Oxidative Addition of Aryl Electrophiles to a Prototypical Nickel(0) Complex: Mechanism and Structure/Reactivity Relationships

Abstract

Detailed kinetic studies of the reaction of a model Ni 0 complex with a range of aryl electrophiles have been conducted. The reactions proceed via a fast ligand exchange preequilibrium, followed by oxidative addition to produce either [Ni I X(dppf)] (and biaryl) or [Ni II (Ar)X(dppf)]; the ortho substituent of the aryl halide determines selectivity between these possibilities. A reactivity scale is presented in which a range of substrates is quantitatively ranked in order of the rate at which they undergo oxidative addition. The rate of oxidative addition is loosely correlated to conversion in prototypical cross-coupling reactions. Substrates that lead to Ni I products in kinetic experiments produce more homocoupling products under catalytic conditions.

Direct Cross‐Coupling of Allylic C(sp3)−H Bonds with Aryl‐ and Vinylbromides by Combined Nickel and Visible‐Light Catalysis

“…However, this mechanism is unlikely because of the inconsistency with the exclusive regioselectivities, as well as the unfavored reduction potential (see the Supporting Information for more details). Although our attempts with aryl chlorides and mesylates turned out to be unsuccessful and only starting materials were recovered, the more reactive aryl iodide 1 p afforded 3 p in 20 % yield. Notably, we were pleased to find out that the reaction efficiency increased dramatically when LiBr was used as bromine additive (Scheme c).…”

The Dual Role of Benzophenone in Visible‐Light/Nickel Photoredox‐Catalyzed C−H Arylations: Hydrogen‐Atom Transfer and Energy Transfer

“…Unexpectedly, 4‐iodobenzonitrile ( 5 a ) exhibited much lower reactivity (39 % NMR yield) compared to its bromo analogue 2 a . This was surprising because the aryl–I bond should be more reactive towards oxidative addition compared to the aryl–Br bond . This observation led us to consider an underlying parallel propagation mechanism where 3 acts as a triplet sensitizer (Scheme c) …”

Direct Cross‐Coupling of Allylic C(sp3)−H Bonds with Aryl‐ and Vinylbromides by Combined Nickel and Visible‐Light Catalysis

“…Zuschriften recovered, the more reactive aryl iodide [16] 1p afforded 3p in 20 %y ield. Notably,w ew ere pleased to find out that the reaction efficiency increased dramatically when LiBr was used as bromine additive (Scheme 2c).…”