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

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

Syn thesis J. Fischer et al. Short Review

dearomatization R2

OH O OH

EnT [2+2] cycloaddition R2 R2 = O

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 organic 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

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 was multistep reactions as these structures can be found in found as an efficient and atom economic alternative to a many natural products. In this short review, we will focus classical phenyliodine diacetate oxidation (Scheme 4). 1 our attention to the recent advances in O2-mediated dearo- matizations of para-substituted phenols. Methodology O As a first example, Kilic and co-workers prepared the OH hydroperoxides 2 after photooxygenation of the acetani- O2 RB (10 mol%) lides 1 towards their syntheses of epoxy quinol antibiotics visible light 14 OH (Scheme 2). This reaction was realized with tetraphenyl- EtOH, pH 10 buffer 15 0 °C, 3 h porphyrin (TPP) as photosensitizer and no control on ste- O TrocN then Me S, 45% 45TrocN 2 O reoselectivity was observed for this oxidation starting from Me Me a chiral substrate 1b. Me Me

Photosensitizer Methodology Cl OH O2 O H TPP (0.8–1.4 mol%) H Cl Cl N R N R visible light RB O acetone/CH2Cl2 or CHCl3 O Cl CO2Na rose bengal –25 °C or RT, 2 d I I ET = 164 kJ/mol Me O2H Me 12 λabs = 490–570 nm

Photosensitizer O O ONa I I Scheme 4 Oxidative dearomatization of 4

N TPP tetraphenylporphyrin NH HN ET = 140 kJ/mol Following these results, Klussmann and co-workers de- λabs = 300–700 nm N scribed in 2013 a sequence of peroxide formation, reduction, and intramolecular cyclization of 6 to give access to oxaspirocycles 7 (Scheme 5).18 This sequence could be exe-

Examples Methodology O O OAc H H OH O O N Me N O2 Me TPP, visible light This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. O O CHCl3, RT, 16–48 h i) reduction n = 2, 3 ii) cyclization Me O H Me O2H 2 X = Cl, CO2Me, OCOMe n O2H O 2a, 62% 2b, 70% n X X 6 7 Scheme 2 Photooxygenation of acetanilides O Application O O O The hydroperoxides 2 were then reduced to the corre- t-Bu t-Bu sponding alcohols, which underwent a Weitz–Scheffer epoxidation with tert-butyl hydroperoxide (TBHP) leading to O O the cis-epoxy quinols 3, important building blocks in Manu- O mycin natural products (Scheme 3). O O 7a (74%, one pot) 7b (50%, one pot) 7c (72%) Oxidative dearomatization was also used by Hoye and co-workers in their synthesis of the polar pharmacophoric Scheme 5 Oxaspirocycles synthesis via a photooxygenation/reduc- subunit of (+)-scyphostatin.16 The singlet-oxygen-mediated tion/cyclization sequence

Syn thesis J. Fischer et al. Short Review

cuted stepwise or more conveniently in a one-pot fashion, O without isolation of the hydroperoxide intermediates. Here OTBS O TBSO O2 again, the key oxidative dearomatization is an atom-eco- NH RB (5 mol%) O O visible light nomic alternative to hypervalent iodine reactants. Although N O PPh3, NaHCO3 O the scope is limited to 3 examples of oxaspirocyclic com- MeCN/H2O (1:1), RT 77% pounds 7, it demonstrated the potential of photosensitized O OH dearomatization of phenols to obtain more complex struc- 10 OH 11 tures. O Singlet-oxygen chemistry has been harnessed in total O TBSO NH 19 synthesis of natural and biologically relevant products. O For instance, an elegant diastereoselective visible-light-in- O duced oxidative dearomatization was observed by the HO group of Li during their synthesis of the tricyclic framework O contained in tiglianes and daphnanes (Scheme 6).20 Using TPP as photosensitizer in chloroform under a strong visible Scheme 7 Multibond-forming process light, irradiation followed by reduction of the corresponding p-peroxy quinol led to the alcohol 9 as a single diaste- cycloaddition that would ultimately render the fused furans reoisomer in a high yield (74%). The relative configuration 13 after reduction. Mechanistic studies were carried out to was established by X-ray diffraction analysis, and it was as- outline the importance of singlet oxygen in the reaction sumed that this stereoselectivity was due to the trimethyl- outcome. silyl ether moiety and its steric hindrance on one face of the phenol, leading singlet oxygen to react on the other face.

