Complex molecule synthesis by electrocatalytic decarboxylative cross-coupling.

Modern retrosynthetic analysis in organic chemistry is based on the principle of polar relationships between functional groups to guide the design of synthetic routes1. This method, termed polar retrosynthetic analysis, assigns partial positive (electrophilic) or negative (nucleophilic) charges to constituent functional groups in complex molecules followed by disconnecting bonds between opposing charges2-4. Although this approach forms the basis of undergraduate curriculum in organic chemistry5 and strategic applications of most synthetic methods6, the implementation often requires a long list of ancillary considerations to mitigate chemoselectivity and oxidation state issues involving protecting groups and precise reaction choreography3,4,7. Here we report a radical-based Ni/Ag-electrocatalytic cross-coupling of substituted carboxylic acids, thereby enabling an intuitive and modular approach to accessing complex molecular architectures. This new method relies on a key silver additive that forms an active Ag nanoparticle-coated electrode surface8,9 in situ along with carefully chosen ligands that modulate the reactivity of Ni. Through judicious choice of conditions and ligands, the cross-couplings can be rendered highly diastereoselective. To demonstrate the simplifying power of these reactions, concise syntheses of 14 natural products and two medicinally relevant molecules were completed.

Ni-electrocatalytic Csp3-Csp3 doubly decarboxylative coupling.

Cross-coupling between two similar or identical functional groups to form a new C-C bond is a powerful tool to rapidly assemble complex molecules from readily available building units, as seen with olefin cross-metathesis or various types of cross-electrophile coupling1,2. The Kolbe electrolysis involves the oxidative electrochemical decarboxylation of alkyl carboxylic acids to their corresponding radical species followed by recombination to generate a new C-C bond3-12. As one of the oldest known Csp3-Csp3 bond-forming reactions, it holds incredible promise for organic synthesis, yet its use has been almost non-existent. From the perspective of synthesis design, this transformation could allow one to agnostically execute syntheses without regard to polarity or neighbouring functionality just by coupling ubiquitous carboxylates13. In practice, this promise is undermined by the strongly oxidative electrolytic protocol used traditionally since the nineteenth century5, thereby severely limiting its scope. Here, we show how a mildly reductive Ni-electrocatalytic system can couple two different carboxylates by means of in situ generated redox-active esters, termed doubly decarboxylative cross-coupling. This operationally simple method can be used to heterocouple primary, secondary and even certain tertiary redox-active esters, thereby opening up a powerful new approach for synthesis. The reaction, which cannot be mimicked using stoichiometric metal reductants or photochemical conditions, tolerates a range of functional groups, is scalable and is used for the synthesis of 32 known compounds, reducing overall step counts by 73%.

Hindered dialkyl ether synthesis with electrogenerated carbocations.

Hindered ethers are of high value for various applications; however, they remain an underexplored area of chemical space because they are difficult to synthesize via conventional reactions1,2. Such motifs are highly coveted in medicinal chemistry, because extensive substitution about the ether bond prevents unwanted metabolic processes that can lead to rapid degradation in vivo. Here we report a simple route towards the synthesis of hindered ethers, in which electrochemical oxidation is used to liberate high-energy carbocations from simple carboxylic acids. These reactive carbocation intermediates, which are generated with low electrochemical potentials, capture an alcohol donor under non-acidic conditions; this enables the formation of a range of ethers (more than 80 have been prepared here) that would otherwise be difficult to access. The carbocations can also be intercepted by simple nucleophiles, leading to the formation of hindered alcohols and even alkyl fluorides. This method was evaluated for its ability to circumvent the synthetic bottlenecks encountered in the preparation of 12 chemical scaffolds, leading to higher yields of the required products, in addition to substantial reductions in the number of steps and the amount of labour required to prepare them. The use of molecular probes and the results of kinetic studies support the proposed mechanism and the role of additives under the conditions examined. The reaction manifold that we report here demonstrates the power of electrochemistry to access highly reactive intermediates under mild conditions and, in turn, the substantial improvements in efficiency that can be achieved with these otherwise-inaccessible intermediates.

Physicochemical Principles of AC Electrosynthesis: Reversible Reactions.

