Kinetics of a Ni/Ir-Photocatalyzed Coupling of ArBr with RBr: Intermediacy of ArNi^II(L)Br and Rate/Selectivity Factors

Abstract

The Ni/Ir-photocatalyzed coupling of an aryl bromide (ArBr) with an alkyl bromide (RBr) has been analyzed using in situ LED-19F NMR spectroscopy. Four components (light, [ArBr], [Ni], [Ir]) are found to control the rate of ArBr consumption, but not the product selectivity, while two components ([(TMS)3SiH], [RBr]) independently control the product selectivity, but not the rate. A major resting state of nickel has been identified as ArNiII(L)­Br, and 13C-isotopic entrainment is used to show that the complex undergoes Ir-photocatalyzed conversion to products (Ar-R, Ar-H, Ar-solvent) in competition with the release of ArBr. A range of competing absorption and quenching effects lead to complex correlations between the Ir and Ni catalyst loadings and the reaction rate. Differences in the Ir/Ni Beer–Lambert absorption profiles allow the rate to be increased by the use of a shorter-wavelength light source without compromising the selectivity. A minimal kinetic model for the process allows simulation of the reaction and provides insights for optimization of these processes in the laboratory.

Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure–Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways

“…19−28 While originally thought to proceed through a Ni(0)/Ni(II) cycle, 6 recent work has instead supported a Ni(I)/Ni(III) cycle in the direct excitation pathway (Figure 1A). 23,29,30 Through analysis of a library of Ni(II)−bpy aryl halide complexes, we have revealed that excited-state Ni(II)−C aryl bond homolysis from the S = 0 square planar Ni(II) ground state features a key ligand-to-metal charge transfer (LMCT) process. This LMCT results in electron excitation between the Ni−aryl σ and σ* orbitals, which lowers the bond order from one to zero, resulting in repulsive homolytic bond cleavage and the generation of an aryl radical and a three-coordinate Ni(I)− bpy halide species.…”

Best Practices for the Collection of Robust Time Course Reaction Profiles for Kinetic Studies

“…Catalyst species can be monitored and quantified via ESI-MS, , MALDI-MS, or various NMR techniques. ,− If direct analysis is not possible, then indications of nonconstant catalyst concentrations can be found in time course data. Slow catalyst activation generally manifests as an induction period in time course profiles, while loss of active catalyst concentration over time in the form of catalyst deactivation can be probed through same excess experiments .…”

In situ Reaction Monitoring in Photocatalytic Organic Synthesis

“…[55] A metallaphotocatalytic cross-electrophile-coupling of aryl bromide 23 and alkyl bromide 24 was investigated using online monitoring with a LED-NMR setup (Figure 16). [56] The initial reaction profile ( 19 F-NMR) showed not only the generation of the desired product, but also led to the identification of two minor side products arising from a coupling reaction of the aryl bromide with the solvent (Ar-solv) and protodehalogenation of 24 (ArÀ H). More importantly, a transient Ni II species (25) was identified.…”

“…This provided evidence that intermediate 25 is responsible for product formation (B). Reproduced from ref [56]. with permission from the American Chemical Society.…”