Cobalt-Catalyzed Preparation of Arylindium Reagents from Aryl and Heteroaryl Bromides

Metal Activation Produces Different Reaction Environments for Intermediates during Oxidative Addition

Commercial zinc metal powder requires activation for consistent and reliable use as a reductant in the formation of organozinc reagents from organohalides, and for the avoidance of supplier and batch-to-batch variability. However, the impact of activation methods on the reaction environments of subsequent intermediates has been unknown. Herein, a fluorescence lifetime imaging microscopy (FLIM) method is developed to bridge this knowledge gap, by imaging and examining reaction intermediates on zinc metal that has been activated by pretreatment through different common methods (i. e., by chemical activation with TMSCl, dibromoethane, or HCl; or by mechanical activation). The group of chemical activating agents, previously thought to act similarly by removing oxide layers, are here shown to produce markedly different reaction environments experienced by subsequent oxidative-addition intermediates from organohalides - data uniquely available through FLIM's ability to detect small quantities of intermediates in situ coupled with its microenvironmental sensitivity. These different microenvironments potentially give rise to different rates of formation, subsequent solubilization, and reactivity, despite the shared "[RZnX]" molecular structure of these intermediates. This information revises models for methods development for oxidative addition to currently sluggish metals beyond zinc by establishing diverse outcomes for pretreatment activation methods that were previously considered similar.

Trimethylsilyl Chloride Aids in Solubilization of Oxidative Addition Intermediates from Zinc Metal

Trimethylsilyl chloride (TMSCl) is commonly used to "activate" metal(0) powders toward oxidative addition of organohalides, but knowledge of its mechanism remains limited by the inability to characterize chemical intermediates under reaction conditions. Here, fluorescence lifetime imaging microscopy (FLIM) overcomes these prior limitations and shows that TMSCl aids in solubilization of the organozinc intermediate from zinc(0) metal after oxidative addition, a previously unknown mechanistic role. This mechanistic role is in contrast to previously known roles for TMSCl before the oxidative addition step. To achieve this understanding, FLIM, a tool traditionally used in biology, is developed to characterize intermediates during a chemical reaction-thus revealing mechanistic steps that are unobservable without fluorescence lifetime data. These findings impact organometallic reagent synthesis and catalysis by providing a previously uncharacterized mechanistic role for a widely used activating agent, an understanding of which is suitable for revising activation models and for developing strategies to activate currently unreactive metals.

Cobalt-N-Heterocyclic Carbene Complexes in Catalysis

Cobalt-NHC complexes have emerged as an attractive class of 3d transition metal catalysts for a broad range of chemical processes, including cross-coupling, hydrogenation, hydrofunctionalization and cycloaddition reactions. Herein, we present a comprehensive review of catalytic methods utilizing cobalt-NHC complexes with a focus on catalyst structure, the role of the NHC ligand, properties of the catalytic system, mechanism and synthetic utility. The survey clearly suggests that the recent emergence of well-defined cobalt-NHC catalysts may have a tremendous utility in the design and application of catalytic reactions using more abundant 3d transition metals.

Origins of Batch-to-Batch Variation: Organoindium Reagents from Indium Metal

Yields of organoindium reagents synthesized from indium metal were previously reported to be highly dependent on metal batch and supplier due to the presence or absence of anticaking agent. Here, single-particle fluorescence microscopy established that MgO, an additive in some batches nominally for anticaking, significantly increased the physisorption of small-molecule organics onto the surface of the resulting MgO-coated indium metal particles. An inert and relatively nonpolar boron dipyrromethene fluorophore with a hydrocarbon tail provided a sensitive probe for this surface physisorption. SEM images revealed markedly different surface properties of indium particles either with or without MgO, consistent with their different physisorption properties observed by fluorescence microscopy. We further documented incomplete commercial bottle labeling regarding the presence and composition of this anticaking agent, both within our laboratory and previously in the literature, which may complicate reproducibility between laboratories. Trimethylsilyl chloride pretreatment, a step employed in a subset of reported synthetic procedures, removed the anticaking agent and produced particles with similar physisorption properties as commercial batches of indium powder distributed without the anticaking agent. These data indicate the possibility of an additional substrate/catalyst physisorption mechanism by which the anticaking agent may be influencing synthetic procedures that generate organoindium reagents from indium metal, in addition to simple anticaking.

Indium-mediated difunctionalization of iodoalkyl-tethered unactivated alkenes via an intramolecular cyclization and an ensuing palladium-catalyzed cross-coupling reaction with aryl halides

A cobalt-catalyzed, indium-mediated difunctionalization of iodoalkyl-tethered unactivated alkenes for accessing five-membered cyclic compounds via a cyclization/cross-coupling sequence was developed.

