Intermediacy of Ni–Ni Species in sp² C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation

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)/PCy-catalyzed silylation of aryl pivalates with CuF/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.

Base‐Mediated Defluorosilylation of C(sp2)−F and C(sp3)−F Bonds

“…Even methoxy residues ( 2 e , 2 j , 2 l ) and nitrogen‐containing heterocycles did not interfere with productive C−Si bond formation ( 2 k , 2 m and 2 n ). These results are particularly noteworthy, as they demonstrate orthogonal reactivity to recent silylation of C−OMe bonds and C−Si bond formations with electron‐rich heterocycles…”

“…Gratifyingly, our LiHMDS‐mediated protocol could be applied to C(sp 3 )−F scission under otherwise identical reaction conditions to those shown in Scheme , invariably affording 4 a – h in good to excellent yields (Scheme ). Nitrogen‐containing heterocycles ( 4 e – g ) and methoxy groups ( 4 a , 4 d ) were perfectly be accommodated . Of particular relevance is the successful preparation of 4 c featuring two C−F bonds possessing different hybridized carbon centers .…”

Visual kinetic analysis

“…VTNA has already been used successfully by academic and industrial research groups in metal-catalysed [25][26][27][28][29][30][31][32][33][34][35][36][37][38] and organocatalytic reactions. [39][40][41][42][43]…”

Evidence for Photocatalyst Involvement in Oxidative Additions of Nickel-Catalyzed Carboxylate O-Arylations

“…Although further studies are required to account for this behavior, it is possible that metal catalyst above a certain threshold engages in some combination or comproportionation, disproportionation, and/or higher-order nickel species known to form in the presence of carboxylic acids. 33 The area at very low concentration of nickel likely would provide a regime in which the nickel catalytic cycle was not limited by access to an excited photocatalytic species. To confirm this was the case, at low concentrations we used Variable Time Normalization Analysis (VTNA) 34 to find a first-order dependence on nickel between 0.15-0.25 mM ( Figure S11).…”