Cobalt Pincer Complexes in Catalytic C–H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

Abstract

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-(iPrPNP)CoH2(BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(iPrPNP)Co(N2)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(iPrPNP)CoBPin (R=H, BPin), were generated by reductive elimination of H2 in the former, through Berry pseudorotation to the cis isomer, and N2 loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C-H oxidative addition of the aromatic substrate (C6H6), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C6H6 is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and re-association of HBPin. Alternative pathways, σ-bond metathesis and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond, are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B2Pin2 and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal-ligand cooperation based on the ligand’s aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand, do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition-row metals that differ from those in heavier metal complexes.

Mechanistic details of the cobalt-mediated dehydrogenative dimerization of aminoquinoline-directed benzamides

“…This work adds to and compliments a growing number of mechanistic and theoretical studies exploring cobalt-mediated organic transformations that have highlighted multiple challenges involved in computational investigations of Cocomplexes, including lower-lying multiple electronic and spin states and the important role of dispersive interactions. [18][19][20][21][22][23][24] Results and discussion…”

Mechanistic Origins of Regioselectivity in Cobalt-Catalyzed C(sp2)-H Borylation of Benzoate Esters and Arylboronate Esters

“…It is worthwhile to note that, in a comprehensive computational study concerning the mechanism of the C(sp 2 )-H borylation of benzene by ( iPr PNP)Co complexes, DFT calculations using the uB97XD functional predict oxidative addition of HBPin to the Co(I)aryl to be the turnover-limiting step for this substrate. 55 Moreover, the subsequent reductive elimination of arylboronate product from the resulting complex was found to occur with a kinetic barrier of 14.8 kcal/mol compared with a barrier of 17.2 kcal/ mol required for the slow step. It is anticipated that the relative barriers to these two slowest steps of the catalytic cycle may change as a function of the arene substituents and substitution pattern, and these variations can, in turn, result in a change in the turnover-limiting step.…”

“…Given all of the available experimental data, a mechanism for the 1-(H) 2 BPin-catalyzed C(sp 2 )-H borylation of benzoate esters is proposed wherein the fundamental steps resemble those previously implicated in the ortho-selectivity borylation of fluorinated arenes, but where differences in the rates of these steps relative to one another result in selective functionalization para to the ester substituent. The cobalt precatalyst 1-(H) 2 BPin eliminates H 2 by a multi-step sequence 55 to generate the C-H activating boryl species, 1-BPin. The species then undergoes C(sp 2 )-H oxidative addition to generate a Co(III)-aryl-hydride-boryl intermediate with mutually cis boryl and hydride ligands, and this species then reductively eliminates HBPin to generate Co(I)-aryl species paraand meta-4f.…”

Direct C−H Borylation of Arenes Catalyzed by Saturated Hydride‐Boryl‐Iridium‐POP Complexes: Kinetic Analysis of the Elemental Steps

“…The first type has been employed by the groups of Chirik, Cui, and Kamitani. Chirik has developed cobalt complexes bearing NNN, NNP, PNP, and PPP ligands, which promote reactions of heterocycles and arenes through a cycle that relies on a cobalt(I)–(III) redox couple. Cui has synthesized a cobalt(II) compound stabilized by a silylene‐pyridine‐silylene pincer, which enables reactions of pyridines, furans, and fluorinated arenes, whereas Kamitani has achieved efficient C−H borylation of arenes and heteroarenes using an iron complex bearing a quinoline‐based PNN ligand.…”