Aerobic Carbon-Carbon Bond Cleavage of Alkenes to Aldehydes Catalyzed by First-Row Transition-Metal-Substituted Polyoxometalates in the Presence of Nitrogen Dioxide

摘要

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 {α_2-Cu(L)P_2W_(17)O_(61)}~(8-) or {[(Cu- (L)]_2WZn(ZnW_9O_(34)_2}~(12-), as catalysts, where L = NO_2. 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 NO_2 or are formed in situ when the reactions are carried with nitroalkanes (for example, nitroethane) as solvents or cosolvents. Nitroethane was shown to release NO_2 under reaction conditions. ~(31)P NMR shows that the Cu-NO_2-substituted polyoxometalates act as oxygen donors to the C-C double bond, yielding a Cu-NO product that is reoxidized to Cu-NO_2 under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {α_2- Co(NO_2)P_2W_(17)O_(61)}~(8-) as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C-C bond cleavage involving two molecules of {α_2-Co(NO_2)P_2W_(17)O_(61)}~(8-) and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by ~(31)P NMR.