Ring opening of a formal cyclopentylmethyl radical in the thermolysis of di(tert-alkyl)(1-norbornyl)methanols

Abstract

In the thermolysis of di(1-adamantyl)(1-norbonyl)methanol, Ad₂NorCOH, 1a, in toluene at 220–265 °C, C–C bond cleavage within the norbornyl group of the first-formed (1-adamantyl)(1-norbornyl)ketyl radical (by loss of Ad˙) leads to ring-opened ketones and several ketonic cross-products. These are isomeric with the secondary alcohol, AdNorCHOH, and with the regular cross-product, AdNorSCOH (S = benzyl), respectively, also present in the product mixture. Formation of the ring-opened thermolysis products is particularly favoured by high temperature and the use of deuteriated solvent, which slows hydrogen transfer from the solvent to the intermediate ketyl radical. The new products are cyclopentane derivatives, formed by cleavage of the norbornyl C(1)–C(2) bond, in agreement with MM2 calculations on the transition states. Self-consistent values for the cage effect have been determined by measuring the extent of 1-[²H] labelling of the adamantane formed in [²H₈] toluene and by scavenging the ketyl radical with benzenethiol in [¹H₈] toluene. The product composition of the scavenger-free reaction in [¹H₈] or [²H₈] toluene has been interpreted by kinetic simulation based on the steady state approximation, a Simplex procedure being used to optimise several rate constants, in particular those for hydrogen transfer from toluene to the ketyl radical and ring opening of the latter. The Arrhenius pre-exponential factor and activation energy are both much greater for ring opening than for hydrogen transfer.