Mechanistic Interrogation of Co/Ni-Dual Catalyzed Hydroarylation

Abstract

Cobalt/nickel-dual catalyzed hydroarylation of terminal olefins with iodoarenes builds complexity from readily available starting materials, with a high preference for the Markovnikov (branched) product. Here, we advance a mechanistic model of this reaction through the use of reaction progress kinetic analysis (RPKA), radical clock experiments, and stoichiometric studies. Through exclusion of competing hypotheses, we conclude that the reaction proceeds through an unprecedented alkylcobalt to nickel direct transmetalation. Demonstration of catalytic alkene prefunctionalization, via spectroscopic observation of an organocobalt species, distinguishes this Csp-Csp cross-coupling method from a conventional transmetalation process, which employs a stoichiometric organometallic nucleophile, and from a bimetallic oxidative addition of an organohalide across nickel, described by radical scission and subsequent alkyl radical capture at a second nickel center. A refined understanding of the reaction leads to an optimized hydroarylation procedure that excludes exogenous oxidant, demonstrating that the transmetalation is net redox neutral. Catalytic alkene prefunctionalization by cobalt and engagement with nickel catalytic cycles through direct transmetalation provides a new platform to merge these two rich areas of chemistry in preparatively useful ways.

RASS‐Enabled S/P−C and S−N Bond Formation for DEL Synthesis

“…These ideas have spurred our labs interest in using a homogeneous silane based reductant in nickel mediated RASS‐based cross couplings . We found the Ni/silane system in DMA to be flexible in its utility.…”

Recent Advances in Four‐Coordinated Planar Cobalt Catalysis in Organic Synthesis

“…Meanwhile, Schenvi and coworkers successfully extended the hydrofunctionalization to a Co‐catalyzed branch‐selective hydroarylation for the cross‐coupling of iodoarenes with alkenes in the presence of hydrosilane (Scheme ) . Mechanistic studies revealed that the cross‐coupling proceeds via a cage‐rebound process between alkyl‐Co(III) and aryl‐Ni(III) complexes within a solvent cage . Thus, a SET process from secondary alkyl‐Co(III) intermediate 77 to aryl‐Ni(III) 78 would provide alkyl‐Co(IV) intermediate 79 and aryl‐Ni(II) 80 .…”

Cycloisomerization of Olefins in Water

“…[15][16][17] Our group previously reported aC o/Ni bimetallic hydroarylation of olefins,w here Co(Sal t-Bu,t-Bu )BF 4 appeared to be superior to Co(Sal t-Bu,t-Bu )Cl. [18,19] We found that at high dilution Co(Sal t-Bu,t-Bu )BF 4 catalyzes cycloisomerization significantly faster than the initially reported Co(Sal t-Bu,t-Bu )Cl. Water, however,h ad al eveling effect on this cationic complex, possibly acting as an apical ligand, so that both complexes reached roughly equal rates ( Figure 2).…”

“…Indeed, we have found N-fluorocollidinium oxidants to rescue off-pathway Co +2 species by reoxidation to Co +3 . [18] Importantly,r ates at high dilution (1 mm) allowed completion of the reaction within only 25 minutes. Initial rates indicated the bimolecular MHATs tep (Path 1, Figure 4) must possess ar ate constant of at least 76 m À1 s À1 , corresponding to an energy barrier of at most 14.9 kcal mol À1 .…”