Abstract
Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C-O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C-O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy3-catalyzed silylation of aryl pivalates with CuF2/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C-O bond cleavage and their relevance in C-Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C-O functionalization reactions employing monodentate phosphines.
Highlights
Prompted by the broad accessibility, thermal stability, and cost efficiency of phenol derivatives, C−O electrophiles have recently gained momentum as alternatives to aryl halides in cross-coupling reactions.[1]
Despite the advances realized in Ni-catalyzed cross-coupling reactions of aryl esters,[1] these processes are poorly understood in mechanistic terms, and progress in the field is typically based on empirical discoveries.[2]
Scheme 4. 1H NMR Evidence for Ni−Naphthyl Bonding posed no problems, which contrasts with a number of C−O cleavage protocols that are limited to extended π systems.[12,13]
Summary
It becomes apparent that the transmetalation process with the Si−B species is turnover-limiting (II-TSIII−IV = 22.4 kcal mol−1) This observation is consistent with the zero-order dependence in 1a and the positive-order dependence in Et3SiBPin.[24] Our experimental data, showed that the mononickel oxidative addition complex is in equilibrium with dinickel 2a. The thermodynamic stability of 2a compared with the mononickel complex was corroborated by calculations, with the equilibrium Gibbs free energy found to be −10.6 kcal mol−1 in favor of 2a (Scheme 14) This result reinforces the role of 2a as an off-cycle species and is consistent with the experimental half-order dependence on [Ni].
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