OH O Methodology O H O2 H 2 OH TPP (5 mol%),visible light PS1 (2 mol%) O R2 xenon light OH CHCl , 0 °C CDCl , 10 °C, 6 h 3 R3 3 3 2 then PPh , 74% R R TMSO 8 3 TMSO 9 then thiourea, MeOH O 16 h, 40 °C 1 Scheme 6 Oxidative dearomatization towards the synthesis of tigliane R1 12HO R 13 13 examples and daphnane diterpenes 1 O2 reduction 30–60%

O O Another diastereoselective oxidative transformation R2 R2 1O was conducted as a key step in the synthesis of erythrina al- R3 2 R3 O kaloids by the group of Kitamura and Fukuyama (Scheme O HOO R1 ABHOO R1 7).21 Here again, the steric hindrance around the phenol has 1 a strong influence on the stereoselective O2 addition. An X- Photosensitizer ray analysis confirmed that the silyloxy group is pointing Me Ph Me toward the phenol moiety. This configuration and the atro- PS1 poisomeric stereochemistry guided singlet oxygen to react I I ET = 145 kJ/mol N N λabs = 450–600 nm specifically on one face of 10 to give selectively a peroxide B Me F F Me

that led to a spontaneous reaction with the amide nitrogen This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. yielding efficiently 11 (77%) as a single diastereomer. Selected examples Inspired by these results, we reported in 2018 a one-pot O O O Bodipy-catalyzed domino photooxygenation.22 A photosensitized singlet-oxygen-mediated phenol dearomatization, Ph 4-MeC6H4 3-MeC6H4 followed by a [4+2] cycloaddition gave a variety of six- O O O membered-ring-fused furans 13 under mild conditions HO Me HO Me HO Me 13a, 53% 13b, 50% 13c, 41% (Scheme 8). The best results were obtained by using the O O O readily available Bodipy PS1 as a photosensitizer.23 Starting from 2-alkenylphenols 12, we envisioned that a first singlet Ph 2,4-Cl2C6H3 Ph oxygen molecule would react as previously described with O O Me O the more electron-rich phenol, giving the corresponding HO Ph HO Me HO Me 13d, 35% 13e, 54% 13f, 60% hydroperoxide A. A second singlet oxygen molecule could then react with these intermediates in a classical [4+2] Scheme 8 Catalyzed domino photooxygenation

Syn thesis J. Fischer et al. Short Review

Methodology The importance of functionalized epoxyquinol frameworks in natural products served to challenge synthetic or- OH O O2 24 1 ganic chemists. Previous reports have shown that phenols R4 R1 RB (1 mol%) R4 R Cs2CO3 (15 mol%) 15 were interesting substrates to get access to epoxyquinols O 17 examples green LED, MeOH 1 R3 R3 11–75% through a O2-mediated dearomatization/reduction/epoxi- RT, 3 h 2 R2 14 R OH dation sequence (Schemes 2 and 3). Nevertheless, the reduction/oxidation tactic lowers the synthetic appeal of this Selected examples Cl strategy. In order to harness the full potential of singlet ox- O ygen by transferring both oxygen atoms, we reported a one- Ph OEt pot synthesis of cis-epoxyquinols 15 starting from phenols O O O O 25 CO2Me 14 (Scheme 9). The key point for the success of the reac-

