Thiols as Powerful Atom Transfer Catalyst: Opportunities in Photoredox‐Mediated Reactions

Abstract

The employing visible light to drive organic transformations is the most promising choice to meet the needs for green synthesis, in which the reactions promoted by visible light may provide a more efficient and greener method. Currently, the most abundant methods for activation and functionalization of reaction substrates have relied on the direct single‐electron transfer (SET) between the excited photocatalyst and substrates, and these wonderful works were summarized and docoumented in many reviews. As a complement to the above reactions, the photoredox‐mediated atom transfer transformations (photocatalyst‐to‐HAT catalyst‐to‐substrates) to generate radical species are showing an explosively growing trend. In this review, we highlight recent significant developments in this rapidly growing area, mainly focusing on the photoinduced base‐to‐substrates charge transfer reactions.

Practical photocatalytic hydroalkylation of alkenes with chloroacetates mediated by the formate ion

“…At the same time, in acetonitrile the reaction was slow because of low solubility of sodium formate. tert -Butyl thiol (10 mol%) was most effective as a HAT mediator, 34 while without thiol the reaction did not proceed. Aromatic thiol was completely ineffective due to facile nucleophilic substitution of chloride by the thiolate [the corresponding by-product was detected by GC-MS analysis] (entry 6).…”

Peptide macrocyclisation via intramolecular interception of visible-light-mediated desulfurisation

“…The resulting thiyl radical can then continue the cycle or be reduced by the catalyst and protonated. 68 Oxygen in the buffer has also been identied as an oxidant for the catalyst during Ru-mediated desulfurisation. 60 The chemoselectivity of this chemistry should ensure that the reaction enables efficient cyclisation of unprotected peptides in aqueous solution.…”

Combined Photocatalysis and Lewis Acid Catalysis Strategy for the Oxa-Pictet–Spengler Reactions of Ethers

“…In our reaction design, the employment of an appropriate terminal oxidant and a thiol-type HAT catalyst could site-selectively generate a radical at the benzylic ether position . Concurrently, the oxidant-derived benzoate radical counterpart would couple to the ether radical to yield the crucial intermediate acetal.…”