Visible light driven, nickel-catalyzed aryl esterification using a triplet photosensitiser thioxanthen-9-one

Abstract

The nickel-catalyzed esterification of carboxylic acids with aryl bromides using thioxanthen-9-one as a photosensitizer provided aryl esters with excellent yields.

General Paradigm in Photoredox Nickel‐Catalyzed Cross‐Coupling Allows for Light‐Free Access to Reactivity

“…The success of aryl esterification is noteworthy given the diminished nucleophilicity of carboxylates relative to other heteroatomic cross‐coupling substrates, as demonstrated by the requirement for super‐stoichiometric amounts of silver salts in a reported Pd‐catalyzed methodology for the esterification of aryl iodides, and a scarcity of examples for the direct cross‐coupling of carboxylic acids with aryl bromides under thermal nickel catalysis. Moreover, reports of this transformation under photochemical conditions unanimously invoke photosensitization pathways that necessitate access to a Ni II excited state as a precondition for reductive elimination, whereas our reactions in the absence of light preclude any possibility of excited‐state formation. These results suggest a Ni I/III pathway as an alternative, and hitherto unexplored, mechanistic strategy for achieving this transformation.…”

“…Our results show that Ni‐catalyzed amination, etherification, and esterification of aryl bromides can all be realized across a wide range of substrates without light under conditions that otherwise bear resemblance to the parent photochemical methodologies. To this end, one may wish to re‐evaluate the growing body of literature that invokes energy transfer as the mechanism for catalysis . As we show here, only a small amount of Ni I can initiate self‐sustained Ni I /Ni III thermal catalysis.…”

“…Furthermore, the majority of photocatalysts used are based on precious metals, such as iridium and ruthenium, and the reaction mechanisms, an understanding of which is critical for rational optimization towards more energy‐efficient and sustainable methodologies, remain largely undefined. Previous work by our group demonstrated that, in contrast to the closed photocycles commonly proposed for photoredox cross‐coupling reactions (Scheme , A and B), a Ni I/III cycle (Scheme , C) may be operative in the reaction between aryl bromides and alcohols to form O ‐aryl ethers, where the photon serves to resuscitate the cycle once the catalytically active Ni I or Ni III species are depleted to form inactive Ni II complexes via a highly exergonic comproportionation reaction, akin to what had been proposed for the related electrochemical amination of aryl bromides . Given the various mechanistic possibilities proposed for photoredox Ni cross‐coupling, it was unclear if similar dark cycles were operative in other reactions.…”

Carbonyl-Photoredox/Metal Dual Catalysis: Applications in Organic Synthesis

“…The first report of this chemistry was in 1976 by Jones and co-workers who discovered the dehydrogenation of alkanes using benzophenone (P1) in conjunction with Cu(OPiv) 2 . 22 This study was followed by a 40 year hiatus until reports began emerging of dual photocatalytic systems that could mediate the homocoupling of aryl halides, 23 arylation and alkylation of C(sp 3 )-H bonds, 24 carboxylation of allylic, benzylic, and aliphatic bonds with CO 2 , 25 acylation of benzylic C(sp 3 )-H bonds with acid anhydride or chlorides, 26 hydroalkylation of olefins, 27 hydroalkylation of imines, 28 arylation of aromatic aldehydes, 29 aryl esterification, 30 and enantioselective oxidation of -keto esters. 31 These recent discoveries are reviewed herein, categorized by reaction X = Br, Y = Me, 70% X = Cl, Y = Me, 14% X = I, Y = Me, 72% X = Br, Y = OMe, 67% X = Br, Y = tBu, 74% X = Br, Y = CN, 52% X = Br, Y = CO 2 Me, 62% X = Br, Y = CF 3 , 84% a X = Br, Y = Ph, 79% X = Br, Y = Bpin, 67% under these reaction conditions, giving the desired product in 67% yield.…”

“…In photoredox cycle (Cycle I), P2 is excited by visible-light irradiation to excited state P2* (E = 1.28 V vs saturated calomel electrode, SCE), 30 which is reduced by iPr 2 NEt (E = 0.68 V vs SCE) 32 to ketyl radical anion P2 •-. The radical anion P2 •is a strong reductant (E 1/2 = -1.70 V vs SCE) 30 and reduces (dtbbpy)Ni II Br 2 (1) (E(Ni II /Ni 0 ) = -1.20 V vs SCE) 23,33 to (dtbbpy)Ni 0 2. The oxidative addition of aryl halide to 2 generates (dtbbpy)Ni II ArBr 3, which is located at the intersection of Cycle II and Cycle III.…”