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.

可见光辅助光催化有机反应近年来作为一种可实现选择性化学转化的通用策略受到广泛关注，这类转化可在温和反应条件下完成C-C键及多种C-X（X=N、O、S）键的构建。此前研究中所使用的光捕获剂主要为钌或铱基光敏剂。与之相对，具备选择性的脂肪族C-C键断裂光催化反应则较为罕见。本研究聚焦于合理设计五价钒氧配合物作为分子光氧化还原催化剂，以实现脂肪族醇类底物中醇羟基邻位C-C键的选择性活化与断裂。我们通过动力学测试与密度泛函理论（DFT）计算，开发出一款催化C-C键活化反应的候选催化剂，其反应速率较我们最初使用的钒配合物提升最高达7倍。此外我们还筛选出一类底物，其C-C键的断裂速率取决于所使用的催化剂，可达基准速率的2.5至17倍。为进一步阐明配体修饰对反应热力学与催化性能的影响，我们通过DFT计算揭示了轨道能级、C-C键断裂过程中的电子跃迁以及活化能垒。综合动力学与计算研究结果表明，在配体的特定位点引入吸电子基团，是开发适用于该C-C键断裂反应的高活性、高稳定性钒基光催化剂的关键。