The homogeneous photochemical functionalization of alkanes with polyoxometalates

The homogeneous photochemical functionalization of alkanes with polyoxometalates is examined in detail at low conversions. The major alkane oxidation products include alkenes, methyl ketones, and dimers. The polyoxometalate is reduced by one or two electrons in this process. With branched alkanes, {dollar}alpha{dollar}-PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}sp{lcub}3-{rcub}{dollar} leads predominantly to the maximally substituted olefins whereas W{dollar}sb{lcub}10{rcub}{dollar}O{dollar}sb{lcub}32{rcub}sp{lcub}4-{rcub}{dollar} produces mainly the minimally substituted olefins in non-acidic media. Electron transfer from the alkane to the polyoxometalate excited state is believed to occur in the primary alkane functionalization step. Data which examines the reaction rates as a function of the light intensity, the concentration of alkane and polyoxometalate, and several other variables is fit to a rate law invoking the steady-state approximation.; The addition of strong acids to polyoxometalate/acetonitrile/alkane/{dollar}hv{dollar} systems causes significant increases in the quantum yields of alkane oxidation. For Q{dollar}sb4{dollar}W{dollar}sb{lcub}10{rcub}{dollar}O{dollar}sb{lcub}32{rcub}{dollar}, quantum yield enhancements to values of {dollar}sim{dollar}1 and a variety of other changes result upon acidification. These changes are attributed to protonation of Q{dollar}sb4{dollar}W{dollar}sb{lcub}10{rcub}{dollar}O{dollar}sb{lcub}32{rcub}{dollar}.; The photooxidation of 1:1 alkanes/alcohol mixtures is examined with {dollar}alpha{dollar}-H{dollar}sb3{dollar}PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar} and Q{dollar}sb4{dollar}W{dollar}sb{lcub}10{rcub}{dollar}O{dollar}sb{lcub}32{rcub}{dollar}. Addition of acids to the Q{dollar}sb4{dollar}W{dollar}sb{lcub}10{rcub}{dollar}O{dollar}sb{lcub}32{rcub}{dollar} systems allows for a greater amount of cyclooctane derived products to be formed relative to 2-butanone. 2-butanol can be rendered still less reactive if trifluoroacetic acid is employed due to partial formation of the trifluoroacetate ester.