Structure–Reactivity Studies, Characterization, and Transformation of Intermediates by Lithium Chloride in the Direct Insertion of Alkyl and Aryl Iodides to Metallic Zinc Powder

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.

Surfactant Micellar and Vesicle Microenvironments and Structures under Synthetic Organic Conditions

Fluorescence lifetime imaging microscopy (FLIM) reveals vesicle sizes, structures, microenvironments, reagent partitioning, and system evolution with two chemical reactions for widely used surfactant-water systems under conditions relevant to organic synthesis, including during steps of Negishi cross-coupling reactions. In contrast to previous investigations, the present experiments characterize surfactant systems with representative organohalide substrates at high concentrations (0.5 M) that are reflective of the preparative-scale organic reactions performed and reported in water. In the presence of representative organic substrates, 2-iodoethylbenzene and 2-bromo-6-methoxypyridine, micelles swell into emulsion droplets that are up to 20 μm in diameter, which is 3-4 orders of magnitude larger than previously measured in the absence of an organic substrate (5-200 nm). The partitioning of reagents in these systems is imaged through FLIM─demonstrated here with nonpolar, amphiphilic, organic, basic, and oxidative-addition reactive compounds, a reactive zinc metal powder, and a palladium catalyst. FLIM characterizes the chemical species and/or provides microenvironment information inside micelles and vesicles. These data show that surfactants cause surfactant-dictated microenvironments inside smaller micelles (<200 nm) but that addition of a representative organic substrate produces internal microenvironments dictated primarily by the substrate rather than by the surfactant, concurrent with swelling. Addition of a palladium catalyst causes the internal environments to differ between vesicles─information that is not available through nor predicted from prior analytical techniques. Together, these data provide immediately actionable information for revising reaction models of surfactant-water systems that underpin the development of sustainable organic chemistry in water.

Reactivity Differences of Rieke Zinc Arise Primarily from Salts in the Supernatant, Not in the Solids

Contrary to prevailing thought, the salt content of supernatants is found to dictate reactivity differences of different preparation methods of Rieke zinc toward oxidative addition of organohalides. This conclusion is established through combined single-particle microscopy and ensemble spectroscopy experiments, coupled with careful removal or keeping of the supernatants during Rieke zinc preparations. Fluorescence microscopy experiments with single-Rieke-zinc-particle resolution determined the microscale surface reactivity of the Rieke zinc in the absence of supernatants, thus pinpointing its inherent reactivity independent of the convoluting supernatant composition. In parallel experiments, scanning electron microscopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma-mass spectrometry characterized the zinc metal chemical composition at the bulk and single-particle levels. Proton nuclear magnetic resonance spectroscopy kinetics characterized bench-scale Rieke zinc reactivity in the presence and absence of different supernatants and exogenous salt additives. Together, these experiments show that the differences in reactivity from sodium-reduced vs lithium-reduced Rieke zinc arise from the residual salts in the supernatant rather than the differing salt compositions of the solids. This supernatant salt also determines the structure of the ultimate organozinc product, generating either the diorganozinc or monoorganozinc halide complex. That different organozinc complexes formed upon direct insertion to different preparations of Rieke zinc was not previously reported, despite Rieke zinc's widespread use. These findings impact organozinc-reagent and nanomaterial synthesis by showing that, unexpectedly, desired Rieke zinc reactivity can be achieved through simple addition of soluble salts to solutions that were used to prepare the metals─a substantially easier synthetic manipulation than solid composition and morphology control.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn⁰ or Mn⁰, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

Single-Micelle and Single-Zinc-Particle Imaging Provides Insights into the Physical Processes Underpinning Organozinc Reactions in Water

Micelles on the surfaces of individual metallic zinc particles are imaged by fluorescence microscopy with sensitivity up to single micelles. These micelles are made fluorescent to enable imaging, through the incorporation of boron dipyrromethene fluorophores as representative organic molecular "cargo". Highlighting an advantage of this in situ and sensitive fluorescence technique, the same micelles are not visible by ex situ scanning electron microscopy/energy dispersive X-ray spectroscopy analysis. Examination of micellar solutions with zinc reveals an aging process: micelles do not immediately adhere to the zinc surfaces upon mixing but rather build up over time. Furthermore, at longer times, smaller zinc particles become fully encased in micelle "shells". Once adhered, micelles remain in the local regions of the zinc surface for the duration of the imaging experiments (>2 h). Single micelles are imaged in solution, and their molecular contents are characterized. Two-color fluorescence crossover experiments show that micelles adhered to the surface of the zinc exchange molecular contents with micelles in solution, achieving molecular exchange equilibrium in ∼2.5 h. Unique (non-ensemble averaged) exchange kinetics are displayed by micelles at different locations on the zinc surface, consistent with exchange kinetics of single micelles or small local clusters of micelles. The aging of the micellar solutions and the rate of exchange while on the surface of the zinc suggest that micelle mass transport processes may contribute to overall reaction barriers in sustainable organozinc cross-coupling reactions in micellar water. The observed aging of the system suggests routes for improvement of preparative, bench-scale synthetic reactions involving micellar preparations of organozinc compounds.

