Mechanism of Ni-Catalyzed Reductive 1,2-Dicarbofunctionalization of Alkenes

Abstract

Ni-catalyzed cross-electrophile coupling reactions have emerged as appealing methods to construct organic molecules without the use of stoichiometric organometallic reagents. The mechanisms are complex: plausible pathways, such as "radical chain" and "sequential reduction" mechanisms, are dependent on the sequence of the activation of electrophiles. A combination of kinetic, spectroscopic, and organometallic studies reveals that a Ni-catalyzed, reductive 1,2dicarbofunctionalization of alkenes proceeds through a "sequential reduction" pathway. The reduction of Ni by Zn is the turnover-limiting step, consistent with Ni(II) intermediates as the catalyst resting-state. Zn is only sufficient to reduce (phen)Ni(II) to a Ni(I) species. As a result, commonly proposed Ni(0) intermediates are absent under these conditions. (Phen)Ni(I)-Br selectively activates aryl bromides via two-electron oxidation addition, whereas alkyl bromides are activated by (phen)Ni(I)-Ar through single-electron activation to afford radicals. These findings could provide insight into achieving selectivity between different electrophiles.

trans‐Selective Aryldifluoroalkylation of Endocyclic Enecarbamates and Enamides by Nickel Catalysis

“…[21] Treatment of complex A with 1 a and 2 c afforded the pyrrolidine 4 e in 44 % yield along with 23 % yield of cross-coupling side product 5 e. Meanwhile, a parallel reaction by reduction of [Ar-Ni II (L1)] (A) with Cp 2 Co [22] was performed to generate [Ar-Ni I (L1)] complex (B) (Scheme 4 D). The formation of complex B was supported by the EPR analysis of the reaction, in which the EPR signal of complex B is similar with the nickel(I) complex reported in the literature [23] (Supporting Information Figure S2). However, no EPR signal was observed in the absence of reductant Cp 2 Co (Supporting Information Figure S1), suggesting that two different nickel species were involved in these two reactions.…”

General Paradigm in Photoredox Nickel‐Catalyzed Cross‐Coupling Allows for Light‐Free Access to Reactivity

“…We also discovered that Ni 0 in combination with Zn 0 gave higher yields than Ni 0 alone (Figure , conditions C), suggesting that inactive Ni II aryl complexes may be reductively resurrected to re‐enter the Ni I/III cycle. This may indicate that Zn 0 can further reduce Ni II aryl complexes to form Ni I aryl species that undergo subsequent oxidative addition to give on‐cycle Ni III intermediates, in a manner analogous to that proposed for Ni‐catalyzed reductive coupling …”

“…exceptionally mild conditions using inexpensive and readily accessible ligands under Ni catalysis). Despite numerous examples of Ni I/III cross‐coupling cycles, the propensity of Ni to undergo one‐electron redox processes has made elucidation of reaction mechanisms challenging in many cases, and the factors that dictate whether a reaction occurs through a Ni 0/II or a Ni I/III cycle are not fully understood. Hence, a general strategy for selectively engaging the Ni I/III redox couple under thermal catalysis has remained largely elusive.…”

Asymmetric Reductive Dicarbofunctionalization of Alkenes via Nickel Catalysis

“…Oxidative addition and reduction of a nickel(I) bromide species to the aryl bromide electrophile results in an arylnickel(I) species. 19 This intermediate undergoes migratory insertion with the alkene substrate. Oxidative addition of another molecule of aryl bromide to the arylnickel(I) makes a nickel(III) species.…”