O O O O tion lies in the use of basic conditions. After optimization studies and mechanistic insights, we Me OH Me OH Me OH Et OH demonstrated that using rose bengal as photosensitizer and 15a, 50% 15b, 55% 15c, 46% 15d, 70% cesium carbonate as base led to a large scope of functional- Proposed mechanism ized epoxyquinols. Cesium carbonate plays a pivotal role by increasing the kinetic of the photooxygenation step while RB* RB O triggering the epoxidation reaction. Ph

3 1 O2 O2 2.2 [2+2] Cycloadditions

Me O2Cs B OCs O Cs A second class of reactions initiated by energy transfer is Ph Ph Cs2CO3 [2+2] cycloaddition reactions. Contrary to photoredox 14a strategies, visible-light photocatalyzed [2+2] cycloadditions – CsHCO3 C A O Me O by energy transfer are not limited by electrochemical po- Me Ph O tentials. This was demonstrated by the breakthrough work 15a of Yoon and co-workers who developed a two-catalyst O strategy for enantioselective cycloadditions of phenol de- 14a 26 D rivatives 16. Indeed, scandium(III) salts in the presence of Me OCs a chiral ligand formed a chiral Lewis acid acting as a catalyst Scheme 9 Epoxyquinol synthesis via photooxygenation under basic for inducing enantioselectivities (Scheme 10).27 A large di- conditions

Methodology Catalytic system

Ru(bpy)3(PF6)2 (2.5 mol%) R3 OH O OH O L = (S,S)-t-Bu-PyBox 2 R (15 mol%) N 3 M = Sc(OTf)3 (10 mol%) R + R N N Ru(bpy)3 R i-PrOAc/MeCN (3:1) R1 2+ E = 208 kJ/mol Ar 23 W CFL, 20 h Ar 1 R2 Ru T 17 R λ N N abs = 180–450 nm 16 12 examples, 66–86% coordination by the up to 4:1 d.r. N 83–98% ee diene chiral Lewis acid (MLn) This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited.

MLn MLn * O O O O EnT O O N R R (S,S)-t-Bu-PyBox N N Ar Ar AB t-Bu t-Bu

OH O Application: selected examples OH O OH O OH O OH O

Br Me Me Me Ph Me Ph Ph Me Me Me Me Me O 17a, 84% 17b, 86% 17c, 92% 17d, 82% 17e, 72% 93% ee, 3:1 d.r. 83% ee, 2:1 d.r. 84% ee, 3:1 d.r. 85% ee, 4:1 d.r. 93% ee, 3:1 d.r. Scheme 10 Enantioselective [2+2] cycloadditions through Lewis acid catalyzed triplet energy transfer

