Experimental and Theoretical Study of Ni^II‐ and Pd^II‐Promoted Double Geminal C(sp³)−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

Abstract

We report the first examples of metal-promoted double geminal activation of C(sp 3 )À H bonds of the NÀ CH 2 À N moiety in an imidazole-type heterocycle, leading to nickel and palladium N-heterocyclic carbene complexes under mild conditions. Reaction of the new electron-rich diphosphine 1,3-bis((di-tert-butylphosphaneyl)methyl)-2,3-dihydro-1Hbenzo[d]imidazole (1) with [PdCl 2 (cod)] occurred in a stepwise fashion, first by single CÀ H bond activation yielding the alkyl pincer complex [PdCl(PC sp 3 H P)] (3) with two trans phosphane donors and a covalent PdÀ C sp 3 bond. Activation of the CÀ H bond of the resulting α-methine C sp 3 HÀ M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC NHC P)]Cl (2). Treatment of 1 with [NiBr 2 (dme)] also afforded a NHC pincer complex, [NiBr(PC NHC P)]Br ( 6), but the reactions leading to the double geminal CÀ H bond activation of the NÀ CH 2 À N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a NÀ CH 2 À N moiety in the NÀ C NHC À N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.

PCP Pincer Carbene Nickel(II) Chloride, Hydride, and Thiolate Complexes: Hydrosilylation of Aldehyde, Ketone, and Nitroarene by the Thiolate Complex

“…As shown in Scheme , the reaction between [NiCl 2 (DME)] and L1 or L2 in the presence of KPF 6 in CH 3 CN afforded the pincer carbene complexes 1 or 2 , respectively, in high yields. In their 1 H NMR spectra, the presence of only two resonances, instead of three, in the CH 2 region of ligand immediately suggested the formation of metal–carbene complexes, which are formed by the double C–H bond activation of the N–CH 2 –N moiety that more likely follows the mechanism recently reported by Braunstein and coworkers . The 31 P­{ 1 H} NMR spectra of complexes 1 and 2 showed downfield shifted singlets at δ 39.3 and 78.5 ppm, respectively, which support the presence of a single environment for the phosphorus atoms in their structures.…”

“…In their 1 H NMR spectra, the presence of only two resonances, instead of three, in the CH 2 region of ligand immediately suggested the formation of metal−carbene complexes, which are formed by the double C−H bond activation of the N−CH 2 −N moiety that more likely follows the mechanism recently reported by Braunstein and coworkers. 28 The 31 P{ 1 H} NMR spectra of complexes 1 and 2 showed downfield shifted singlets at δ 39.3 and 78.5 ppm, respectively, which support the presence of a single environment for the phosphorus atoms in their structures. The coordination-caused chemical shift (Δδ) is around 61 ppm, which is approximately 9 ppm higher than that (52 ppm) observed for the analogous hexahydropyrimidine-based carbene complex.…”

Carbonylative Formal Cycloaddition between Alkylarenes and Aldimines Enabled by Palladium-Catalyzed Double C–H Bond Activation

“…Thus, the development of catalytic paradigms furnishing such double C–H functionalization promises great value to the field of chemical synthesis. In this context, a variety of double C–H bond activations have been developed to generate the metal-carbene complexes, One of the challenges of developing such reactions is that the carbon atom must be tethered with multiple strong coordination groups to facilitate double C–H activations on a single metal, which significantly prevents further catalytic transformations (Scheme B). As a result, alternative paradigms for double C–H bond activation on the same carbon that can be utilized for further transformation is an appealing yet elusive goal.…”

“…Carbene-CO complexes can be utilized as sources of ketene intermediates in ketene–imine or ketene–alkene cycloadditions or electrophilic substitution reactions . Nonetheless, the catalytic formation of ketenes or its equivalents via C–H bond activation and CO insertion has been largely unexplored, although the metal–carbene complexes can be generated through double C–H activation and ketene ligands have been observed on single metal centers after coupling of carbene and CO ligands . Herein, we communicate a novel strategy for the generation of ketene-equivalents via palladium-catalyzed sequential double C–H bond activation and carbonylation in the presence of CO, which allows for the straightforward and efficient synthesis of β-lactams from readily accessible alkylarenes and imines (Scheme C).…”