Achieving Nickel Catalyzed C–S Cross-Coupling under Mild Conditions Using Metal–Ligand Cooperativity

Abstract

A simple and efficient approach of C–S cross-coupling of a wide variety of (hetero)­aryl thiols and (hetero)­aryl halides under mild conditions, mostly at room temperature, catalyzed by well-defined singlet diradical Ni­(II) catalysts bearing redox noninnocent ligands is reported. Taking advantage of ligand centered redox events, the high-energetic Ni(0)/Ni­(II) or Ni­(I)/Ni­(III) redox steps were avoided in the catalytic cycle. The cooperative participation of both nickel and the coordinated ligands during oxidative addition/reductive elimination steps allowed us to perform the catalytic reactions under mild conditions.

C−N Cross‐Coupling Reactions Under Mild Conditions Using Singlet Di‐Radical Nickel(II)‐Complexes as Catalyst: N‐Arylation and Quinazoline Synthesis

“…Recently, we reported CÀ S cross-coupling reactions of a broad range of aryl halides and thiols under mild reaction conditions catalyzed by singlet di-radical nickel(II)-complexes [Ni II {(L 1À 3 ) *À } 2 ] (1-3) featuring redox-active diamine based ligands (Scheme 1). [16,17] In this work we demonstrated that the energetically demanding nickel-centered redox processes can be averted via cooperative involvement of both nickel and ligand-centered redox processes during catalytic turnover even in absence of the expensive photosensitizers. DFT studies indicated that during oxidative addition (of aryl halides) and reductive elimination nickel and one of the di-amine ligands in catalyst 3 undergo single electron oxidation and reduction respectively, and thus avoid the energetically unfavorable nickel(II)/nickel(IV) (during oxidative addition) and nickel(II)/nickel(0) (during reductive elimination) oxidation states.…”

“…[2] To obtain the optimal reaction parameters for the N-arylation reactions, we studied the reaction of 1H-pyrazole (4 a) and iodobenzene (5 a) under different reaction conditions using the nickel(II)complexes, [Ni II {(L 1 ) *À } 2 ](1), [Ni II {(L 2 ) *À } 2 ](2), and [Ni II {(L 3 ) *À } 2 ](3) as catalysts ( Table 1). [16,17] Optimiza- (3) as the catalyst revealed that the highest yield (82%) of 1-phenyl-1H-pyrazole (6 a) was obtained when the reaction was carried out in an CH 3 CN/DMF (5:1) solvent mixture at room temperature in presence of 8.0 mol% of the catalyst 3 and 1.40 equiv. of NaO t Bu as base (Table 1, entry 6).…”

“…In order to expand the scope of our hypothesis of avoiding energetically unfavorable nickel-centered redox states via synergistic involvement of both ligandand nickel-centered redox events, we decided to explore the possibility of CÀ N cross-coupling reactions using the nickel(II)-complexes [Ni II {(L 1À 3 ) *À } 2 ] (1-3) as catalysts (Scheme 1). [16,17] To our delight, catalyst 3 showed promising activity in the N-arylation of a diverse range of nitrogen nucleophiles with aryl halides under mild reaction conditions. Moreover, our synthetic protocol has also been found suitable for the synthesis of six member N-heterocycles (quinazolines) via CÀ N cross-coupling of benzamides and amidines with 2-bromobenzylamine and 2-bromobenzylbromide respectively, under relatively mild reaction conditions.…”

“…We envisioned that nickel-catalyzed CÀ N cross-coupling followed by intramolecular cyclization would afford the desired quinazolines. [17,18] To our delight, the reaction of benzamide (8 a) and 2-bromobenzylamine (9 a) under the pre-optimized conditions (Table S1, entry 3) afforded 2-phenylquinazoline (10 a) in 34% yield. After further optimization of the reaction conditions, the highest yield (71%) of 10 a was obtained at 70°C in DMF with 8.0 mol% catalyst and 0.6 equivalents NaO t Bu loading (see supplementary material for details).…”

Nickel‐Catalyzed Synthesis of Pyrimidines via Dehydrogenative Functionalization of Alcohols

“…For catalyst 1 a inert condition and slightly higher temperature is required to get the products in satisfactory yields while the catalyst 2 a affords the respective pyrimidines in even higher yields under comparatively mild aerobic conditions. Moreover, the metal‐ligand cooperativity and active involvement of ligand centered radical in 2 a allow to achieve pyrimidines in good yields even starting from unactivated alcohols bearing strongly electron‐withdrawing substituents. Overall, the metal‐ligand cooperative approach via active involvement of the ligand centered redox process during catalysis allow to avoid the energetically demanding high energy nickel centered redox processes and allow to achieve the reactions under mild and aerobic conditions.…”