Discovery of N-X anomeric amides as electrophilic halogenation reagents.

Electrophilic halogenation is a widely used tool employed by medicinal chemists to either pre-functionalize molecules for further diversity or incorporate a halogen atom into drugs or drug-like compounds to solve metabolic problems or modulate off-target effects. Current methods to increase the power of halogenation rely on either the invention of new reagents or activating commercially available reagents with various additives such as Lewis or Brønsted acids, Lewis bases and hydrogen-bonding activators. There is a high demand for new reagents that can halogenate otherwise unreactive compounds under mild conditions. Here we report the invention of a class of halogenating reagents based on anomeric amides, taking advantage of the energy stored in the pyramidalized nitrogen of N-X anomeric amides as a driving force. These robust halogenating methods are compatible with a variety of functional groups and heterocycles, as exemplified on over 50 compounds (including 13 gram-scale examples and 1 flow chemistry scale-up).

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.

Overcoming the limitations of Kolbe coupling with waveform-controlled electrosynthesis.

The Kolbe reaction forms carbon-carbon bonds through electrochemical decarboxylative coupling. Despite more than a century of study, the reaction has seen limited applications owing to extremely poor chemoselectivity and reliance on precious metal electrodes. In this work, we present a simple solution to this long-standing challenge: Switching the potential waveform from classical direct current to rapid alternating polarity renders various functional groups compatible and enables the reaction on sustainable carbon-based electrodes (amorphous carbon). This breakthrough enabled access to valuable molecules that range from useful unnatural amino acids to promising polymer building blocks from readily available carboxylic acids, including biomass-derived acids. Preliminary mechanistic studies implicate the role of waveform in modulating the local pH around the electrodes and the crucial role of acetone as an unconventional reaction solvent for Kolbe reaction.

Electrochemical PINOylation of Methylarenes: Improving the Scope and Utility of Benzylic Oxidation through Mediated Electrolysis.

A mediated electrosynthetic method has been developed for selective benzylic oxidation of methylarenes. Phthalimide-N-oxyl (PINO) radical generated by proton-coupled electrochemical oxidation of N-hydroxypthalimide serves as a hydrogen atom-transfer (HAT) mediator and as a radical trap for the benzylic radicals generated in situ. This mediated electrolysis method operates at much lower anode potentials relative to direct electrolysis methods for benzylic oxidation initiated by single-electron transfer (SET). A direct comparison of SET and mediated-HAT electrolysis methods with a common set of substrates shows that the HAT reaction exhibits a significantly improved substrate scope and functional group compatibility. The PINOylated products are readily converted into the corresponding benzylic alcohol or benzaldehyde derivative under photochemical conditions, and the synthetic utility of this method is highlighted by the late-stage functionalization of the non-steroidal anti-inflammatory drug celecoxib.

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.

Expanding the Repertoire for "Large Small Molecules": Prodrug ABBV-167 Efficiently Converts to Venetoclax with Reduced Food Effect in Healthy Volunteers.

Since gaining approval for the treatment of chronic lymphocytic leukemia (CLL), the BCL-2 inhibitor venetoclax has transformed the treatment of this and other blood-related cancers. Reflecting the large and hydrophobic BH3-binding groove within BCL-2, venetoclax has significantly higher molecular weight and lipophilicity than most orally administered drugs, along with negligible water solubility. Although a technology-enabled formulation successfully achieves oral absorption in humans, venetoclax tablets have limited drug loading and therefore can present a substantial pill burden for patients in high-dose indications. We therefore generated a phosphate prodrug (3, ABBV-167) that confers significantly increased water solubility to venetoclax and, upon oral administration to healthy volunteers either as a solution or high drug-load immediate release tablet, extensively converts to the parent drug. Additionally, ABBV-167 demonstrated a lower food effect with respect to venetoclax tablets. These data indicate that beyond-rule-of-5 molecules can be successfully delivered to humans via a solubility-enhancing prodrug moiety to afford robust exposures of the parent drug following oral dosing.

Scalable Flow Electrochemical Alcohol Oxidation: Maintaining High Stereochemical Fidelity in the Synthesis of Levetiracetam.

An electrochemical flow process has been developed for an alcohol oxidation step in the synthesis of the generic epilepsy drug levetiracetam. A crucial metric in this process is the retention of high enantiomeric purity as the oxidation of the primary alcohol to the carboxylic acid proceeds via an epimerizable aldehyde intermediate. Here, three different reactor configurations are compared: undivided batch, undivided flow, and divided flow cells. The divided flow cell accesses the highest rate, throughput, and enantiomeric fidelity among the three configurations. This approach is showcased in a 200-gram scale process that retains ≥97% enantiomeric purity and highlights a unique advantage of flow electrolysis.

