How and Why Does Ni⁰ Promote Smooth Etheric CO Bond Cleavage and CC Bond Formation? A Theoretical Study

Abstract

Ni-catalyzed cross-coupling between aryl alkyl ethers (ArOR) and Grignard reagents (RMgBr), known since 1979, proceeds under mild conditions in many cases. Although the reaction routes of various synthetic protocols involving transition-metal-catalyzed C-O bond activation have been elucidated, the mechanism of this etheric Kumada-Tamao-Curriu reaction remains enigmatic. This is because oxidative addition of inert etheric C-O to Ni(0) is thermodynamically and kinetically unfavorable, making it hard to explain the observed high reactivity of ether toward Ni catalysts. In this work, we used DFT calculations to identify a plausible reaction pathway by the Ni(0)-ate complex, which enables smooth C-O bond cleavage and R-group transfer with reasonable activation barriers; this mechanism also accounts for the ineffectiveness of Pd catalysts. These results throw new light on both C-O activation and cross-coupling, and should be valuable for further rational development of the methodologies.

Ethereal C–O Bond Cleavage Mediated by Ni(0)-Ate Complex: A DFT Study

“…46) Computational studies by Nakamura and colleagues, 55) Liu and colleagues, 56) Martin and colleagues, 46) and us 14) clearly demonstrated that OA of a E C-O bond generally involves a very high activation barrier (Chart 2(1)), even considering the Lewis acidity of Mg (Chart 2(2)). This could be attributed to the high stability of the E C-O bond and the weak interaction between the E C-O moiety and Ni(0) species.…”

“…Considering the nucleophilicity of Ni (0) Cornella and Martin, 19) and our group 14) have suggested that, instead of the neutral Ni(0) or anionic ] − species, the [C-Ni(0)]-ate species [57][58][59][60] is involved. In 2015, we reported the first systematic mechanistic study on the Ni-catalyzed…”

“…Single point energy considering the solvent effect of i Pr 2 O (Self-Consistent Reaction Field (SCRF) method using the Polarizable Continuum Model (PCM) model [71][72][73] ) was obtained via calculation of the B3LYP geometries with M06/6-311++G** (H, Li, C, O, P, Li) & SDD (Ni, Zn). Gibbs free energy (kcal/mol) was calculated based on M06 single point energy and B3LYP frequency 74,75) Ni(PCy 3 ) 2 was chosen as the active catalyst species based on previous reports, 14,46,55,56) from which the initial geometry of PCy 3 was also taken. NBO analysis was performed with NBO5.9.…”

“…These findings well explain why many reactions that involve breaking E C-O bonds, such as Suzuki-Miyaura coupling, amination, reduction, and so on, need vigorous heating. However, this understanding seems incomplete, because other Ni-catalyzed etheric cross-couplings, such as Kumada-Tamao-type, Negishi-type, and Murahashi-type reactions, take place under mild conditions in many cases.Considering the nucleophilicity of Ni (0) Cornella and Martin, 19) and our group 14) have suggested that, instead of the neutral Ni(0) or anionic ] − species, the [C-Ni(0)]-ate species [57][58][59][60] is involved. In 2015, we reported the first systematic mechanistic study on the Ni-catalyzed…”

“…As early as in 1979, Wenkert et al 9,10) reported the first Ni-catalyzed Kumada-Tamao type (Mg) [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] reaction, in which aryl methyl ether (ArOMe) acted as the electrophilic partner, and this is now recognized as the first example of Ni-catalyzed activation of an inert C-O bond. However, this breakthrough was overlooked for decades, until quite recently.…”