The photochemical dehydrogenation of alkanes is catalyzed in solution by trans-Rh(PMe3)2(CO)Cl with high efficiency; quantum yields up to 0.10 and turnover numbers as high as 5000 are achieved with cyclooctane as substrate. The intramolecular regioselectivity of the reaction is investigated with methyl-, ethyl-, and isopropylcyclohexane. In competition experiments, cyclooctane is found to be 17 times as reactive as cyclohexane; under carbon monoxide atmosphere, the selectivity is enhanced to a factor of 130. A kinetic isotope effect, kH/kD = 5.3, is found for the dehydrogenation of C6H12/C6D12. Both intra- and intermolecular selectivities are consistent with a pathway involving a reversible C-H oxidative addition followed by a β-hydrogen elimination. trans-Rh(PMe3)2(CO)Cl is demonstrated to be the only significant photoactive species in solution. The dehydrogenation reaction is quenched by carbon monoxide with Stern-Volmer kinetics. On the basis of these results, a mechanism is proposed in which the energy needed to drive these thermodynamically unfavorable dehydrogenations is obtained only from Rh-CO bond photolysis.
|Number of pages||6|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1989|
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