Photochemical dehydrogenation of alkanes catalyzed by trans-carbonylchlorobis(trimethylphosphine)rhodium

Aspects of selectivity and mechanism

John A. Maguire, William T. Boese, Alan S Goldman

Research output: Contribution to journalArticle

125 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)7088-7093
Number of pages6
JournalJournal of the American Chemical Society
Volume111
Issue number18
Publication statusPublished - 1989

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Rhodium
Alkanes
Dehydrogenation
Carbon Monoxide
Paraffins
Carbon monoxide
Regioselectivity
Hydrogen
Kinetics
Photolysis
Quantum yield
Cyclohexane
Isotopes
Atmosphere
Substrates
Experiments

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Photochemical dehydrogenation of alkanes catalyzed by trans-carbonylchlorobis(trimethylphosphine)rhodium : Aspects of selectivity and mechanism. / Maguire, John A.; Boese, William T.; Goldman, Alan S.

In: Journal of the American Chemical Society, Vol. 111, No. 18, 1989, p. 7088-7093.

Research output: Contribution to journalArticle

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AB - 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.

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