Microscopy Reveals: Impact of Lithium Salts on Elementary Steps Predicts Organozinc Reagent Synthesis and Structure

Lithium chloride is known to promote the direct insertion of metallic zinc powder into organohalides in the practical synthesis of organozinc reagents, but the reason for its special ability is poorly understood. Pioneering a combined approach of single-metal-particle fluorescence microscopy with ¹H NMR spectroscopy, we herein show that the effectiveness of different lithium salts toward solubilizing intermediates on the surface of zinc metal establishes a previously unknown reactivity correlation that predicts the propensity of that salt to promote macroscale reagent synthesis and also predicts the solution structure of the ultimate organozinc reagent. A salt-free pathway is also identified. These observations of an organometallic surface intermediate, its elementary-step reactivity, and the impact of various synthetic conditions (salt, salt-free, temperature, stirring, and time) on its persistence, are uniquely available from the sensitivity and spatial localization ability of the microscopy technique. These studies unify previously disparate observations under a single unified mechanistic framework. This framework enables the rational prediction of salt effects on multiple steps in organozinc reagent synthesis and reactivity. This is an early example of single-particle microscopy characterization of elementary steps providing predictive power in reaction development by gaining a sensitive and selective spectral handle on an important intermediate, highlighting the role of this next generation of analytical tools in the development of synthetic chemistry.

Metallic salt-catalyzed direct indium insertion into alkyl iodides and their applications in cross-coupling reactions

An efficient method for the synthesis of alkyl indium reagents via an indium(iii) or lead(ii) halide-catalyzed direct insertion of indium into alkyl iodides is developed. NMR and ESI-MS analyses indicated the formation of a mixture of an alkyl indium dihalide (RInX₂) and a dialkyl indium halide (R₂InX).

Triorganoindium Reagents in Rh-Catalyzed C⁻H Activation/C⁻C Cross-Coupling Reactions of 2-Arylpyridines

The activation of C–H bonds through catalytic reactions using transition metals is an important challenge in organic chemistry in which the intermediates are related to those produced in the classical cross-coupling reactions. As part of our research program devoted to the development of metal-catalyzed reactions using indium organometallics, a protocol for the C–H activation and C–C coupling of 2-arylpyridines with triorganoindium reagents under Rh(I) catalysis is reported. Under the optimized conditions, we found that Me₃In and Ar₃In reagents reacted with 2-arylpyridines and related compounds in the presence of Rh(PPh₃)₃Cl, in PhCl/THF (9:1), at 120 °C for 48 h, to afford the ortho-coupling products in moderate to good yields. The nitrogen atom in the pyridine ring acts as a directing group to assist the functionalization at the ortho position of the aryl group forming a new C–C bond at this position.

Synthesis of bench-stable solid triorganoindium reagents and reactivity in palladium-catalyzed cross-coupling reactions

Triorganoindium reagents can be isolated as bench-stable solid R₃In(DMAP) complexes and show excellent reactivity in palladium-catalyzed cross-coupling reactions.

Sequential In-catalyzed intramolecular hydroarylation and Pd-catalyzed cross-coupling reactions using bromopropargyl aryl ethers and amines

A sequential one-pot In-catalyzed intramolecular hydroarylation and Pd-catalyzed cross-coupling reaction using bromopropargyl aryl ethers and amines has been developed.

Transition metal-catalyzed cross-coupling reactions using organoindium reagents

This review highlights the versatility and significance of transition metal-catalyzed cross-coupling reactions employing mild and unique organoindium reagents with exceptional functional group compatibility and sometimes remarkable chemo- and stereoselectivities.

Transition-Metal-Free Cross-Coupling of Indium Organometallics with Chromene and Isochroman Acetals Mediated by BF3·OEt2

A transition-metal-free coupling of triorganoindium reagents with benzopyranyl acetals mediated by a Lewis acid has been developed. The reaction of R3In with chromene and isochroman acetals in the presence of BF3·OEt2 afforded 2-substituted chromenes and 1-substituted isochromans, respectively, in good yields. The reactions proceed with a variety of triorganoindium reagents (aryl, heteroaryl, alkynyl, alkenyl, alkyl) using only 50 mol % of the organometallic, thus demonstrating the efficiency of these species. Preliminary mechanistic studies indicate the formation of an oxocarbenium ion intermediate in the presence of the Lewis acid.

Synthesis of Highly Functionalized Triarylbismuthines by Functional Group Manipulation and Use in Palladium- and Copper-Catalyzed Arylation Reactions

Organobismuthines are an attractive class of organometallic reagents that can be accessed from inexpensive and nontoxic bismuth salts. Triarylbismuthines are particularly interesting due to their air and moisture stability and high functional group tolerance. We report herein a detailed study on the preparation of highly functionalized triarylbismuth reagents by triple functional group manipulation and their use in palladium- and copper-catalyzed C-, N-, and O-arylation reactions.

A 2-((4-Arylpiperazin-1-yl)methyl)phenol ligated Pd(ii) complex: an efficient, versatile catalyst for Suzuki–Miyaura cross-coupling reactions

N,N,O-Tridentate palladium(ii) complex 4a was found to be an efficient catalyst for the Suzuki cross-coupling reaction of aryl halides (iodo-, bromo- and chloro-), which afforded cross-coupling products in good to excellent yields.