Electrolysis integrates renewable energy into chemical manufacturing and is key to sustainable chemistry. Controlling the waveform beyond direct current (DC) addresses the long-standing obstacle of chemoselectivity, yet it also expands the parameter set to optimize, creating a demand for theoretical predictions. Here, we report the first analytical theory for predicting chemoselectivity in an alternating current (AC) electrosynthesis. The mechanism is a selective reversal of the unwanted redox reaction during periods of opposite polarity, reflected in the final reaction outcome as a time-averaged effect. In the ideal scenario of all redox reactions being reversible, square AC waveform biases the outcome toward more overoxidation/over-reduction, whereas sine AC waveform exhibits the opposite effect. However, in a more realistic scenario of some redox reactions being quasi-reversible, sine AC may behave mostly like square AC. These predictions are in numerical agreement with model experiments employing acetophenone and align qualitatively with the literature precedent. Collectively, this study provides theoretical proof for a growing trend that promotes changing waveforms to overcome limitations challenging to address by varying canonical electrochemical parameters.

Simplified Modular Access to Enantiopure 1,2-Aminoalcohols via Ni-Electrocatalytic Decarboxylative Arylation.

Chiral aminoalcohols are omnipresent in bioactive compounds. Conventional strategies to access this motif involve multiple-step reactions to install the requisite functionalities stereoselectively using conventional polar bond analysis. This study reveals that a simple chiral oxazolidine-based carboxylic acid can be readily transformed to substituted chiral aminoalcohols with high stereochemical control by Ni-electrocatalytic decarboxylative arylation. This general, robust, and scalable coupling can be used to synthesize a variety of medicinally important compounds, avoiding protecting and functional group manipulations, thereby dramatically simplifying their preparation.

Simplifying Access to Targeted Protein Degraders via Nickel Electrocatalytic Cross-Coupling.

C-C linked glutarimide-containing structures with direct utility in the preparation of cereblon-based degraders (PROTACs, CELMoDs) can be assessed in a single step from inexpensive, commercial α-bromoglutarimide through a unique Brønsted-acid assisted Ni-electrocatalytic approach. The reaction tolerates a broad array of functional groups that are historically problematic and can be applied to the simplified synthesis of dozens of known compounds that have only been procured through laborious, wasteful, multi-step sequences. The reaction is scalable in both batch and flow and features a trivial procedure wherein the most time-consuming aspect of reaction setup is weighing out the starting materials.

Stereocontrolled Radical Thiophosphorylation.

The first practical, fully stereoselective P(V)-radical hydrophosphorylation is presented herein by using simple, limonene-derived reagent systems. A set of reagents have been developed that upon radical initiation react smoothly with olefins and other radical acceptors to generate P-chiral products, which can be further diversified (with conventional 2e- chemistry) to a range of underexplored bioisosteric building blocks. The reactions have a wide scope with excellent chemoselectivity, and the unexpected stereochemical outcome has been supported computationally and experimentally. Initial ADME studies are suggestive of the promising properties of this rarely explored chemical space.

Small Change, Big Impact: Reversal of Diastereoselection in Cuprate Conjugate Additions to α,ß-Unsaturated Lactams and Identification of a Competing Mechanism.

A seemingly minor change to a reactant is shown to cause a change in reaction mechanisms. Conjugate addition of organocopper reagents to bicyclic α,ß-unsaturated lactams derived from pyroglutaminol is determined by the nature of the aminal group. Aminals derived from aldehydes give anti addition; those from ketones give syn addition. Divergence in diastereoselection occurs because the substrates react by different mechanisms, ultimately due to a small but significant difference in pyramidalization of the aminal nitrogen.

Scalable Electrochemical Decarboxylative Olefination Driven by Alternating Polarity.

A mild, scalable (kg) metal-free electrochemical decarboxylation of alkyl carboxylic acids to olefins is disclosed. Numerous applications are presented wherein this transformation can simplify alkene synthesis and provide alternative synthetic access to valuable olefins from simple carboxylic acid feedstocks. This robust method relies on alternating polarity to maintain the quality of the electrode surface and local pH, providing a deeper understanding of the Hofer-Moest process with unprecedented chemoselectivity.

Chemoselective (Hetero)Arene Electroreduction Enabled by Rapid Alternating Polarity.

