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

Abstract

Self-sustained Ni I/III cycles are established as ap otentially general paradigm in photoredox Ni-catalyzed carbonheteroatom cross-coupling reactions through as trategy that allows us to recapitulate photoredox-like reactivity in the absence of light across aw ide range of substrates in the amination, etherification, and esterification of aryl bromides, the latter of whichh as remained, hitherto,e lusive under thermal Ni catalysis.M oreover,t he accessibility of esterification in the absence of light is especially notable because previous mechanistic studies on this transformation under photoredox conditions have unanimously invoked energytransfer-mediated pathways.

Evidence for Photocatalyst Involvement in Oxidative Additions of Nickel-Catalyzed Carboxylate O-Arylations

“…Specifically, we observed an unexpected “peak” where catalysis has a much higher rate, around 1 mM. Although further studies are required to account for this behavior, it is possible that, above a certain threshold, the metal catalyst engages in some combination of comproportionation 35 , 36 and/or higher-order nickel species thought to be catalytically relevant. 37 , 38 …”

Light‐Promoted Nickel Catalysis: Etherification of Aryl Electrophiles with Alcohols Catalyzed by a NiII‐Aryl Complex

“…[15] The observations above suggest that the etherification in question involves Ni I species and light is required throughout to achieve high product yields. On the basis of these results and the studies of Doyle, Scholes, and Nocera et al, [14,19] a simplified mechanistic pathway is tentatively suggested for the etherification under question (Scheme 6). Irradiation of A at 390-395 nm would excite the Ni II complex to a 3 MLCT state [21] that probably decays to a long-lived 3 d-d state, which could then undergo homolysis of the nickel À carbon bond to generate Ni I and aryl radicals.…”

“…Notably, for aryl chloride bearing a chloro substituent, the mono-etherified product (10) was dominant, thus providing the possibility for subsequent transformation. Of further interest is that a range of (hetero)aryl chlorides were compatible, delivering the desired products containing quinolines (12)(13)(14)(15), pyridine (16,17), benzothiophene (18), benzoxazole (19), and benzothiazole (20), which are important building blocks in drug molecules, with good to excellent yields. As shown in Scheme 2, various alcohols can be used.…”

“…As shown in Scheme 5, when complex A was irradiated at 390-395 nm for 1 h without substrates, significant amounts of arenes were observed, thus indicating concomitant formation of Ni I (Scheme 5 A). [14d] When the in situ prepared Ni 0 complex Ni(dtbbpy)(cod) [19] was used as the catalyst under the standard conditions, the reaction proceeded with high yields; however, when the light was omitted, the desired product was not observed (Scheme 5 B), thus highlighting that light is required to enable the C À O coupling, regardless of whether the pre-catalyst is Ni II or Ni 0 . [15] Next, the Ni II complex A was irradiated under long-wave UV light for 30 min in the absence of substrates and then the irradiation was stopped.…”

“…= 2.07 (SI, Figure 8), [15] which indicates the formation of a d 9 Ni I species. [19,20] Moreover, when the Ni I complex, prepared in situ through the comproportionation of catalyst A and Ni(dtbbpy)(cod) by following the procedure of Nocera and co-workers, [19] was used as catalyst, the desired product was obtained only in 16 % yield under thermal conditions (Scheme 5 D). The EPR signal of the resulting Ni species is similar to that of the irradiated Ni II catalyst A ( Figure S9 in the Supporting Information).…”