Mechanisms of Cobalt/Phosphine‐Catalyzed Cross‐Coupling Reactions

Abstract

>The combination of CoCl2 with bidentate phosphines is known to catalyze challenging cross‐coupling and Heck‐type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray‐ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low‐valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br−. In cross‐coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck‐type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single‐electron transfer processes and the involvement of radicals in these transformations.

Low‐Cost Transition Metal‐Catalyzed Heck‐Type Reactions: An Overview

“…Using in situ ESI mass spectroscopy analysis of intermediates produced, Koszinowski et al . presented a mechanistic insight into Cobalt/Phosphine‐Catalyzed Heck‐Coupling in 2019 [58] . Analysis of the speciation of CoCl 2 and bidentate phosphine ligands L (L=dppe, dppp, dpph and dppbz) in THF indicated L 2 CoCl + and LCoCl 3− (L=dppe) as the major ions and these species were strikingly similar to complexes previously detected by Severin et al .…”

“…Using in situ ESI mass spectroscopy analysis of intermediates produced, Koszinowski et al presented a mechanistic insight into Cobalt/Phosphine-Catalyzed Heck-Coupling in 2019. [58] Analysis of the speciation of CoCl 2 and bidentate phosphine ligands L (L = dppe, dppp, dpph and dppbz) in THF indicated L 2 CoCl + and LCoCl 3À (L = dppe) as the major ions and these species were strikingly similar to complexes previously detected by Severin et al in crystals of CoCl 2 /dppe. [59] On adding Grignard or organolithium reagents, the cobalt precatalyst gets reduced to Co À I , Co 0 , and Co I species by a sequence of double transmetallation, reductive elimination, and redox disproportionation (Scheme 30).The strong evidence of involvement of reductive elimination was obtained using gas-phase fragmentation experiments of mass-selected ions, with cobalt in different oxidation states.…”

Synthesis of dendrimer assisted cobalt nanoparticles and catalytic application in Heck coupling reactions in ionic liquid

“…The mechanistic investigations of cobalt catalyzed Heck coupling reaction carried out recently by numerous researchers [51][52][53] described that alkyl halides induce the generation of the corresponding radical intermediates with the catalyst with the cobalt catalyst whereas alkenyl and aryl halides undergo non-radical oxidative addition in the mechanistic pathway. Based on these observations, we have proposed a plausible mechanism of catalyst 4 as following.…”

A Cobalt-Catalyzed Enantioconvergent Radical Negishi C(sp3)–C(sp2) Cross-Coupling with Chiral Multidentate N,N,P-Ligand

“…The result showed a complete inhibition of the desired cross-coupling reaction of 1b and TEMPO-trapped product 4 was obtained in 35% yield, which supported the formation of the alkyl radical species (Scheme A). On the basis of the literature reports, , we proposed a plausible mechanism as shown in Scheme B. The precatalyst Co II was first be reduced by the arylzinc reagent to the active catalytic species LnCo m for the initiation of the reaction.…”