Mechanistic Investigation of Visible-Light-Induced Intermolecular [2 + 2] Photocycloaddition Catalyzed with Chiral Thioxanthone

Abstract

The recent thioxanthone-sensitizer-catalyzed intermolecular [2 + 2] cycloaddition induced by visible-light irradiation set the stage for the future development of feasible photocycloadditions. Nonetheless, the mechanism of this reaction still remains under debate, especially on the activation mode of the thioxanthone photosensitizer (energy transfer, bielectron exchange, and hydrogen transfer are all possible mechanisms). To settle this issue, systematic density functional theory calculations have been carried out. The results indicate that the energy-transfer pathway is more favorable than the bielectron-exchange and the hydrogen-transfer pathways. Meanwhile, the overall transformations involve the complexation and excitation of photosensitizer, the first C-C bond formation, the dissociation of the sensitizer, the triplet-to-singlet electronic state crossing, and the second C-C bond formation. The first C-C bond formation is the rate- and selectivity-determining step, and synergistic energy and electron transfer from photosensitizer to substrate moieties takes place along this process. On this basis, the effect of olefin substrates (ethyl vinyl ketone vs vinyl acetate) on the stereoselectivity was finally analyzed.

Thioxanthone: a powerful photocatalyst for organic reactions

“…62 The complexation of chiral thioxanthone with the quinolone core, which occurs via the formation of hydrogen bonding, is responsible for the enantioselective approach of the alkene. Mechanistic and computational studies by Yu et coworkers, 63 revealed that the enantioselectivity is governed by the quick and stable formation of the complex between thioxanthone derivative 35 and the quinolone core of the substrate, in a similar fashion as shown in Scheme 10, A. The lower triplet energy of the thioxanthone derivative (263 kJ/mol), compared to the previous reported xanthone derivative (316 kJ/mol), increases the selectivity of the [2+2] photocycloaddition.…”

Pivotal Electron Delivery Effect of the Cobalt Catalyst in Photocarboxylation of Alkynes: A DFT Calculation

“…According to Figure , the Z -carboxylate product of Pro-2b is favorably formed, predominantly due to the anti-hydrogen transfer and the syn -carboxylation steps. Herein, it is noteworthy that in the presence of the photocatalyst Ir III , the configuration changes from the Z - to E -carboxylate product could occur easily via the energy transfer process (see Figure S4 for the details). − In this context, β-carboxylation occurs prior to α-carboxylation, and both Z - and E -carboxylate products could be formed. The results are consistent with the experimental outcome reported by Wu et al…”

Understanding the Mechanism of Triplet-Triplet Energy Transfer in the Photocatalytic [2+2] Cycloaddition: Insights from Quantum Chemical Modeling

“…[19][20][21][22] After the TTEnT, the substrate accesses its triplet manifold and undergoes the bond-forming events in a stepwise manner. 23,24 Previously, we examined the origins of stereoselectivity in intra-and intermolecular…”