We report the non-adiabatic dynamics of VIIICl3(ddpd), a complex based on the Earth-abundant first-row transition metal vanadium with d2 electronic configuration which is able to emit phosphorescence in solution in...
In
concert with carbonyl compounds, Lewis acids have
been identified
as a versatile class of photocatalysts. Thus far, research has focused
on activation of the substrate, either by changing its photophysical
properties or by modifying its photochemistry. In this work, we expand
the established mode of action by demonstrating that UV photoexcitation
of a Lewis acid–base complex can lead to homolytic cleavage
of a covalent bond in the Lewis acid. In a study on the complex of
benzaldehyde and the Lewis acid BCl3, we found evidence
for homolytic B–Cl bond cleavage leading to formation of a
borylated ketyl radical and a free chlorine atom only hundreds of
femtoseconds after excitation. Both time-dependent density functional
theory and transient absorption experiments identify a benzaldehyde-BCl2 cation as the dominant species formed on the nanosecond time
scale. The experimentally validated B–Cl bond homolysis was
synthetically exploited for a BCl3-mediated hydroalkylation
reaction of aromatic aldehydes (19 examples, 42–76% yield).
It was found that hydrocarbons undergo addition to the CO
double bond via a radical pathway. The photogenerated chlorine radical
abstracts a hydrogen atom from the alkane, and the resulting carbon-centered
radical either recombines with the borylated ketyl radical or adds
to the ground-state aldehyde-BCl3 complex, releasing a
chlorine atom. The existence of a radical chain was corroborated by
quantum yield measurements and by theory. The photolytic mechanism
described here is based on electron transfer between a bound chlorine
and an aromatic π-system on the substrate. Thereby, it avoids
the use of redox-active transition metals.
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