Sequential Reduction of Nitroalkanes Mediated by CS2 and Amidine/Guanidine Bases: A Controllable Nef Reaction.

In this letter, we describe a mild, functional group-tolerant reductive Nef reaction that utilizes CS2 and an amidine or guanidine base to sequentially cleave N-O bonds. These conditions transform secondary nitroalkanes to ketones via an isolable oxime with minimal erosion at labile stereogenic carbons, show excellent compatibility with groups sensitive to oxidizing or reducing conditions, display good scalability, and are well-suited for generating useful 3-pyrrolidinone motifs from readily accessible 1,3-dipolar cycloaddition products.

A Laser Driven Flow Chemistry Platform for Scaling Photochemical Reactions with Visible Light.

Visible-light-promoted organic reactions can offer increased reactivity and selectivity via unique reaction pathways to address a multitude of practical synthetic problems, yet few practical solutions exist to employ these reactions for multikilogram production. We have developed a simple and versatile continuous stirred tank reactor (CSTR) equipped with a high-intensity laser to drive photochemical reactions at unprecedented rates in continuous flow, achieving kg/day throughput using a 100 mL reactor. Our approach to flow reactor design uses the Beer-Lambert law as a guideline to optimize catalyst concentration and reactor depth for maximum throughput. This laser CSTR platform coupled with the rationale for design can be applied to a breadth of photochemical reactions.

Macrocyclic bis-thioureas catalyze stereospecific glycosylation reactions.

Carbohydrates are involved in nearly all aspects of biochemistry, but their complex chemical structures present long-standing practical challenges to their synthesis. In particular, stereochemical outcomes in glycosylation reactions are highly dependent on the steric and electronic properties of coupling partners; thus, carbohydrate synthesis is not easily predictable. Here we report the discovery of a macrocyclic bis-thiourea derivative that catalyzes stereospecific invertive substitution pathways of glycosyl chlorides. The utility of the catalyst is demonstrated in the synthesis of trans-1,2-, cis-1,2-, and 2-deoxy-ß-glycosides. Mechanistic studies are consistent with a cooperative mechanism in which an electrophile and a nucleophile are simultaneously activated to effect a stereospecific substitution reaction.

The Development of Multidimensional Analysis Tools for Asymmetric Catalysis and Beyond.

In most modern organic chemistry reports, including many of ours, reaction optimization schemes are typically presented to showcase how reaction conditions have been tailored to augment the reaction's yield and selectivity. In asymmetric catalysis, this often involves evaluation of catalyst, solvent, reagent, and, sometimes, substrate features. Such an article will then detail the process's scope, which mainly focuses on its successes and briefly outlines the "limitations". These limitations or poorer-performing substrates are occasionally the result of obvious, significant changes to structure (e.g., a Lewis basic group binds to a catalyst), but frequently, a satisfying explanation for inferior performance is not clear. This is one of several reasons such results are not often reported. These apparent outliers are also commonplace in the evaluation of catalyst structure, although most of this information is placed in the Supporting Information. These practices are unfortunate because results that appear at first glance to be peculiar or poor are considerably more interesting than ones that follow obvious or intuitive trends. In other words, all of the data from an optimization campaign contain relevant information about the reaction under study, and the "outliers" may be the most revealing. Realizing the power of outliers as an entry point to entirely new reaction development is not unusual. Nevertheless, the concept that no data should be wasted when considering the underlying phenomena controlling the observations of a given reaction is at the heart of the strategy we describe in this Account. The idea that one can concurrently optimize a reaction to expose the structural features that control its outcomes would represent a transformative addition to the arsenal of catalyst development and, ultimately, de novo design. Herein we outline the development of a recently initiated program in our lab that unites optimization with mechanistic interrogation by correlating reaction outputs (e.g., electrochemical potential or enantio-, site, or chemoselectivity) with structural descriptors of the molecules involved. The ever-evolving inspiration for this program is rooted in outliers of classical linear free energy relationships. These outliers encouraged us to ask questions about the parameters themselves, suggest potential interactions at the source of the observed effects, and, of particular applicability, identify more sophisticated physical organic descriptors. Throughout this program, we have integrated techniques from disparate fields, including synthetic methodology development, mechanistic investigations, statistics, computational chemistry, and data science. The implementation of many of these strategies is described, and the resulting tools are illustrated in a wide range of case studies, which include data sets with simultaneous and multifaceted changes to the reagent, substrate, and catalyst structures. This tactic constitutes a modern approach to physical organic chemistry wherein no data are wasted and mechanistic hypotheses regarding sophisticated processes can be developed and probed.

