Visible-Light-Driven Transformations of Phenols via Energy Transfer Catalysis

Abstract

In the past decade, the field of visible-light-mediated photocatalysis has been particularly thriving by offering innovative synthetic tools for the construction of functionalized architectures from simple and readily available substrates. One strategy that has been of interest is energy transfer catalysis, which is a powerful way of activating a substrate or an intermediate by using the combination of light and a relevant photosensitizer. This review deals with recent advances in energy transfer catalysis applied to phenols, which are ubiquitous in chemistry both as starting materials and as high-added-value products. Processes involving energy transfer from the excited photosensitizer to ground state oxygen and to phenol-containing substrates will be described.1 Introduction2 Intermolecular Processes2.1 Reactions with Singlet Oxygen2.2 [2+2] Cycloadditions3 Intramolecular Transformations4 Conclusions and Outlook

A Green Blue LED-Driven Two-Liquid-Phase One-Pot Procedure for the Synthesis of Estrogen-Related Quinol Prodrugs

“…Substrate scope of novel blue LED-driven two-liquid-phase photo-oxygenation a . The mechanism of dye-mediated photo-oxygenation of phenols has been reviewed and discussed; it includes the transfer of the excited state from the photosensitizer to the substrate (Type I mechanism), or alternatively the inter-crossing system between the photosensitizer and dioxygen, with formation of singlet oxygen ( 1 O2) (Type II mechanism) [38,[41][42][43]. Under our experimental conditions, the Type I mechanism was most probably not operating, as suggested by the loss of reactivity of the substrate in the absence of meso-TPP (Table 1, entry 5), associated with the low absorption coefficient of estrogens in the interaction with blue-LED photons (Figures S3-S6) [44][45][46][47].…”

“…The tentative reaction pathway for the photo-oxygenation of compounds 1a-d is reported in Scheme 2, it includes: (i) blue-LED photo-activation of meso-TPP to form the singlet excited state ( 1 meso-TPP*); (ii) intersystem crossing (ISC) to form the triplet excited state ( 3 meso-TPP*); (iii) energy transfer, and formation of singlet oxygen ( 1 O2); (iv) selective insertion of 1 O2 on substrate to yield an unstable adduct A (not isolated in our case); and (v) rearrangement of adduct A to yield the corresponding hydro-peroxide. The mechanism of dye-mediated photo-oxygenation of phenols has been reviewed and discussed; it includes the transfer of the excited state from the photosensitizer to the substrate (Type I mechanism), or alternatively the inter-crossing system between the photosensitizer and dioxygen, with formation of singlet oxygen ( 1 O2) (Type II mechanism) [38,[41][42][43]. Under our experimental conditions, the Type I mechanism was most probably not operating, as suggested by the loss of reactivity of the substrate in the absence of meso-TPP (Table 1, entry 5), associated with the low absorption coefficient of estrogens in the interaction with blue-LED photons (Figures S3-S6) [44][45][46][47].…”

Gd(III)-Catalyzed Regio-, Diastereo-, and Enantioselective [4 + 2] Photocycloaddition of Naphthalene Derivatives

“…Visible-light-mediated photocatalysis has made significant developments in the past decade . The highly reactive radicals and excited-state intermediates generated under the irradiation with visible light became an impetus for the dearomatization reactions of inert arenes, generally in a racemic manner . Notably, CADA reactions involving the excited state of arenes were disclosed in recent years.…”

Ex Situ Generation of 18O2 and 17O2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen

“…The photooxidation of substituted phenols was also investigated by irradiating substrate 15 under green light in the presence of catalytic amounts of rose bengal and cesium carbonate while being exposed to *O-labeled dioxygen generated from 2 equiv of [*O 2 ]­2 . A mixture of quinol [*O 1 ]­16a and epoxyquinol [*O 2 ]­16b was obtained in both cases, with overall yields of 32% to 53% (Scheme h).…”