14072-82-3Relevant articles and documents
The Interaction of π orbitals with a carbocation over three σ bonds
Lambert, Joseph B.,Ciro, Sol M.
, p. 1940 - 1945 (2007/10/03)
The semi-π analogue of double hyperconjugation ("hyperconjugation/conjugation") has been examined in 4-isopropylidenecyclohexyl mesylate (4-OMs) by comparison with the saturated analogue, trans-4-isopropylcyclohexyl mesylate (5-OMs). The unsaturated substrate reacts in 97% trifluoroethanol only four times faster than the saturated substrate. Raber-Harris plots indicate that both substrates react by ks mechanisms; i.e., solvolysis occurs with solvent assistance rather than carbocation formation. These results are consistent with the absence of a direct, through-bond interaction of the double bond with the reactive center. The absence is caused at least in part by less than ideal overlap of the γ,δ π orbitals with the α,β σ orbitals. In contrast, an electron-rich tin atom attached to the 4-position provides a large rate enhancement and changes the mechanism to carbocation formation through double hyperconjugation.
Photochemical Dehydrogenation of Alkanes Catalyzed by trans-Carbonylchlorobis(trimethylphosphine)rhodium: Aspects of Selectivity and Mechanism
Maguire, John A.,Boese, William T.,Goldman, Alan S.
, p. 7088 - 7093 (2007/10/02)
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.
