The photochemical dehydrogenation of alkanes is catalyzed in solution by trans-Rh(PMe3),(CO)C1 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 C6Hlz/C,Dlz. Both intra-and intermolecular selectivities are consistent with a pathway involving a reversible C-H oxidative addition followed by a P-hydrogen elimination. trans-Rh(PMe,),(CO)CI 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.The selective and catalytic functionalization of alkanes remains one of the most important and challenging goals of organometallic chemists.l Many homogeneous transition-metal alkane activation reactions have been discovered in the past 10 years, but there have been relatively few examples of catalytic derivatizations.2-8 Recently, Tanaka reported9 the development of a photochemical rhodium system, related to Eisenberg's benzene carbonylation systems,1° which effects the terminal carbonylation of n-pentane with moderate efficiency.

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Intermediates relevant to the carbonylation of metal alkyl complexes have been generated by laser flash photolysis of the manganese acyl complexes RC(0)Mn(C0)5 (R = CH3, CD3, CH2CH3, CH2F, CF3). This results in immediate CO photodissociation to give intermediate acyl complexes which were observed by time resolved infrared (TRIR) and time resolved optical (TRO) spectroscopy. In the presence of added ligands, such intermediates are trapped to form stable as-substituted octahedral complexes in competition with alkyl migration from the acyl group to give the alkyl pentacarbonyl complexes RMn(CO)s. The spectra and reactivity of the intermediate (I) derived from CH3C(0)Mn(C0)s (A) indicate that this exists as the chelated acyl complex (?/2 3-CH3CO)Mn(CO)4 in weakly coordinating solvents such as cyclohexane but as the solvento species as-CH3C(0)Mn(C0)4(THF) in tetrahydrofuran. Comparisons with thermal reaction kinetics support the assertion that the intermediates generated photochemically are indeed relevant to understanding the mechanism for CH3Mn(CO)s carbonylation. The CF3 and CH2F analogs of I are much more reactive than I in cyclohexane solution, and this has been interpreted in terms of the ?y2-acyl configuration being destabilized by these electron-withdrawing groups. Solvent effects on the rates of methyl migration and ligand trapping reactions of the intermediate species are described and analyzed in terms of their relevance to the migratory insertion mechanism.

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