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 (-c arbenoid) intermediates generated from decomposition of the diazo prec ursors (1-adamantyl)diazophenylmethane (7) and methyl (1-adamantyl)dia zoacetate (10). Decomposition to 4-phenyl- and 4-methoxycarbonyl-subst ituted homoadamant-3-enes 1 and 2 was much more efficient via catalysi s with Rh-2(OAc)(4) in dichloromethane than by photolysis or thermolys is (FVP; in the case of 7, indane-fused homoadamantane was produced by a phenylcarbene rearrangement followed by insertion to a bridged meth ylene). In the Rh catalysis, reactions of 7 and 10 in hexane and with Rh-2(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 conside rably stable even at 0 degrees C to room temperature (more than half o f 1 and 2 survived in solution at room temperature after 12 and 1 h, r espectively), while parent homoadamant-3-ene was recorded to be unstab le at -20 degrees C. Therefore, after decomposition of the diazo precu rsors was complete, reagents (electrophies for 1 and nucleophiles for 2) were allowed to react at these temperatures to give 3,4-disubstitut ed homoadamantane derivatives, including some cycloadducts. With atmos pheric 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 structu re to have a greater effect in reducing the strain energy. This notion was verified by examining longer carbon-carbon double bonds using spe ctroscopy and PM3 calculations.