SYNTHESIS0039-78811437-210X © Georg Thieme Verlag Stuttgart · New York 2020, 52, 1617–1624 short review 1617 en Syn thesis J. Fischer et al. Short Review Visible-Light-Driven Transformations of Phenols via Energy Transfer Catalysis Jérôme Fischer Pierrick Nun Vincent Coeffard* 0000-0003-2982-7880 Université de Nantes, CEISAM UMR CNRS 6230, 44000 Nantes, France [email protected] Received: 28.11.2019 atively received less attention. Energy transfer catalysis in- Accepted after revision: 26.02.2020 volves the combination of a photosensitizer (PS), a sub- Published online: 02.04.2020 4 DOI: 10.1055/s-0039-1708005; Art ID: ss-2019-z0654-sr strate (S), and light. This photochemical process is based on the deactivation of an excited-light absorbing sensitizer Abstract In the past decade, the field of visible-light-mediated photo- (PS*) by transferring its energy to a substrate S leading to catalysis has been particularly thriving by offering innovative synthetic the formation of the excited substrate S* (Figure 1). Unlike tools for the construction of functionalized architectures from simple and readily available substrates. One strategy that has been of interest photoredox catalysis, no single-electron transfer occurs be- is energy transfer catalysis, which is a powerful way of activating a sub- tween the excited photocatalyst and the substrate or any strate or an intermediate by using the combination of light and a rele- intermediates from the reaction mixture. vant 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 Introduction 2 Intermolecular Processes 2.1 Reactions with Singlet Oxygen 2.2 [2+2] Cycloadditions 3 Intramolecular Transformations Figure 1 Energy transfer and photoredox catalysis 4 Conclusions and Outlook Key words photochemistry, phenol, dearomatization, cycloaddition, Breakthroughs in energy transfer catalysis have been singlet oxygen driven by the design of new photosensitizers and the im- plementation of innovative synthetic transformations with This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. 1 Introduction applications in a range of contexts.5 By careful examination of the reaction conditions, visible-light-induced energy The concept of harnessing visible light to trigger chemi- transfer catalysis has been harnessed in a series of transfor- cal transformations is a powerful tool in organic chemistry.1 mations such as [2+2] photocycloadditions, alkene pho- Nevertheless, most of the organic compounds do not pos- toisomerizations, sensitization of metal complexes, oxida- sess the relevant absorption features for a direct photoexci- tive processes through the formation of singlet oxygen, or tation strategy under visible light.2 Within this context, vis- the activation of azide and diazo compounds to form het- ible-light-mediated photocatalysis has significantly con- erocyclic compounds.4 Amongst the substrates tested, phe- tributed to the advent of photochemistry by enabling the nol derivatives were deemed worthy of investigation be- reaction of photoexcited species with organic molecules cause of their unique chemical behavior. Besides classical through electron, atom or energy transfer processes. While reactivities such as electrophilic aromatic substitutions6 enormous achievements have been made in photo-initiated and C–O bond forming reactions,7 considerable attention electron transfer catalytic processes (photoredox cataly- has been given in recent years to dearomatization process- sis),3 the field of energy transfer catalysis (EnT) has compar- es8 and direct C–H functionalizations of the aromatic ring.9 © 2020. Thieme. All rights reserved. Synthesis 2020, 52, 1617–1624 Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany 1618 Syn thesis J. Fischer et al. Short Review O dearomatization R2 R1 OH O OH EnT [2+2] cycloaddition R2 R2 = O R1 Vincent Coeffard (left) was born in Nantes (France) in 1981. He stud- O ied chemistry at Université de Nantes and completed his Ph.D. studies cyclization 1 in 2007 in the group of Prof. Jean-Paul Quintard. He then moved to Uni- R versity College Dublin (Ireland) to work as a postdoctoral research fel- low for two years in asymmetric catalysis under the guidance of Prof. Figure 2 Phenol transformations via energy transfer catalysis Pat Guiry. He then spent ten months in Spain for a second postdoctoral experience in the group of Prof. Antonio M. Echavarren to investigate gold chemistry. In 2010, he was appointed CNRS researcher in the formation of the highly electrophilic species singlet oxygen 1 group of Prof. Christine Greck at Université de