Syn thesis J. Fischer et al. Short Review

versity of asymmetric [2+2] cycloadditions was conducted Methodology on diversely substituted phenol derivatives 16 and dienes. Ru(bpy)3(PF6)2 OH O After mechanistic and computational investigations, OH O (5 mol%) Me Sc(OTf)3/L3-PiEt2Me (1:1, 10 mol%) they demonstrated that complexation of 16a with Sc(III) + R R i-PrCN, 5 Å MS salts results in a dramatic decrease in the energy of the Me Ar 23 W CFL, RT Ar Me Me triplet state, from ET(exp) = 54 kcal/mol to 32 kcal/mol. No 16 19 reaction occurred in the absence of any Lewis acid as well 13 examples O O 56–99% L3-PiEt2Me as in the absence of light. The formation of the chiral com- N N up to 6.1:1 d.r. Ar = 2,6-Et-4-MeC6H2 plex A involving the Sc(III) salt, the chiral PyBox ligand, and NHAr O O NHAr up to >92% ee the two oxygens of the 2′-hydroxychalcone 16 is probably Selected examples the key intermediate, leading the [2+2] cycloaddition to OH O OH O proceed in an enantioselective way. OH O Very recently, Chen, Shen, and co-workers carried out a Me Me detailed computational study on the mechanism of this Me Me F 28 Ph Me Me transformation. Among many other data, they explained Me F the origin of chirality in this reaction by energy changes in 19a, 88% 19b, 93% 19c, 85% the triplet state depending of the attacked side of the coor- 84% ee, 4.4:1 d.r. 88% ee, 4.8:1 d.r. 37% ee, 5:1 d.r. dinated chalcone. Scheme 12 [2+2] Photocycloaddition mediated by N,N′-dioxide-metal In a further study, the group of Yoon extended the scope complexes of the reaction to various styrenes and diene systems, ob- taining high enantiomeric excesses and giving a direct access to diarylcyclobutane natural products (Scheme 11).29 Reaction conditions are quite similar but tetradentate Following Yoon’s pioneering works, Feng, Liu and co-work- N,N′-dioxide platforms were used to form a chiral cluster ers also reported an enantioselective [2+2] cycloaddition with Sc(III). Results are comparable to those obtained by involving 2′-hydroxychalcones 16 (Scheme 12).30 Yoon in term of yields and enantiomeric excesses, but are generally better in term of diastereoselectivities, with ratios Methodology up to 6.1:1. Ru(bpy)3(PF6)2 (2.5 mol%) OH O OH O (S,S)-t-Bu-PyBox (15 mol%) Sc(OTf)3 (10 mol%) + R 3 Intramolecular Transformations R i-PrOAc/MeCN (3:1) R1 1 Ar 23 W CFL, 20 h Ar R 16 18 Although limited, intramolecular visible-light-photo- 32 examples, 18–97% up to >10:1 d.r. catalyzed transformations of phenols by energy transfer up to >99% ee have already been reported. As highlighted in the previous Application: selected examples section, alkenes are prone to undergo [2+2] cycloadditions OH O OH O by energy transfer catalysis. In 2012, the group of Yoon ex- plored the [2+2] photocycloaddition of alkenes 20 performed in the presence of 1 mol% of PS2 under visible light Ph 31 Ph Ph (Scheme 13, top). This transformation was applied to the 18a, 88% 18b, 87% expedient synthesis of racemic cannabiorcicyclolic acid, >99% ee, 2:1 d.r. 93% ee, 2:1 d.r. B(pin)

OH O which is a natural product belonging to the family of canna- This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. binoids (Scheme 13, down). Condensation of the phenol 22 OH O and citral 23 gave rise to the cycloadduct precursor 24 in

Ph 54% yield. The photocycloaddition was performed in the presence of 1 mol% of PS2 in DMSO under visible light. After Ph SPh F3C 8 hours of irradiation, the desired cyclobutane-containing 18c, 86% 18d, 93% compound 25 was isolated in 86% yield and subsequent hy- 93% ee, 1:1 d.r. 96% ee, 2:1 d.r. drolysis of the ester function afforded the natural product OH O OH O in 97% yield. In previous examples, the energy transfer took place be-

Ph tween the reactants and the excited photosensitizers. The Me Ph Ph Cl concept of energy transfer catalysis has been also applied to O 18e, 42% 18f, 84% transition metal complexes providing a wealth of untapped 94% ee, 2:1 d.r. 91% ee, 2:1 d.r. synthetic potential in organometallic chemistry.32 This Scheme 11 Photocycloadditions of olefins

