Thermolysis of highly congested tri-tert-alkylmethanes. Rearrangement of a 3-noradamantylmethyl radical

Abstract

Activation energies for C–Ad fission in the thermolysis of di-1-adamantyl-tert-alkylmethanes and 1-adamantyl-di-1-bicyclo[2.2.2]octylmethane, AdR¹R²CH, in toluene are best correlated with the strain energy difference (MMP2 force field) between the methane and the corresponding radical, R¹R²C·H; difficulties were encountered in the application of MM3 to certain of these trialkylmethanes. Normally, the major thermolysis product is the di-tert-alkylmethane, R¹R²CH₂, but when a 3-noradamantyl group is present (1d) the initially formed radical ring opens to give 1,2′-biadamantyl in amounts which depend on the temperature and the solvent (normal or octadeuteriated). This rearrangement is readily explained by MMP2 calculations. Since the crossproduct yield is low (less than 3%, even in deuteriated solvent at the highest temperature) the thermodynamic parameters for the hydrogen transfer and ring opening reactions of the 1adamantyl-3-noradamantylmethyl radical can be compared directly. Both the activation enthalpy and entropy are much greater for ring opening than for hydrogen abstraction from the solvent. Isotope effects on hydrogen abstraction are high and satisfy certain criteria for tunnelling, as do data on the analogous reaction of Ad₂C·H. A more sophisticated treatment of the product composition for 1d thermolysis, using kinetic simulation, leads to essentially the same conclusions as the simpler treatment.