Thermal fragmentation of 1-substituted spiro[adamantane-2,2'-adamantane] derivatives

Thermolysis of 1-hydroxy- or 1-acetoxy[1]diadamantane gives mixtures of 3-(adamantylidene)-bicyclo[3.3.1]non-6-ene, 3, 3-(2-adamantyl)bicyclo[3.3,1]nona-2,6- and -2,7-dienes, 4, and 2-(3-noradamantyl)-2,4-dehydroadamantane 7. At low conversion 3 and 7 are the major products, while one isomer of 4, the 2,6-diene, predominates in the equilibrium mixture. Pd/C-catalysed hydrogenation of the dienes in ethanol gives first 3-(2-adamantyl)bicyclo[3.3.1]non-2-ene, 5, and then endo-3-(2-adamantyl)bicyclo[3.3.1]nonane, 6. Hydrocarbon 7 is not hydrogenated under these conditions; it is converted into a mixture of 3 and 4(2,6) upon heating. Mechanisms for the fragmentation of the 1-[1] diadamantyl carbocation are discussed. While a 1,4-hydride shift may contribute to the reactions of the [1]diadamantane system, molecular mechanics calculations suggest that the ready formation of an olefin from 1-hydroxyspiro[adamantane-2,9'-bicyclo[3.3.1]nonane] is best interpreted in terms of a 1,5-hydride shift involving the chair-boat conformation of the bicyclononane system.