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.
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 singleparticle microscopy and ensemble spectroscopy experiments, coupled with careful removal or keeping of the supernatants during Rieke zinc preparations. Fluorescence microscopy experiments with single-Riekezinc-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 metalsa substantially easier synthetic manipulation than solid composition and morphology control.
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