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 trans-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 trans-stilbene that is sufficiently stable
at room temperature to be observed by 31P NMR.