Kinetics and DFT Studies of Photoredox Carbon–Carbon Bond Cleavage Reactions by Molecular Vanadium Catalysts under Ambient Conditions

Visible
light assisted photocatalytic organic reactions have recently
received intense attention as a versatile approach to achieve selective
chemical transformations, including C–C and several C–X
(X = N, O, S) bond formations under mild reaction
conditions. The light harvesters in previous reports predominantly
comprise ruthenium or iridium photosensitizers. In contrast, selective,
photocatalytic aliphatic C–C bond cleavage reactions are scarce. The present study focuses on rationally designing
VV oxo complexes as molecular, photoredox catalysts toward
the selective activation and cleavage of a C–C bond adjacent
to the alcohol group in aliphatic alcoholic substrates. We have employed
kinetics measurements and DFT calculations to develop a candidate
for the catalytic C–C bond activation reaction that is up to
7 times faster than our original vanadium complex. We have also identified
a substrate where the C–C bond cleaves at rates 2.5–17
times faster, depending on the catalyst used. In order to better understand
the effects of ligand modification on the thermodynamics and catalysis,
DFT calculations were employed to reveal the orbital energies, the
electronic transitions during the C–C bond cleavage, and the
activation barriers. Our combined kinetics and computational studies
indicate that the incorporation of electron-withdrawing groups at
select sites of the ligand is essential for the development of active
and stable vanadium photocatalysts for our C–C bond cleavage
reactions.