Mechanistic Insights into C−S Cross-Coupling Reactions Catalyzed by Nickel Bis(phosphinite) Pincer Complexes

Abstract

Nickel bis(phosphinite) pincer complexes [2,6-(R2PO)2C6H3]NiCl (R = Ph, 1a; R = Me, 1b; R = iPr, 1c; R = tBu, 1d) show catalytic activity in cross-coupling of aryl iodides and aryl thiols. The optimal catalytic conditions involve 1 mol % of 1a and 2 equiv of KOH (with respect to aryl thiols) in DMF at 80 °C and tolerate a variety of functional groups in the substrates. The potential intermediates in these catalytic reactions, such as nickel thiolate complexes [2,6-(Ph2PO)2C6H3]NiSAr (Ar = Ph, 2a; Ar = p-MeC6H4, 3a; Ar = p-MeOC6H4, 4a), have been synthesized and spectroscopically characterized. The reaction between 2a and PhI in DMF-d7 is slow enough to argue against 2a being directly involved in the C−S bond-forming step. NMR studies suggest that the pincer ligand framework in complexes 1a, 3a, and 4a is destroyed by KOH via the cleavage of P−O bonds to release Ph2POK, and further decomposition leads to Ph3P and other phosphorus-containing products. The cross-coupling reactions are more effectively cataly...