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)­P₂W₁₇O₆₁}^8– or {[(Cu­(L)]₂WZn­(ZnW₉O₃₄)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 = H₂O) 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)­P₂W₁₇O₆₁}^8– as oxidant and <i>trans</i>-stilbene derivatives as substrates point toward a reaction mechanism for C–C bond cleavage involving two molecules of {α₂-Co­(NO₂)­P₂W₁₇O₆₁}^8– and one molecule of <i>trans</i>-stilbene that is sufficiently stable at room temperature to be observed by ³¹P NMR