Efficient Aryl Migration from an Aryl Ether to a Carboxylic Acid Group To Form an Ester by Visible‐Light Photoredox Catalysis

Abstract

We have developed ahighly efficient aryl migration from an aryl ether to ac arboxylic acid group through retro-Smiles rearrangement by visible-light photoredoxc atalysis at ambient temperature.T ransition metals and as toichiometric oxidant and base are avoided in the transformation. Inspired by the high efficiency of this transformation and the fundamental importance of C À Ob ond cleavage,w ed eveloped anovel approachtothe C À Ocleavage of abiaryl ether to form two phenolic compounds,a sd emonstrated by ao ne-pot, twostep gram-scale reaction under mild conditions.T he aryl migration exhibits broad scope and can be applied to the synthesis of pharmaceutical compounds,s uch as guacetisal. Primary mechanistic studies indicate that the catalytic cycle occurs by ar eductive quenchingpathway.Various aryl-migration reactions have been developed and display broad application. [1] Notably,a ryl migration from an aryl ether to form an ester is rare.In1955, this migration was first observed upon the thermal decomposition of 2-phenoxybenzoyl peroxide in 25 %y ield by DeTar and Hlynsky. [2] Recently,t he migration was promoted by silver catalysis at 130 8 8CbyHossian and Jana, who used acarboxylic acid as the starting material. [3] However,t he stoichiometric strong oxidant K 2 S 2 O 8 and ab ase were required. Furthermore,t he transformation only resulted in up to 64 %y ield. No clear electronic effects on this aryl migration were observed. Thus, the transformation was identified as aSmiles rearrangement. Herein, we demonstrate ahighly efficient aryl migration from an aryl ether to form the ester of acarboxylic acid by visiblelight photoredox catalysis at ambient temperature without an oxidant [Scheme 1, Eq. (1)].O nly 1mol %o faphotosensitizer (PDI or Acr + -Mes ClO 4 À )a nd 10 mol %o fabase (K 2 HPO 4 )w ere required. Notably,e lectron-donating groups on the migrating aryl ring generally induced higher yields in this transformation, which differs from the typical Smiles rearrangement [4] and can be understood as ar etro-Smiles rearrangement. [4a,5] As this developed aryl migration is the retro-reaction of the reported Smiles rearrangement involving aryl migration from an ester to form an aryl ether. [6] And the Smiles rearrangement always requires ah eteroatom as the nucleophilic component and an electrophilic arene activated by strong electron-withdrawing groups,acarbonbased nucleophile,orfree-radical attack on the ipso-position of sulfonates or sulfonamides. [4] By taking advantage of visible-light photoredox catalysis,transition metals and astoichiometric amount of an oxidant and abase are avoided, thus leading to an environmentally benign and highly atom economical transformation. Moreover,i nspired by the high efficiency of this aryl migration and the fundamental importance of CÀOb ond cleavage, [4i, 7] we demonstrate an ovel approach to CÀOb ond cleavage in biaryl ethers to give two phenolic compounds in ao ne-pot, two-step reaction under mild conditions as as ubsequent transformation [Scheme 1, Eq. (2)].Visible...

Deoxygenative Arylation of Carboxylic Acids by Aryl Migration

“…In the last five years, visible-light-mediated radical Smiles aryl rearrangement has been recognizeda sapowerful structural reversal tactic to construct new CÀCb onds (Scheme 1b). [7] Althought he radicalS miles rearrangement [8] through af ivemembered transition-state has been recently established for 1,4-arylm igration, [9] photoredox1 ,5-aryl migrationl eading to CÀCb ond formation [10] has been rarely developed possibly due to the higher energy barrier that is involved. We questioned if an unprecedented deoxygenative arylation protocol by photoredox-catalyzed 1,5-arylm igration could be ar eliable methodt of urnish versatile o-aminobenzophenones as well as the synthetically useful o-hydroxybenzophenones( Scheme 1c).…”

Perylene Diimide Based Imine Cages for Inclusion of Aromatic Guest Molecules and Visible‐Light Photocatalysis

“…Thus it may act as noble metal‐free candidates for photocatalysis in visible‐light‐driven reactions. In order to examine its photocatalytic activity visible‐light‐driven Smiles rearrangement of 2‐aryloxybenzoic acids to aryl salicylates was chosen (Scheme ) . 2‐Phenoxybenzoic acid was first employed as a model substrate to optimize the reaction conditions (Table S1).…”

“…2‐Phenoxybenzoic acid was first employed as a model substrate to optimize the reaction conditions (Table S1). The binding constant K sv of 2‐phenoxybenzoic acid with ( RRRR )‐PDI‐TFBE is calculated to be 1530 L mol −1 according to the Stern‐Volmer analysis by conducting the titration experiment of ( RRRR )‐PDI‐TFBE in MeOH (1.25×10 −7 mol L −1 ) with 2‐phenoxybenzoic acid (0.003 mol L −1 ) (Figure S49) . The best photocatalytic results were obtained using MeCN as solvent (20 mL used), substrate (0.1 mmol), 0.5 mol % imine cage catalyst, 15 mol % K 2 HPO 4 as base and irradiation with blue LEDs ( λ max =453 nm, 15 W) at ambient temperature.…”

“…The reduction potential of E (PDI‐TFBE/PDI‐TFBE − ) was calculated to be −0.40 V vs. SCE in CH 3 CN (Figure S50), and the oxidation potential of E (PhOPhCO 2 • /PhOPhCO 2 − ) was quoted as +1.90 V vs. SCE in CH 3 CN according to the literature, while the excited state energy E 00 was 2.37 eV determined from the cross‐point of the absorption and fluorescence spectra of ( RRRR )‐PDI‐TFBE at 524 nm (Figure S51). According to the Rehm‐Weller equation, the free energy of electron transfer Δ G ET = E (PhOPhCO 2 • /PhOPhCO 2 − )‐ E (PDI‐TFBE/PDI‐TFBE − )‐ E 00 =−0.07 eV.…”

“…The results show that the formation of ( RRRR )‐PDI‐TFBE − and the carboxylic acid radical of the substrates is thermodynamically feasible via single‐electron transfer (SET) between ( RRRR )‐PDI‐TFBE* and the carboxylic acid anion of the substrates. Based on the results and the literature, the catalytic mechanism is proposed in Scheme . As indicated, the substituents on the substrate may remarkably change the catalyst‐substrate (host‐guest) interaction, thus causing different catalytic activity.…”