Comproportionation and disproportionation in nickel and copper complexes

Abstract

This review covers factors that contribute to comproportionation and disproportionation reactions in transition metal complexes and provide insight into the importance of these electron transfer events in Ni- and Cu-catalyzed transformations.

Light Activation and Photophysics of a Structurally Constrained Nickel(II)–Bipyridine Aryl Halide Complex

“…Untethered 1 exhibits precedented reactivity; the accumulation of the Ni­(I)–bpy chloride intermediate complex is fully suppressed by mesityl bromide (Figure b, left). The proposed pathway for this reactivity consists of oxidative addition of mesityl bromide to Ni­(I) to yield a high-energy Ni­(III) intermediate, ,− which can then undergo comproportionation with another Ni­(I) in each reaction cycle to produce a new photoactive Ni­(II)–bpy aryl halide and a photoinactive Ni­(II)–bpy dihalide in a 1:1 ratio. , With extended irradiation, all of the starting Ni­(II)–bpy aryl halide complexes are expected to ultimately convert to a triplet d 8 Ni­(II)–bpy dihalide (Scheme S8), which does not possess distinctive UV–vis absorption features in the visible region (Figure S4). Note that the sloping UV–vis spectra in Figure b (left) is indicative of either the precipitation of Ni­(II)–bpy dihalide or formation of an off-pathway [Ni­(I)–bpy halide] 2 binuclear species from two Ni­(I)–bpy halide units …”

“…Prolonged irradiation of 1 culminated in the formation of Ni(II)−bpy dihalide, as monitored by the disappearance of the Ni(II)−bpy aryl halide UV−vis absorption features, which is consistent with the previously proposed oxidative addition and comproportionation reaction mechanism. 16,30,33 Contrastingly, 2 yielded a new Ni species with a unique UV−vis spectrum (Figure 3b, right) in addition to a Ni(II)− Ph bpy dihalide. A species with a similar spectrum was accessed via spectroelectrochemical one-electron reduction of the bromine analogue of 2 and was attributed to the tethered, tridentate Ni(I)−bpy phenyl complex.…”

“…Photoredox catalysis has motivated modern synthetic organic chemistry through the development of selective bond transformations using light. − At its core, metallaphotoredox catalysis utilizes photoactive transition metal complexes to facilitate intramolecular charge transfer and/or outer-sphere electron transfer processes to generate catalytically reactive intermediates. − Nickel­(II)–bipyridine (bpy) aryl halide complexes have emerged as a prominent class of light activated cross-coupling catalysts, − wherein direct photoexcitation leads to population of singlet metal-to-ligand charge transfer ( 1 MLCT) excited-state potential energy surfaces and, ultimately, Ni­(II)–C bond homolysis to yield reactive Ni­(I) species . Two bond homolysis mechanisms are outlined in Figure a and feature either (1) relaxation to a triplet Ni­(II) ligand-field ( 3 LF) state followed by thermal Ni­(II)–C bond homolysis or (2) intersystem crossing to repulsive ligand-to-metal charge transfer (LMCT) states (referred to herein as ISC/LMCT). , Assessing the interplay between these two (or other) possibilities remains challenging, however, as the quantum yields for Ni­(II)–C bond homolysis are small, preventing direct observation using transient spectroscopies.…”

Rate Equations for Reversible Disproportionation Reactions and Fitting to Time-Course Data

“…To test this, the disproportionation of 4-acetamidoTEMPO was also investigated and good agreement with previously reported values for k f and k b were obtained by this method (see Supporting Information, section S9). We look forward to the application of these equations to nonaminoxyl systems as well, such as the reversible disproportionations of nickel complexes that are relevant to understand their catalytic activity . Another class of reactions where these equations are useful is in the speciation of N -chloro amines and sulfonamides, which are important in both synthetic chemistry and biology. , This speciation takes the form of a disproportionation into the amine and dichloroamine, as shown in eq .…”

Photoredox/copper-catalyzed formal cyclopropanation of olefins

“…Meanwhile, substrate 1a reacts with alkyl radical R˙ to produce intermediate 6 , which is captured by the Cu( ii ) complex to give Cu( iii ) species 7 . Subsequently, intermediate 7 undergoes reductive elimination to give γ-bromoketone 8 and the Cu( i ) species, which undergoes a disproportionation reaction 21 to give Cu(0) and Cu( ii ). Both Cu( ii ) and 8 generate the intermediate 9 in the presence of a base.…”