Transition metal oxo species are key intermediates for the activation of strong C‒H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo...
Biology employs exquisite control over proton, electron, H-atom, or H2 transfer. Similar control in synthetic systems has the potential to facilitate efficient and selective catalysis. Here we report a dihydrazonopyrrole...
The
selective hydroxylation of aliphatic C–H bonds remains
a challenging but broadly useful transformation. Nature has evolved
systems that excel at this reaction, exemplified by cytochrome P450
enzymes, which use an iron-oxo intermediate to activate aliphatic
C–H bonds with k
1 > 1400 s–1 at 4 °C. Many synthetic catalysts have been
inspired by these enzymes and are similarly proposed to use transition
metal-oxo intermediates. However, most examples of well-characterized
transition metal-oxo species are not capable of reacting with strong,
aliphatic C–H bonds, resulting in a lack of understanding of
what factors facilitate this reactivity. Here, we report the isolation
and characterization of a new terminal CoIII-oxo complex,
PhB(AdIm)3CoIIIO. Upon oxidation,
a transient CoIV-oxo intermediate is generated that is
capable of hydroxylating aliphatic C–H bonds with an extrapolated k
1 for C–H activation >130 s–1 at 4 °C, comparable to values observed in cytochrome P450 enzymes.
Experimental thermodynamic values and DFT analysis demonstrate that,
although the initial C–H activation step in this reaction is
endergonic, the overall reaction is driven by an extremely exergonic
radical rebound step, similar to what has been proposed in cytochrome
P450 enzymes. The rapid C–H hydroxylation reactivity displayed
in this well-defined system provides insight into how hydroxylation
is accomplished by biological systems and similarly potent synthetic
oxidants.
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