Use of excited-state and ground-state redox properties of polyoxometalates for selective transformation of unactivated carbon-hydrogen centers remote from the functional group in ketones

Article abstract of DOI:10.1021/ja00029a022

Two types of processes are described which involve the selective transformation of unactivated carbon-hydrogen bonds in a ketone, cis-2-decalone, cis-1, which possesses conventionally far more reactive bonds. The first type of process involves irradiation of decatungstate, W₁₀O₃₂^4- in the presence of cis-1 producing, trans-2-decalone, trans-1, the product resulting from epimerization of an unactivated tertiary C-H bond remote from the carbonyl group, in high selectivity at high conversion of substrate, eq 1. The second type of reaction involves irradiation of the heteropolytungstate, α-P₂W₁₈O₆₂^6- or α-PW₁₂O₄₀^3-, in the presence of cis-1 producing two monounsaturated ketones (octalones) in high selectivity with the nonthermodynamic isomer, 2, in comparable or greater quantity than the conventional thermodynamic (conjugated) isomer, 3, eq 2. Both types of processes are independent of wavelength over the principal range of absorption of the complexes (250-380 nm). The reactions are first order in W₁₀O₃₂^4- under optically dilute conditions. The primary kinetic isotope of the corresponding decalin hydrocarbons evaluated under the same reaction conditions as eq 1, k_{cis-decalin-H18}/k_{cis-decalin-D18} ～ 2, and the solvent kinetic isotope effect, k_CH3CN/k_CD3CN = 1.0. The photochemical reaction of decatungstate with α,α,α′,α′-D₄-cis-1 leads exclusively, even at moderate conversion of substrate (25%), to α,α,α′,α′-D₄-trans-1. These data, an isotope crossover experiment in which decatungstate was irradiated in the presence of a 50/50 molar mixture of deuterated and protiated cis-decalin in CD₃CN, reactions of cis-1 and cis-decalin with an authentic localized ground-state radical, t-BuO^?, and a number of other experiments are consistent with initial H atom abstraction in all cases. The dramatically different products seen with the different polyoxometalate systems are dictated by the relative rates of epimerization, oxidation, and escape of the cisoid tertiary bridgehead radicals in the initial radical cage and, to a lesser extent, by the rates of conventional radical-radical reactions and other processes.