Mechanistic Interrogation of Alkyne Hydroarylations Catalyzed by Highly Reduced, Single-Component Cobalt Complexes

Abstract

Highly reactive catalysts for ortho-hydroarylations of alkynes have previously been reported to result from activation of CoBr2 by Grignard reagents, but the operative mechanism and identity of the active cobalt species have been undefined. A mechanistic analysis of a related system, involving hydroarylations of a (N-aryl)­aryl ethanimine with diphenylacetylene, was performed using isolable reduced Co complexes. Studies of the stoichiometric reaction of Co­(I) or Co­(II) precursors with CyMgCl implicated catalyst initiation via a β-H elimination/deprotonation pathway. The resulting single-component Co­(-I) complex is proposed as the direct pre-catalyst. Michaelis–Menten enzyme kinetic studies provide mechanistic details regarding the catalytic dependence on substrate. The (N-aryl)­aryl ethanimine substrate exhibited saturation-like behavior, whereas alkyne demonstrated a complex dependency; rate inhibition and promotion depend on the relative concentration of alkyne to imine. Activation of the aryl C–H bond occurred only in the presence of coordinated alkyne, which suggests operation of a concerted metalation–deprotonation (CMD) mechanism. Small primary isotope effects are consistent with a rate-determining C–H cleavage. Off-cycle olefin isomerization catalyzed by the same Co­(-I) active species appears to be responsible for the observed Z-selectivity.

C–H Bond Activation Facilitated by Bis(phosphinoamide) Heterobimetallic Zr/Co Complexes

“…A ubiquitous strategy to enhance both the reactivity and selectivity of C–H activation reactions is the use of a directing group that is able to coordinate to the transition metal prior to C–H bond activation, facilitating the activation of a single C–H bond within the substrate. − The ready coordination of pyridine to transition metals, combined with the weakened C–H bonds in heteroaromatic compounds, renders pyridine derivatives popular candidates for directed C–H bond activation. When a Lewis acid is incorporated into the ligand framework of a transition metal complex, C–H activation is facilitated by coordination of the directing groups to the appended Lewis acid, appropriately positioning the substrate for C–H activation by the transition metal. − For example, Ozerov reported an iridium complex that selectively activates the o -C–H bond of pyridine via coordination of pyridine to a Lewis acidic boryl fragment incorporated into a pincer ligand framework (Chart A) .…”

Interfacial Interactions between Co‐Based Cocatalysts and Semiconducting Light Absorbers for Solar‐Light‐Driven Redox Reactions

“…[ 33 ] For example, the well‐studied hydroarylation of alkynes usually undergoes low‐valent cobalt catalysis, in which the active catalysis species is generated by the treatment of cobalt salts (e.g., CoCl 2 , CoBr 2 ) with Grignard reagents in the presence of added ligands. [ 34 ] Inspired by that, Chirik's group synthesized cobalt complexes with an electron‐rich Co center to enable C–H activation and coupling to construct CB bond that relied on a Co(I)–Co(III) redox couple. [ 35 ] In contrast, bench‐stable Cp*Co III complexes are commonly involved in high‐valent cobalt catalysis.…”

Olefin Hydroarylation Catalyzed by a Single-Component Cobalt(-I) Complex

“…Studies in this laboratory on the mechanism of alkyne hydroarylations catalyzed by cobalt led to identification of a well-defined Co(-I) complex, [(PPh 3 ) 3 Co(N 2 )]Li(THF) 49 Herein, the use of Co-Li as a versatile catalyst for olefin hydroarylations is described. The two general catalytic conditions employed in this study were based on initial optimizations described for the alkyne hydroarylation system.…”

“…The two general catalytic conditions employed in this study were based on initial optimizations described for the alkyne hydroarylation system. 49 Catalytic reactions were examined under dilute substrate concentrations (0.1 M) in toluene with 10 mol % catalyst loading at either 25 or 80 °C.…”

“…Treatment of an equimolar mixture of acetophenone and p-anisidine with catalytic quantities of Co-Li resulted in rapid N−H deprotonation to generate the inactive complex, (PPh 3 ) 3 Co-(N 2 )H, as has been discussed in a previous report. 49 The identity of the solvent does not affect the catalytic efficacy in Scheme 2. Olefin Hydroarylations with (N-Aryl)aryl Imines Catalyzed by Co-Li a,b a Reaction conditions: 1 (0.1 M, 1 equiv), 2 (0.1 M, 1 equiv), and Co-Li (10 mol %) in toluene at either 25 or 80 °C under N 2 .…”