Using physical organic parameters to correlate asymmetric catalyst performance.

A classic strategy of physical organic chemists is to probe reaction mechanisms using linear free energy relationships. Identifying such relationships in asymmetric catalytic reactions provides substantial insight into the key factors controlling enantioselectivity, which in turn increases the predictability and applicability of these reactions. The focus of this JOCSynopsis is to highlight several recent examples in which various parameters were identified and applied to the elucidation of LFERs.

Prediction of catalyst and substrate performance in the enantioselective propargylation of aliphatic ketones by a multidimensional model of steric effects.

The effectiveness of a new asymmetric catalytic methodology is often weighed by the number of diverse substrates that undergo reaction with high enantioselectivity. Here we report a study that correlates substrate and ligand steric effects to enantioselectivity for the propargylation of aliphatic ketones. The mathematical model is shown to be highly predictive when applied to substrate/catalyst combinations outside the training set.

Multidimensional steric parameters in the analysis of asymmetric catalytic reactions.

Although asymmetric catalysis is universally dependent on spatial interactions to impart specific chirality on a given substrate, examination of steric effects in these catalytic systems remains empirical. Previous efforts by our group and others have seen correlation between steric parameters developed by Charton and simple substituents in both substrate and ligand; however, more complex substituents were not found to be correlative. Here, we review and compare the steric parameters common in quantitative structure activity relationships (QSAR), a common method for pharmaceutical function optimization, and how they might be applied in asymmetric catalysis, as the two fields are undeniably similar. We re-evaluate steric/enantioselection relationships, which we previously analysed with Charton steric parameters, using the more sophisticated Sterimol parameters developed by Verloop and co-workers in a QSAR context. Use of these Sterimol parameters led to strong correlations in numerous processes where Charton parameters had previously failed. Sterimol parameterization also allows for greater mechanistic insight into the key elements of asymmetric induction within these systems.

Three-dimensional correlation of steric and electronic free energy relationships guides asymmetric propargylation.

Chemical reaction outcomes are often rationalized on the basis of independent analyses of steric and electronic effects. We applied three-dimensional free energy relationships correlating steric and electronic effects to design and optimize a ligand class for the enantioselective Nozaki-Hiyama-Kishi propargylation of ketones. The resultant mathematical model describing the steric and electronic parameter relationship is highly reliant on the synergistic interactions of these two effects.

Predicting and optimizing asymmetric catalyst performance using the principles of experimental design and steric parameters.

Using a modular amino acid based chiral ligand motif, a library of ligands was synthesized systematically varying the substituents at two positions. The effects of these changes on ligand structure were probed in the enantioselective allylation of benzaldehyde, acetophenone, and methylethyl ketone under Nozaki-Hiyama-Kishi conditions. The resulting three-dimensional datasets allowed for the construction of mathematical surface models which describe the interplay of substituent effects on enantioselectivity for a given reaction. The surface models were both extrapolated and manipulated to predict the enantioselective outcomes of several previously untested ligands. Analyses were also used to predict optimal ligand structure of a minimal dataset. Within the dataset, a linear free energy relationship was also discovered and a direct comparison of both the linear prediction as well as the three-dimensional prediction illustrates the potential predictive power of using a three-dimensional model approach to asymmetric catalyst development.

Phase-transfer-catalyzed asymmetric acylimidazole alkylation.

2-Acylimidazoles are alkylated under phase-transfer conditions with cinchonidinium catalysts at -40 degrees C with allyl and benzyl electrophiles in high yield with excellent enantioselectivity (79 to >99% ee). The acylimidazole substrates are made in three steps from bromoacetic acid via the N-acylmorpholine adduct. The catalyst is made in high purity allowing for S-product formation (6-20 h) under mild conditions, consistent with an ion-pair mechanism. The products are readily converted to useful ester products using methyltriflate and sodium methoxide, via a dimethylacylimidazolium intermediate without racemization. The process is efficient, direct, and amenable to other electrophiles and transformations that proceed through an enolate intermediate.