Conventional chemical and even electrochemical Birch-type reductions suffer from a lack of chemoselectivity due to a reliance on alkali metals or harshly reducing conditions. This study reveals that a simpler avenue is available for such reductions by simply altering the waveform of current delivery, namely rapid alternating polarity (rAP). The developed method solves these issues, proceeding in a protic solvent, and can be easily scaled up without any metal additives or stringently anhydrous conditions.

Scalable, Chemoselective Nickel Electrocatalytic Sulfinylation of Aryl Halides with SO2.

Simple access to aryl sulfinates from aryl iodides and bromides is reported using an inexpensive Ni-electrocatalytic protocol. The reaction exhibits a broad scope, uses stock solution of simple SO2 as sulfur source, and can be scaled up in batch and recycle flow settings. The limitations of this reaction are clearly shown and put into context by benchmarking with state-of-the-art Pd-based methods.

Chemoselective Electrosynthesis Using Rapid Alternating Polarity.

Challenges in the selective manipulation of functional groups (chemoselectivity) in organic synthesis have historically been overcome either by using reagents/catalysts that tunably interact with a substrate or through modification to shield undesired sites of reactivity (protecting groups). Although electrochemistry offers precise redox control to achieve unique chemoselectivity, this approach often becomes challenging in the presence of multiple redox-active functionalities. Historically, electrosynthesis has been performed almost solely by using direct current (DC). In contrast, applying alternating current (AC) has been known to change reaction outcomes considerably on an analytical scale but has rarely been strategically exploited for use in complex preparative organic synthesis. Here we show how a square waveform employed to deliver electric current-rapid alternating polarity (rAP)-enables control over reaction outcomes in the chemoselective reduction of carbonyl compounds, one of the most widely used reaction manifolds. The reactivity observed cannot be recapitulated using DC electrolysis or chemical reagents. The synthetic value brought by this new method for controlling chemoselectivity is vividly demonstrated in the context of classical reactivity problems such as chiral auxiliary removal and cutting-edge medicinal chemistry topics such as the synthesis of PROTACs.

N-Ammonium Ylide Mediators for Electrochemical C-H Oxidation.

The site-specific oxidation of strong C(sp3)-H bonds is of uncontested utility in organic synthesis. From simplifying access to metabolites and late-stage diversification of lead compounds to truncating retrosynthetic plans, there is a growing need for new reagents and methods for achieving such a transformation in both academic and industrial circles. One main drawback of current chemical reagents is the lack of diversity with regard to structure and reactivity that prevents a combinatorial approach for rapid screening to be employed. In that regard, directed evolution still holds the greatest promise for achieving complex C-H oxidations in a variety of complex settings. Herein we present a rationally designed platform that provides a step toward this challenge using N-ammonium ylides as electrochemically driven oxidants for site-specific, chemoselective C(sp3)-H oxidation. By taking a first-principles approach guided by computation, these new mediators were identified and rapidly expanded into a library using ubiquitous building blocks and trivial synthesis techniques. The ylide-based approach to C-H oxidation exhibits tunable selectivity that is often exclusive to this class of oxidants and can be applied to real-world problems in the agricultural and pharmaceutical sectors.

A Survival Guide for the "Electro-curious".

The appeal and promise of synthetic organic electrochemistry have been appreciated over the past century. In terms of redox chemistry, which is frequently encountered when forging new bonds, it is difficult to conceive of a more economical way to add or remove electrons than electrochemistry. Indeed, many of the largest industrial synthetic chemical processes are achieved in a practical way using electrons as a reagent. Why then, after so many years of the documented benefits of electrochemistry, is it not more widely embraced by mainstream practitioners? Erroneous perceptions that electrochemistry is a "black box" combined with a lack of intuitive and inexpensive standardized equipment likely contributed to this stagnation in interest within the synthetic organic community. This barrier to entry is magnified by the fact that many redox processes can already be accomplished using simple chemical reagents even if they are less atom-economic. Time has proven that sustainability and economics are not strong enough driving forces for the adoption of electrochemical techniques within the broader community. Indeed, like many synthetic organic chemists that have dabbled in this age-old technique, our first foray into this area was not by choice but rather through sheer necessity. The unique reactivity benefits of this old redox-modulating technique must therefore be highlighted and leveraged in order to draw organic chemists into the field. Enabling new bonds to be forged with higher levels of chemo- and regioselectivity will likely accomplish this goal. In doing so, it is envisioned that widespread adoption of electrochemistry will go beyond supplanting unsustainable reagents in mundane redox reactions to the development of exciting reactivity paradigms that enable heretofore unimagined retrosynthetic pathways. Whereas the rigorous physical organic chemical principles of electroorganic synthesis have been reviewed elsewhere, it is often the case that such summaries leave out the pragmatic aspects of designing, optimizing, and scaling up preparative electrochemical reactions. Taken together, the task of setting up an electrochemical reaction, much less inventing a new one, can be vexing for even seasoned organic chemists. This Account therefore features a unique format that focuses on addressing this exact issue within the context of our own studies. The graphically rich presentation style pinpoints basic concepts, typical challenges, and key insights for those "electro-curious" chemists who seek to rapidly explore the power of electrochemistry in their research.

