Development of vanadium complexes for selective photocatalytic carbon-carbon bond cleavage
Date of Issue2018-12-31
School of Physical and Mathematical Sciences
Activation of carbon-carbon single bonds remains a long-standing pursuit in synthetic chemistry due to their high stability. However, the ability to selectively break down C-C bonds under mild conditions and with inexpensive catalysts would be beneficial for rapid development of complex molecules as well as for valorizing neglected feedstocks such as biomass and plastics. To address this issue, we have prepared a series of vanadium photocatalysts that are able to effect highly selective C-C bond activation in aliphatic alcohol substrates under exceptionally mild conditions and visible light irradiation. Improving on our group’s initial catalyst VO-10, we began with the systematic tuning of electronic properties of the ligand backbone by introducing strong electron-withdrawing groups at specific ligand sites. This approach led to the development of new photocatalysts with significantly enhanced reactivity. Detailed kinetics studies of photodegradation of representative non-phenolic, β-O-4-containing lignin model substrates 5 and 25 revealed VO-14 as the fastest catalyst, performing up to 7 and 17 times faster than the original VO-10, with respective substrates. Furthermore, the computational DFT studies supported our initial hypothesis that stabilization of the HOMO level of the complex could significantly improve the catalytic rate of the C-C bond cleavage. In the subsequent work, we have been able to apply this unique reactivity of catalyst VO-14 to a wide range of substrates. Alcohols that generate highly stabilized benzyl radicals upon C-C bond cleavage were especially suitable. Additionally, mechanistic screening and substrate modifications revealed that various functional groups are well tolerated under optimized reaction conditions. Moreover, even simple, commercially available alcohols, which generate less stabilized tertiary, secondary, and even primary radicals underwent C-C bond cleavage under photocatalytic conditions. Remarkably, in some instances, a second C-C bond cleavage of the initially formed alcohol product occured, indicating a possibility of applying a cascade-type C-C bond activation in specific substrates. Lastly, the ease of preparation of hydrazone-amide ligands and their corresponding complexes allowed us to synthesize a significant number of new vanadium catalysts with various functional groups on the ligand. This work shows that fine-tuning the ligand design can effect distinct structural, as well as photophysical properties in the resulting vanadium complexes. Depending on the type, and the position of ligand substitution, complexes with different reactivity, as well as distinct light absorbing, and emitting properties may be obtained.