Aerobic carbon-carbon bond cleavage of alkenes to aldehydes catalyzed by first-row transition-metal-substituted polyoxometalates in the presence of nitrogen dioxide

A new aerobic carbon-carbon bond cleavage reaction of linear di-substituted alkenes, to yield the corresponding aldehydes/ketones in high selectivity under mild reaction conditions, is described using copper(II)-substituted polyoxometalates, such as {α₂-Cu(L)P₂W ₁₇O₆₁}^8- or {[(Cu(L)]₂WZn(ZnW ₉O₃₄)₂}^12-, as catalysts, where L = NO₂. A biorenewable-based substrate, methyl oleate, gave methyl 8-formyloctanoate and nonanal in >90% yield. Interestingly, cylcoalkenes yield the corresponding epoxides as products. These catalysts either can be prepared by pretreatment of the aqua-coordinated polyoxometalates (L = H ₂O) with NO₂ or are formed in situ when the reactions are carried with nitroalkanes (for example, nitroethane) as solvents or cosolvents. Nitroethane was shown to release NO₂ under reaction conditions. ³¹P NMR shows that the Cu-NO₂-substituted polyoxometalates act as oxygen donors to the C-C double bond, yielding a Cu-NO product that is reoxidized to Cu-NO₂ under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {α₂-Co(NO₂)P₂W₁₇O ₆₁}^8- as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C-C bond cleavage involving two molecules of {α₂-Co(NO₂)P₂W ₁₇O₆₁}^8- and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by ³¹P NMR.