Versailles Saint-Quentin- ( O2). The two strategies will be discussed in the following en-Yvelines (France) to work on the design of organocatalysts and im- examples. plementation of catalytic technologies to access enantioenriched or- ganic architectures. In 2016, he moved to Université de Nantes where his current research interests include asymmetric organocatalysis and 2.1 Reactions with Singlet Oxygen photochemistry. For many decades now, it has been demonstrated that Pierrick Nun (middle) was born in Brest (France) in 1982. After under- 1 graduate studies at Université de Nantes (France), he completed his singlet oxygen ( O2) is a powerful reactant toward electron- 11 –1 Ph.D. in 2009 under the supervision of Dr. Frédéric Lamaty at Université rich substrates. An energy of 94 kJ·mol is required to de Montpellier (France), working on mechanical activation and solvent- form the highly electrophilic singlet oxygen from ground free synthesis. He then joined the group of Prof. Steven P. Nolan at the state oxygen. Many photosensitizers (tetraphenylporphy- University of St. Andrews (Scotland) as a post-doctoral researcher. Fol- rin, rose bengal, methylene blue to cite only a few) possess a lowing a second post-doctoral fellowship with Prof. Annie-Claude –1 Gaumont at the Ecole Nationale Superieure d’Ingenieurs of Caen triplet energy (ET) equal or greater than 94 kJ·mol re- (France), he was appointed Assistant Professor at Université de Nantes quired to efficiently sensitize triplet oxygen into its singlet and he is currently working on singlet oxygen applications in organic state. In synthetic chemistry, electron-rich phenols are synthesis and medicine. prone to react with singlet oxygen and this research field Jérôme Fischer (right) was born in 1994 in Mulhouse (France). After fin- has received considerable attention from a mechanistic and ishing his high school diploma, he studied chemistry at the Ecole Natio- synthetic point of view. nale Supérieure de Chimie of Mulhouse and joined the group of Dr. In 2019, Dlugogorski and co-workers reported a mecha- Vincent Coeffard for his Ph.D. studies in 2017. His research focuses on the design of new photosensitizers for applications in selective photoo- nistic and kinetic study on the photooxygenation of phenol 12 xidative processes. (Scheme 1, R = H). Based on DFT calculations and electron paramagnetic resonance (EPR), their results provided evi- dence for the 1,4-endoperoxide intermediate, obtained af- In addition, recent advances regarding valorization of bio- ter a [4+2] cycloaddition, yielding p-benzoquinone as the mass to produce phenols have made these compounds at- only detected product after ring opening. Indeed, the 1,4- 10 tractive platforms in organic chemistry. The manifold re- This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. activity of phenols with the synthetic appeal of energy transfer catalysis offers a powerful strategy to reach novel molecular architectures (Figure 2). This is the subject of this O review, which does not intend to be comprehensive, but R = H rather to point out the synthetic potential and future chal- – H2O 3 1 OH O2 EnT O2 lenges of this methodology. O p-benzoquinone OH O O 2 Intermolecular Processes O R R = Alk, 1,4-endoperoxide (Het)Ar R In intermolecular transformations, the reaction can be initiated by energy transfer from the excited triplet photo- R O2H p-peroxy quinols sensitizer to the phenol-containing substrate or the reac- 3 Scheme 1 Singlet-oxygen-mediated oxidation of phenols tant such as ground state triplet oxygen ( O2) leading to the © 2020. Thieme. All rights reserved. Synthesis 2020, 52, 1617–1624 1619 Syn thesis J. Fischer et al. Short Review 1 cycloaddition of O2 at the ipso- and para-positions requires O O –1 H H a 17–22 kJ·mol lower energy barrier than the correspond- N R 1) Me2S, Ti(Oi-Pr)4 N R CH2Cl2, RT, 1 h ing addition at the ortho and meta carbons. O O 2) TBHP, DBU O When singlet oxygen reacts with para-substituted phe- CH2Cl2, RT Me O2H Me OH nols, the in situ opening of the corresponding 1,4-endoper- 2a,b 3a (65%) oxides results in the formation of p-peroxy quinols, that 3b (66%) could then be reduced smoothly with triphenylphosphine Scheme 3 Synthesis of cis-epoxy quinols or dimethyl sulfide to give the corresponding alcohols.13 The photooxidative dearomatization of para-substituted 1 17 phenols with O2 led to highly oxygenated frameworks and oxidation of 4 with rose bengal as photosensitizer under was subsequently applied as a key reaction in numerous basic conditions followed by reduction with Me2S