Chemoselective, Scalable Nickel-Electrocatalytic O-Arylation of Alcohols.

The formation of aryl-alkyl ether bonds through cross coupling of alcohols with aryl halides represents a useful strategic departure from classical SN 2 methods. Numerous tactics relying on Pd-, Cu-, and Ni-based catalytic systems have emerged over the past several years. Herein we disclose a Ni-catalyzed electrochemically driven protocol to achieve this useful transformation with a broad substrate scope in an operationally simple way. This electrochemical method does not require strong base, exogenous expensive transition metal catalysts (e.g., Ir, Ru), and can easily be scaled up in either a batch or flow setting. Interestingly, e-etherification exhibits an enhanced substrate scope over the mechanistically related photochemical variant as it tolerates tertiary amine functional groups in the alcohol nucleophile.

Electrochemically Driven, Ni-Catalyzed Aryl Amination: Scope, Mechanism, and Applications.

C-N cross-coupling is one of the most valuable and widespread transformations in organic synthesis. Largely dominated by Pd- and Cu-based catalytic systems, it has proven to be a staple transformation for those in both academia and industry. The current study presents the development and mechanistic understanding of an electrochemically driven, Ni-catalyzed method for achieving this reaction of high strategic importance. Through a series of electrochemical, computational, kinetic, and empirical experiments, the key mechanistic features of this reaction have been unraveled, leading to a second generation set of conditions that is applicable to a broad range of aryl halides and amine nucleophiles including complex examples on oligopeptides, medicinally relevant heterocycles, natural products, and sugars. Full disclosure of the current limitations and procedures for both batch and flow scale-ups (100 g) are also described.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance.

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Synthetic Organic Electrochemistry: Calling All Engineers.

Unmet potential: Electrochemistry is the most simple and basic way of altering the redox-states of organic molecules. Despite extensive studies and its demonstrated promise, it has yet to take off in mainstream synthesis. The reason is due to engineering challenges in instrument design.

Red-Shifted Fluorogenic Substrate for Detection of lacZ-Positive Cells in Living Tissue with Single-Cell Resolution.

The Escherichia coli lacZ gene encoding ß-galactosidase is a widely used reporter, but few synthetic substrates are available for detecting its activity with single-cell resolution in living samples. Our recently reported fluorogenic substrate SPiDER-ßGal is suitable for this purpose, but its hydrolysis product shows green fluorescence emission, and a red-shifted analogue is therefore required for use in combination with green fluorescent protein (GFP) markers. Herein, we describe the development of a red-shifted fluorogenic substrate for ß-galactosidase, SPiDER-Red-ßGal, based on a silicon rhodol scaffold and a carboxylic group as the intramolecular nucleophile. LacZ-positive cells were successfully labeled with SPiDER-Red-ßGal at single-cell resolution in living samples, which enabled us to visualize different cell types in combination with GFP markers.

Scalable, Electrochemical Oxidation of Unactivated C-H Bonds.

A practical electrochemical oxidation of unactivated C-H bonds is presented. This reaction utilizes a simple redox mediator, quinuclidine, with inexpensive carbon and nickel electrodes to selectively functionalize "deep-seated" methylene and methine moieties. The process exhibits a broad scope and good functional group compatibility. The scalability, as illustrated by a 50 g scale oxidation of sclareolide, bodes well for immediate and widespread adoption.