- Efficient transfer-dehydrogenation of alkanes catalyzed by rhodium trimethylphosphine complexes under dihydrogen atmosphere
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RhL2Cl(CO) (1; L = PMe3), a known catalyst for the photodehydrogenation of alkanes, is found to catalyze the highly efficient thermal (nonphotochemical) transfer-dehydrogenation of alkanes under high-pressure hydrogen atmosphere. The proposed mechanism involves addition of H2, loss of CO, and transfer of H2 to a sacrificial acceptor, thereby generating RhL2Cl, the same catalytically active fragment formed by photolysis of 1. Consistent with this proposal, we report that photochemically inactive species, RhL2ClL′ (L′ = P'Pr3, PCy3, PMe3) and [RhL2Cl]2, are also thermochemical catalyst precursors. These species demonstrate much greater catalytic activity than RhL2Cl(CO), particularly under moderate hydrogen pressures (ca. 500 times greater under 800 Torr of H2 at 50°C). The dependence of the turnover rates on hydrogen pressure is consistent with the proposed role of hydrogen, i.e., displacement of L′ from the four-coordinate complexes or fragmentation of H2Rh2L4Cl2, giving H2RhL2Cl, which is dehydrogenated by olefin to give RhL2Cl. Selectivity studies provide further support for the characterization of the active fragment.
- Maguire, John A.,Petrillo, Angelo,Goldman, Alan S.
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p. 9492 - 9498
(2007/10/02)
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- Photochemical Dehydrogenation of Alkanes Catalyzed by trans-Carbonylchlorobis(trimethylphosphine)rhodium: Aspects of Selectivity and Mechanism
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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 thedehydrogenation 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 enrgy needed to drive these thermodynamically unfavorable dehydrogenations is obtained only from Rh-CO bond photolysis.
- Maguire, John A.,Boese, William T.,Goldman, Alan S.
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p. 7088 - 7093
(2007/10/02)
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- Alkylation of Allylic Derivatives. 11. Copper(I)-Catalyzed Cross Coupling of Allylic Carboxylates with Grignard Reagents
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Reactions of allylic carboxylates with Grignard reagents containing catalytic amounts (1-10 mol percent) of cuprous salts give high yields of cross-coupled products.With alkyl Grignard reagents, regiochemistry can be controlled by choice of cuprous salt.With cuprous halides, little regiospecificity is observed.There is a small excess of γ-coupling in unbiased systems such as 5-methyl-2-cyclohexenyl (1), 2-cyclohexenyl (3), and β-phenylallyl (5) carboxylates.With CuCN, complete regiospecificity (exclusive γ-coupling) is observed with all alkyl Grignard reagents in unbiased systems, and with n-butylmagnesium halide >97percent γ-coupling results with α-methyl-γ-phenylallyl pivalate (7-OPiv) which is biased in favour of coupling at the α-position.In sharp contrast to alkyl Grignard reagents, phenyl and vinyl Grignard reagents containing CuCN show no regiospecificity.
- Tseng, Chung Chyi,Paisley, Steven D.,Goering, Harlan L.
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p. 2884 - 2891
(2007/10/02)
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- A New Reducing System: Calcium Metal in Amines. Reduction of Aromatic Hydrocarbons
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A new reducing system consisting of calcium dissolved in a mixture of amines (methylamine-ethylenediamine) is described.Representative aromatic hydrocarbons have been reduced by this new reagent largely to monoalkenes.Hydrocarbons like tetralin, m- and p-xylene, and indan are reduced in excellent yields by the calcium system to a crude product containing 88percent or better of a single alkene.A new technique involving oxymercuration-demercuration is used to purify two of the monoalkene isomer mixtures obtained in these reductions.Unexpectedly, durene is reduced by the calcium reagent to 1,2,4,5-tetramethyl-1,4-cyclohexadiene in excellent yield.Likewise anthracene is reduced in one step to 1,2,3,4,5,6,7,8,9,10-decahydroanthracene.Experiments designed to elucidate why the calcium system does not reduce durene or anthracene to monoalkenes are described.Similarities and differences between the calcium-amine and the lithium-amine reducing systems are discussed.
- Benkeser, Robert A.,Belmonte, Frank G.,Kang, Jahyo
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p. 2796 - 2802
(2007/10/02)
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