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

Abstract

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-η(2):η(2)-C6H6)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η(2)-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl-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 η(2)-carbonyl complex, phenyl esters were found to predominantly undergo Caryl-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 Caryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.

The Importance of Ligand‐Induced Backdonation in the Stabilization of Square Planar d10 Nickel π‐Complexes

“…DFT calculations were performed on each of the species in Table using simplified diphosphine ligands (see Figure SI 07 for details, Supporting Information). Molecular structures derived from B3LYP/def2‐TZVP calculations of the dtbpe complexes, as well as those using a simplified diphosphine ligand (dmpe=1,2‐bis(dimethylphosphino) ethane), yield good agreement with solid‐state molecular structures of 1 – 12 . The effect of decreasing the steric bulk and electron donation in the supporting diphosphine ligand does not affect the general structural trends and conclusions, which are consistent with those observed in the experimental data.…”

“…In previous work, we noted that the 31 P{ 1 H} NMR spectroscopic data of a number of (dtbpe)Ni [dtbpe=1,2‐bis(di‐ tert ‐butyl)phosphinoethane] π‐complexes were consistent with typical d 10 Ni 0 complexes (Table ) . In contrast, we also noted that the distorted square planar geometry with significant elongation of the π‐bond was most consistent with a d 8 Ni II formulation, in keeping with the metallaepoxide extreme of the Dewar–Chatt–Duncanson (DCD) model of bonding (Scheme ).…”

“…A common approach for evaluating the oxidation state of metals in diphosphine complexes involves using the magnitude of P,P‐scalar coupling constants ( 2 J P,P ). In nickel chemistry, it is generally observed that small 2 J P,P (i.e., 2–30 Hz) correspond to nickel(II) complexes, whereas larger 2 J P,P (i.e., 45–80 Hz) correspond to nickel(0) complexes . However, exceptions to this trend have been reported by ourselves and others .…”

“…Complex 1 does not react with MeLi, even upon prolonged reflux in benzene, but under the same conditions, complex 2 reacts with MeLi to form trace amounts of EtSSEt. In addition, complex 2 is susceptible to cross‐coupling with phenylboronic acid, forming PhSEt in moderate (35 %) yield . No such cross‐coupling reactivity was observed with complex 1 .…”

“…Ni(COD) 2 ( 13 ) was purchased from Strem and used as received. Compounds 1 – 2 , 3 , 4 , 5 – 6 , 7 , 8 – 10 , 11 , 12 , 14 , and 15 were prepared according to literature procedures. All other chemicals were purchased from commercial suppliers and used as received.…”