Mechanistic Insight into Catalytic Redox-Neutral C–H Bond Activation Involving Manganese(I) Carbonyls: Catalyst Activation, Turnover, and Deactivation Pathways Reveal an Intricate Network of Steps

Abstract

Manganese(I) carbonyl-catalyzed C−H bond functionalization of 2-phenylpyridine and related compounds containing suitable metal directing groups has recently emerged as a potentially useful synthetic methodology for the introduction of various groups to the ortho position of a benzene ring. Preliminary mechanistic studies have highlighted that these reactions could proceed via numerous different species and steps and, moreover, potentially different catalytic cycles. The primary requirement for typically 10 mol % catalyst, oftentimes the ubiquitous precursor catalyst, BrMn(CO) 5 , has not yet been questioned nor significantly improved upon, suggesting catalytic deactivation may be a serious issue to be understood and resolved. Several critical questions are further raised by the species responsible for providing a source of protons in the protonation of vinyl−manganese(I) carbonyl intermediates. In this study, using a combination of experimental and theoretical methods, we provide comprehensive answers to the key mechanistic questions concerning the Mn(I) carbonyl-catalyzed C−H bond functionalization of 2-phenylpyridine and related compounds. Our results enable the explanation of alkyne substrate dependencies, i.e., internal versus terminal alkynes. We found that there are different catalyst activation pathways for BrMn(CO) 5 , e.g., terminal alkynes lead to the generation of Mn I −acetylide species, whose formation is reminiscent of Cu I −acetylide species proposed to be of critical importance in Sonogashira cross-coupling processes. We have unequivocally established that alkyne, 2-phenylpyridine, and water can facilitate hydrogen transfer in the protonation step, leading to the liberation of protonated alkene products.

Metathesis by Partner Interchange in σ‐Bond Ligands: Expanding Applications of the σ‐CAM Mechanism

“…In conclusion, the σ‐CAM concept represents an instructive approach to reaction mechanism that brings together metathesis reactions involving the formation of a variety of metal‐element bonds through partner interchange of σ‐bond complexes. It is supported through experimental measurements and computational studies of stoichiometric and catalytic reactions that are becoming increasingly sophisticated [147, 150, 151] . The key concept that defines a metathesis reaction as a σ‐CAM process is the presence of two σ‐bond complexes as intermediates.…”

“…In conclusion, the s-CAM concept represents an instructive approach to reaction mechanism that brings together metathesis reactions involving the formation of av ariety of metal-element bonds through partner interchange of s-bond complexes.I ti ss upported through experimental measurements and computational studies of stoichiometric and catalytic reactions that are becoming increasingly sophisticated. [147,150,151] Thek ey concept that defines am etathesis reaction as a s-CAM process is the presence of two s-bond complexes as intermediates.They must retain the metal in the same oxidation state and must be connected by a single transition state.T he nature of this transition state,h owever, does not define whether it is a s-CAM process or not. This definition allows for experimental and computational investigation of the intermediates,w hile allowing flexibility and nuance in the nature of the transition states.…”

Light‐ and Manganese‐Initiated Borylation of Aryl Diazonium Salts: Mechanistic Insight on the Ultrafast Time‐Scale Revealed by Time‐Resolved Spectroscopic Analysis

“…[32][33][34][35][36][37][38][39][40][41] Despite this precedentt here has been little reported confirming that Mn 2 (CO) 10 generates reactive aryl radicals. [42] Given our interesti ni nitiating chemical transformations with Mn I -carbonyl complexes, [43][44][45] we envisaged that the facile reductiono fa ryl diazoniums alts to aryl radicals could be achieved with Mn 2 CO 10 ,l eadingt og eneration of aryl pinacol boronates upon treatment with B 2 pin 2 .C redibility for this proposal comes from Ackermann andc o-workers'r ecent study,w ith the synthesis of an exemplar aryl boronate (Scheme1), using Mn 2 (CO) 10 and blue LEDs through aM n I -mediated process. [42] We recognized that this single example could be broadened out into agenerals ynthetic methodology, but that the reaction manifold allowed us to ask arguably am ore fundamentally important question about the involvement of Mn species in araft of low oxidation state Mn-mediated transformations, [42,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] particularly those invoking redox neutralMn I species.…”

Manganese(I)‐Catalyzed Site‐Selective C6‐Alkenylation of 2‐Pyridones Using Alkynes via C−H Activation

“…In 2013, the Wang group first described the alkenylation of (hetero)arene C−H bonds with terminal alkynes using Mn(CO) 5 Br as the catalyst [5a] . Thereafter, terminal alkynes have been intensively explored in Mn‐catalyzed (hetero)arene C−H bond alkenylation by the groups of Lei and Li, [5b] Wang, [5c,g] Ackermann, [5d] and others, [5e,f,h–j] and Mn(CO) 5 Br catalyst in combination with a base or acid additive or both worked well for these transformations. In contrast, less success has been achieved with the more challenging internal alkynes, and only a handful of procedures are available [6]…”