Transition metal-substituted Dawson anions as chemo- and regio-selectiveoxygen transfer catalysts for H_2O_2in the epoxidation of allylic alcoholst

摘要

Organic-soluble transition metal-substituted Dawson compounds [(n-C_4H_9)_4N]_9[P_2W_(17)O_(61)M(Br)](M~(n+) = Co~(2+), Ni~(2+), Cu~(2+) and Zn~(2+)), [(n-C_4H_9)_4N]_7[HP_2W_(17)O_(61)M(Br)] (M~(n+) = Cr~(3+), Mn~(3+) and Fe~(3+)) and[K/(n-C_4H_9)_4N]_(10-n)[P_2W_(17)O_(61)M(H_2O)] (M~(n+) =Ir~(4+),Ru~(3+) and Pd~(2+)) have been investigated as oxygentransfer agents for H_2O_2to a series of primary allylic alcohols to generate epoxides under biphasicreaction conditions (1,2-dichloroethane/H_2O) at 30 or 35 °C, such that the effect of variations in thesubstituted transition metals could be evaluated. The allylic alcohols involved the speciesR_1R_2C=C(R_3)CH_2-OH (where R_1,R_2andR_3=H or Me), as well as cyclic (2-cyclohexen-l-ol),bicyclic [(R-)-(-)-myrtenol and (R-)-(-)-nopol] and species with two unsaturated sites (geraniol andnerol). The reactions are highly chemoselective and regioselective. The order of reactivity for theM(II)-substituted species is Pd(II) > Zn(II) > Co(II) > Ni(II), and for M(III) and M(IV) substitution isMn(III) Ir(IV) > Fe(III) > Cr(III). The observed orders are consistent with the formation ofmetal(n+)-alcohol species as part of the reaction mechanism. For the more polarizing Ir(IV), however,Ir(IV)-alcoholate species are likely involved in the mechanism. Formation constants for the Mn(III)and Co(II)-phosphopolyoxotungstate-alcohol species with all of the above alcohols have been evaluatedin 1,2-dichloroethane at 25 °C and range from 19.0-3.5 M~(-1). The most likely transition state involvescoordination of the alcohol to the transition metal substituted at the lacunary site, or alkoxide in thecase of Ir(IV), along with interaction of the double bond of the alcohol with a peroxo group located at aW(VI) site adjacent to the substituted transition metal.