Hydrogenation of [5]- and [6]metacyclophane: Reactivity and thermochemistry

The course of the hydrogenation of [5]- and [6]metacyclophane (Ib and Ic) a nd their thermochemistry is described. Both compounds are hydrogenated rapi dly (within 10 s) to furnish the bridgehead olefins 13b and 12c. The accomp anying hydrogenation enthalpies are -220 and -141 kJ mol(-1), respectively. Strain energies (SE) and olefinic strains (OS) of a number of bridgehead o lefins have been evaluated by DFT calculations; it was concluded that 13b b elongs to the class of hyperstable olefins which correlates nicely with its reluctance to undergo hydrogenation. By combining experimental hydrogenati on enthalpies and DFT calculations, SE of 187 and 121 kJ mol(-1) were deriv ed for Ib and 1 c.