KINETICS OF PHOTOCHEMICAL ALKANE DEHYDROGENATION CATALYZED BY RH(PME3)(2)(CO)CL - IMPLICATIONS CONCERNING THE C-H BOND ACTIVATION STEP

The mechanism of photochemical alkane dehydrogenation catalyzed by Rh( PMe3)(2)(CO)Cl has been further probed with an emphasis on characteriz ing the initial C-H activation step and understanding the effect of ad ded CO on selectivity. While pure cyclooctane and pure cyclohexane are dehydrogenated at the same rate (same quantum yields), cyclooctane sh ows much greater reactivity in mixtures of the two solvents. The produ ct ratio (cyclooctene:cyclohexene) is highly dependent upon the partia l pressure of CO, ranging from 12 in the absence of CO, to 75 in the l imit of high CO pressure (> ca. 400 torr). The kinetic isotope effect for the dehydrogenation of c-C6H12/c-C6D12 is also found to be depende nt upon CO pressure, ranging from 10 in the absence of CO to 4.2 under high CO pressure. The results support our earlier conclusion that the intermediate responsible for C-H activation is ground state [Rh(PMe,) ,CI]. It is also concluded that inhibition of the reaction by CO opera tes primarily via addition of CO to the intermediate alkyl hydrides, ( R)(H)Rh(PMe3)(2)Cl. Addition of CO prior to C-H bond addition is appar ently not a kinetically significant process, even under high CO pressu re. (C) 1998 Elsevier Science S.A.