N-Heterocyclic Carbene Ligand-Controlled Regioselectivity for Nickel-Catalyzed Hydroarylation of Vinylarenes with Benzothiazoles

Abstract

A facile regioselective switch for nickel-catalyzed hydroarylation of vinylarenes with benzothiazoles has been developed, which relies on the simple structural variation of novel Ni­(II) complexes of the type Ni­(NHC)­[P­(OR)3]­Br2. Using magnesium turnings as the reductant, Ni­(IMes)­[P­(OEt)3]­Br2 afforded branched products, while Ni­(IPr*OMe)­[P­(OEt)3]­Br2 created steric demand to afford linear products. This work also provides a rare example of the rational design of heteroleptic Ni­(II) complexes that display the required air stability, reactivity, and regioselectivity via synergism between NHC and phosphite ligands.

The Synthesis and Catalytic Activity of New Mixed NHC-Phosphite Nickel(0) Complexes

“…A catalytic mixture consisting of Ni­(COD) 2 , IPr (NHC) and P­(OPh) 3 , applied to reductive alkene/aldehyde coupling reactions, has recently been described in which a nickel NHC-phosphite (1:1) intermediate is proposed as part of the catalytic cycle . Additionally, nickel-catalyzed hydroarylation of benzothiazoles has very recently been explored with halogenated Ni­(II) precursors containing P­(OEt) 3 and NHC ligands . If nickel(0) NHC-phosphite complexes could be isolated, their further catalytic ability could be fully explored in a range of reactions.…”

“…31 Additionally, nickel-catalyzed hydroarylation of benzothiazoles has very recently been explored with halogenated Ni(II) precursors containing P(OEt) 3 and NHC ligands. 32 If nickel(0) NHC-phosphite complexes could be isolated, their further catalytic ability could be fully explored in a range of reactions. Herein, we report the synthesis and characterization of the first isolable, low-valent, mixed nickel NHC-phosphite systems.…”

Recent Developments in Transition-Metal Catalyzed Direct C–H Alkenylation, Alkylation, and Alkynylation of Azoles

“…The authors were able to isolate and characterize the active catalyst by single-crystal X-ray crystallography (Scheme 59). Similarly, Sun and coworkers also employed a NHCs-based nickel catalytic system for the regioselective hydroarylation of vinylarenes with benzothiazoles, affording the desired product in high yield [92]. In this reaction, the regioselectivity is controlled by the sterically demanding Ni(IMes)[P(OEt)3]Br2 catalyst (Scheme 60).…”

“…Mechanistic studies indicated that the main step is the formation of a Cu(I) carbene species, followed by migratory insertion and protonation to produce the final products, as shown in Scheme 61. Similarly, Sun and coworkers also employed a NHCs-based nickel catalytic system for the regioselective hydroarylation of vinylarenes with benzothiazoles, affording the desired product in high yield [92]. In this reaction, the regioselectivity is controlled by the sterically demanding Ni In 2017, Mandal and coworkers developed a Ni(COD)2 catalyzed regioselective hydroheteroarylation of vinylarenes with benzoxazoles, providing an exclusively wide range of 1,1diarylethane products [91].…”

An Unusual Perpendicular Metallacycle Intermediate is the Origin of Branch Selectivity in the Rh(II)-Catalyzed C–H Alkylation of Aryl Sulfonamides with Vinylsilanes

“…Since the first example of the ortho -C–H alkylation of aromatic ketones with alkenes using Ru catalysis, a large number of reports on directed C–H alkylation with alkenes have appeared. However, most of these reactions were based on a linear-selective transformation, and only a limited number of studies have dealt with a branch-selective transformation. , In addition, the reported branch-selective alkylation reactions mostly involved the use of activated alkenes such as styrenes, acrylate esters, and vinyl esters. Our group recently reported the first example of the branch-selective alkylation of sulfonamides with vinylsilanes (Scheme B) .…”