Syn thesis J. Fischer et al. Short Review

Methodology (4-CF3C6H4)3PAuCl PS2 (1 mol%) (5 mol%) Ar R1 Ar R1 visible light PS2 (1 mol%) X 1,10-phenanthroline O DMSO OH I 2 R2 + (10 mol%) R 2 X R 1 1 K2CO3 R 20 21 R 3 R (2.5 equiv) 27 X = O, NTs, CH , -(CH ) - 23 examples 2 2 2 2 R MeCN, blue LED 18 examples 64–90% 26 RT, overnight 25–83% 3 Photosensitizer R 2 C(sp )–C(sp) 3 – cyclization R I CF3 PF6 coupling F t-Bu PS2 * N O O Ir(dF(CF )ppy) (dtbbpy)·PF N 3 2 6 R2 R2 IrIII F + ET = 255 kJ/mol Ir R1 R1 λmax = 389 nm F I energy transfer catalysis I N Au -L Au -L N AB t-Bu F Scheme 14 Merging electrophilic gold catalysis and photosensitiza- CF3 tion to prepare alkynylbenzofurans Application OH

EtO2C 22 intermediate for triggering the oxidative addition. Gold(I) OH Me complexes are rather reluctant to oxidative addition and Ca(OH) EtO C Me OH 2 2 140 °C Me + therefore, specific reactions conditions are generally re- 54% OHC Me O 24 quired to provide the corresponding gold(III) intermediates. Me Me Given that triplet sensitization can promote oxidative addi- Me Me PS2 (1 mol%) tion under convenient conditions, this strategy should offer 23 visible light 86% DMSO, 8 h new synthetic opportunities in the near future. OH Me Me OH Me Me H H H H HO2C LiOH EtO2C 60 °C 4 Conclusions and Outlook 97% Me O Me O Me Me cannabiorcicycloic acid 25 This review has brought to light recent advances in visi- Scheme 13 Synthesis of cannabiorcicycloic acid using a [2+2] photo- ble-light-mediated transformation of phenols by energy cycloaddition as the key step transfer catalysis. The unique properties of phenols enabled their transformation into synthetically interesting direc- strategy has been harnessed by Mouriès-Mansuy, Ollivier, tions. The hydroxyl group directly attached to the aromatic Fensterbank, and co-workers for the Au(I)-catalyzed forma- ring makes phenols electron rich compounds and this prop- tion of alkynylbenzofuran derivatives 27 (Scheme 14).33 erty was harnessed in dearomatization reactions by means The combination of electrophilic gold catalysis and photo- of singlet oxygen. The hydroxyl group was also involved in sensitization by means of PS2 allowed the formation of the O-cyclization reactions and in chelate complexes as key in- decorated benzofurans 27 starting from phenols 26 and termediates to deliver [2+2] photocyclized adducts with ex- alkynyl iodide partners. The optimized reaction conditions cellent selectivities. As energy transfer is not limited by the were applied to a series of substrates and moderate-to-ex- redox properties of the substrates, energy transfer catalysis

cellent yields of the desired alkynylbenzofurans were ob- has a bright future in organic synthesis and therefore in This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. tained. While a wide substrate scope was observed, no re- phenol chemistry. For instance, the design of new catalytic action occurred with an iodoalkyne bearing a 4-nitroaryl systems is expected to tackle the challenge of asymmetric group and the protodeauration cyclization product was photooxygenation of aromatic compounds such as phenols formed. This review aimed at collecting examples of visible-light- Detailed mechanistic investigations were carried out to driven transformations of phenols via energy transfer catal- shed light on the formation of 27 by performing control ex- ysis and in the near future, we hope that new examples will periments, photophysical analyses and modelling studies. show off the synthetic potential of this strategy. The reaction sequence would start by the 5-endo-dig gold- catalyzed O-cyclization of 26 to afford the vinylgold(I) spe- Funding Information cies A. A Dexter-type energy transfer between this interme- 3 diate and the long-lived triplet state [Ir-F] would occur to This work was supported by The Région Pays de la Loire (NANO2 proj- afford the excited state of the vinylgold(I) species B, a key ect) which financed a Ph.D. grant for J.F. We also thank University of Nantes and CNRS for financial support.Conseil Régiona ldes Pays de la Loire (NANO2)Université de Nantes ()Centre Nationa lde la Recherche Scientifique ()

Syn thesis J. Fischer et al. Short Review

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