Mechanism of an Elusive Solvent Effect in Organozinc Reagent Synthesis

Hanada, Erin M.; Jess, Kristof; Blum, Suzanne A.

doi:10.1002/chem.202002214

Cited by 11 publications

(17 citation statements)

References 30 publications

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“…While the individual values of τ are stochastic, the probability distribution is defined by the underlying corrosion mechanism and associated kinetic and statistical parameters. For example, single-molecule kinetics can be used to determine if there are multiple pathways that would be possible for corrosion of iron at the single-molecule level and provide insight into the deviations from linearity observed in Figure C, similar to what has been achieved in single-molecule studies of catalysis, polymers, and organic synthesis. − …”

Section: Results and Discussionmentioning

confidence: 85%

Fluorophores “Turned-On” by Corrosion Reactions Can Be Detected at the Single-Molecule Level

Saini

Messenger

Kisley

2020

ACS Appl. Mater. Interfaces

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We demonstrate that fluorogenic molecules that “turn-on” upon redox reactions can sense the corrosion of iron at the single-molecule scale. We first observe the cathodic reduction of nonfluorescent resazurin to fluorescent resorufin in the presence of iron in bulk solution. The progression of corrosion is seen as a color change that is quantified as an increase in fluorescence emission intensity. We show that the fluorescence signal is directly related to the amount of electrons that are available due to corrosion progression and can be used to quantify the catalyzed increase in the rate of corrosion by NaCl. By using modern fluorescence microscopy instrumentation we detect real-time, single-molecule “turn-on” of resazurin by corrosion, overcoming the previous limitations of microscopic fluorescence corrosion detection. Analysis of the total number of individual resorufin molecules shows heterogeneities during the progression of corrosion that are not observed in ensemble measurements. Finally, we discuss the potential for single-molecule kinetic and super-resolution localization analysis of corrosion based on our findings. Single-molecule florescence microscopy opens up a new spatiotemporal regime to study corrosion at the molecular level.

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Section: Results and Discussionmentioning

confidence: 85%

Fluorophores “Turned-On” by Corrosion Reactions Can Be Detected at the Single-Molecule Level

Saini

Messenger

Kisley

2020

ACS Appl. Mater. Interfaces

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“…The control imaging agents were used to assess whether the imaging agent was merely physiosorbed onto the zinc surface, without formation of a chemical bond through oxidative addition into the carbon–iodide bond, because the control imaging agents lacked carbon–iodide bonds. If bright green fluorescent “hotspots” were formed with the organohalide imaging agent ( 1 , 3 , or 5 ) but not with the corresponding control imaging agent ( 2 or 4 ), then the green “hotspots” were assigned as oxidative addition intermediates. − However, if the corresponding control experiment displayed similar green “hotspots”, then it was concluded that the surface of the zinc physiosorbed both the organohalide imaging agent and the control imaging agent similarly and that there is no evidence to support oxidative addition intermediates (i.e., the signal is consistent with physisorption). Control and noncontrol images were acquired with identical camera settings and laser power and displayed at identical brightness–contrast settings to enable direct comparisons between samples.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2022

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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 metalsa substantially easier synthetic manipulation than solid composition and morphology control.

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“…Perhaps as a consequence, a wide range of methods are utilized to promote reduction events in XEC reactions, often based on empirical optimization. For example, alkali metal halide additives are often used to promote reduction. , The introduction of such additives may serve practical reactivity purposes, but they tend to convolute the catalytic mechanism because, in addition to promoting reduction events, they can also engage in other roles during catalysis. For instance, alkali metal halide additives can impact catalyst speciation by either behaving as an ancillary ligand or engaging in salt metathesis reactions. ,, Additionally, the alkali metal can act as a Lewis acid cocatalyst, or the halide can alter the speciation and reactivity of substrates. , Electron transfer from homogeneous reductants, however, can be more readily investigated and modulated in catalysis because electron-transfer kinetics are generally related to the thermodynamic potential of the reductant, as governed by Marcus and related theories .…”

Section: Potential Mechanistic Advantages Of Homogeneous Organic Redu...mentioning

confidence: 99%

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

et al. 2022

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Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn 0 or Mn 0 , 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.

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Mechanism of an Elusive Solvent Effect in Organozinc Reagent Synthesis

Abstract: Scheme1.(a) Schematic of the broader mechanistic problem with traditional ensembleanalytical techniques. (b) Twom echanistic hypotheses for solvent effects in organozinc synthesis.

Cited by 11 publications

References 30 publications

Fluorophores “Turned-On” by Corrosion Reactions Can Be Detected at the Single-Molecule Level

Fluorophores “Turned-On” by Corrosion Reactions Can Be Detected at the Single-Molecule Level

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

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

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