Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex

Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}₂(μ-η²:η²-C₆H₆)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η²-carbonyl complexes. In contrast, acetylthioesters underwent rapid C_acyl-S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η²-carbonyl complex, phenyl esters were found to predominantly undergo C_aryl-O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean C_aryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive. The reactivity of a low-valent nickel complex with esters and thioesters is reported. Trifluoromethylthioesters were found to form η²-carbonyl complexes, while acetylthioesters were found to undergo further C_acyl-S oxidative addition and decarbonylation to generate methylnickel complexes. In contrast, the same nickel precursor was found to induce C_aryl-O bond cleavage in aryl esters to generate arylnickel acetate complexes (see scheme).