Deuteriation and deuterogenation of naphthalene and two octalins

Abstract

Saturation of naphthalene on a Group VIII metal yields trans-decalin by cis addition of hydrogen to an appropriate octalin intermediate rather than by direct trans addition. In like manner, dihydronaphthalene may be an intermediate in the formation of tetralin but with less obvious stereochemical consequences. Treatment of naphthalene and the two stereochemically significant octalins with deuterium gave evidence of a veritable ballet of steps wherein exchange (deuteriation) may be separate from or accompany saturation (deuterogenation). The mechanism of exchange on the aromatic ring or about the double bond seems to be largely “dissociative” with Pt and Ir, partly “associative” with Pd, and in-between with Ru and Rh. Saturation, accompanied by exchange and double-bond isomerization, and the exchange accompanying the double-bond shift seem to conform to the classical Horiuti-Polanyi associative mechanism. The “dissociative” process is retarded by increased pressure and accelerated by increased temperature. The elusive, but mechanistically important, isomerized olefin that would establish dissociative reversal from the “half-hydrogenated” state has been detected at elevated temperatures on Pd and Rh catalysts. “Release” of intermediates from the surface by desorption may be a matter of degree—complete enough to achieve exchange or isomerization, but not always complete enough for flipover. Diffusion effects cannot be ignored. Large differences in selectivities of the Group VIII metals may not necessarily reflect differences in mechanisms but rather in relative rates of otherwise identical steps in a complicated sequence.