophenylmethane (7) and methyl (1-adamantyl)diazoacetate (10). Decomposition to 4-phenyl- and 4-methoxycarbonyl-substituted homoadamant-3-enes 1 and 2 was much more efficient via catalysis with Rh2(OAc)4 in dichloromethane than by photolysis or thermolysis (FVP; in the case of 7, indane-fused homoadamantane was produced by a phenylcarbene rearrangement followed by insertion to a bridged methylene). In the Rh catalysis, reactions of 7 and 10 in hexane and with Rh2(NHCOCH3)4 did not promote the formation of 1 and 2, suggesting that the polarized structure of the Rh−carbene complex participated in the 1,2-carbon shift. The substituted bridgehead olefins were considerably stable even at 0 °C to room temperature (more than half of 1 and 2 survived in solution at room temperature after 12 and 1 h, respectively), while parent homoadamant-3-ene was recorded to be unstable at −20 °C. Therefore, after decomposition of the diazo precursors was complete, reagents (electrophies for 1 and nucleophiles for 2) were allowed to react at these temperatures to give 3,4-disubstituted homoadamantane derivatives, including some cycloadducts. With atmospheric oxygen, addition and subsequent bond cleavage occurred smoothly to give bicyclo[3.3.1]nonanones. The remarkable stability of 1 and 2 was considered to be the result of conjugation with the substituents, along with some steric protection, which allowed the polarized structure to have a greater effect in reducing the strain energy. This notion was verified by examining longer carbon−carbon double bonds using spectroscopy and PM3 calculations.