Mechanism of an Elusive Solvent Effect in Organozinc Reagent Synthesis

Solvent effects are often difficult to understand in cases where reaction intermediates, and thus their differential behavior in different solvents, are not directly observable by traditional ensemble analytical techniques. Herein, the sensitivity of single-particle fluorescence microscopy uniquely enables direct observation of organozinc intermediates and solvent effects on their build-up and persistence. When combined with NMR spectroscopy, these imaging data pinpoint the previously elusive mechanistic origin of solvent effects in the synthesis of widely used organozinc reagents. These findings characterize the acceleration of oxidative addition of the starting organoiodide to the surface of zinc metal in DMSO relative to THF, but once formed, surface intermediates display similar persistence in either solvent. The current studies are the first demonstration of a highly sensitive, single-particle fluorescence microscopy technique to pinpoint otherwise elusive solvent effects in synthetic chemistry.

Pyrrole-Protected β-Aminoalkylzinc Reagents for the Enantioselective Synthesis of Amino-Derivatives

Chiral β-aminoalkylzinc halides were prepared starting from optically pure commercial β-amino-alcohols. These amino-alcohols were converted to the corresponding N-pyrrolyl-protected alkyl iodides which undergo a zinc insertion in the presence of LiCl (THF, 25 °C, 10-90 min). Subsequent Negishi cross-coupling or acylation reactions with acid chlorides produced amino-derivatives with retention of chirality. Diastereoselective CBS-reductions of some prepared N-pyrrolyl-ketones provided 1,3-subsituted N-pyrrolyl-alcohols with high diastereoselectivity. Additionally, a deprotection procedure involving an ozonolysis allowed the conversion of the pyrrole-ring into a formamide without loss of optical purity.

Organic and Organometallic Chemistry at the Single-Molecule, -Particle, and -Molecular-Catalyst-Turnover Level by Fluorescence Microscopy

Mechanistic studies have historically played a key role in the discovery and optimization of reactions in organic and organometallic chemistry. However, even apparently simple organic and organometallic transformations may have surprisingly complicated multistep mechanisms, increasing the difficulty of extracting this mechanistic information. The resulting reaction intermediates often constitute a small fraction of the total reaction mixture, for example, creating a long-term analytical challenge of detection. This challenge is particularly pronounced in cases where the positions of intermediates on the reaction energy surface mean that they do not "build up" to the quantities needed for observation by traditional ensemble analytical tools. Thus, their existence and single-step elementary reactivity cannot be studied directly. New approaches for obtaining this otherwise-missing mechanistic information are therefore needed. Single-turnover, single-molecule, single-particle, and other subensemble fluorescence microscopy techniques are ideally suited for this role because of their sensitivity and spatiotemporal resolution. Inspired by the robust development of single-molecule fluorescence microscopy tools for studying enzyme catalysis, our laboratory has developed analogous fluorescence microscopy techniques to overcome mechanistic challenges in synthetic chemistry, with sensitivity as high as the single-complex, single-turnover, and single-molecule level. These techniques free the experimenter from the previous restriction that intermediates must "build up" to quantities needed for detection by ensemble analytical tools and are suited to systems where synchronization through flash photolysis or stopped flow would be inconvenient or inaccessible. In this process, the techniques transform certain previously "unobservable" intermediates and their elementary single-step reactivities into "observable" ones through sensitive and selective spectral handles. Our program has focused on imaging reactions in small-molecule, organic, and polymer synthetic chemistry with an accent on the reactivity of molecular transition metal complexes and catalysts. Our laboratory initiated studies in this area in 2008 with the imaging of individual palladium complexes that were tagged with spectator fluorophores. To enable imaging, we started with fluorophore selection and development, overcame challenges with imaging in organic solvents, and developed strategies compatible with air-sensitive chemistry and concentrations of reagents generally used in small-molecule synthesis. These studies grew to include characterization of previously unknown organometallic intermediates in the synthesis of organozinc reagents and the direct study of their elementary-step reactivity. The ability to directly observe this behavior generated predictive power for selecting salts that accelerated organozinc reagent formation in synthesis, including salts that had not yet been reported synthetically. In 2017 we also developed the first single-turnover imaging of molecular (chemo)catalysts, which through the technique's spatiotemporal resolution revealed abruptly time-variable polymerization kinetics wherein molecular ruthenium ring-opening metathesis polymerization (ROMP) catalysts changed rates independently from other catalysts less than 1 μm away. Individual catalytic turnovers, each corresponding to one single-chain-elongation reaction arising from insertion of single ROMP or enyne monomers at individual Grubbs II molecular ruthenium catalysts, were spatiotemporally resolved as green flashes in growing polymers. In this Account, we discuss the development of this technique from idea to application, including challenges overcome and strategies created to image synthetic organic and organometallic molecular chemistry at the highest levels of detection sensitivity. We also describe challenges not yet solved and provide an outlook for this growing field at the intersection of microscopy and synthetic/molecular chemistry.

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.