Kinetic Analysis of the Redox-Neutral Catalytic Mitsunobu Reaction: Dehydration, Kinetic Barriers, and Hopping between Potential Energy Surfaces

Denton's redox-neutral catalytic Mitsunobu reaction is remarkable in that it translates a reaction traditionally driven by the consumption of sacrificial chemical reagents to an additive-free catalytic manifold. Rational attempts to improve the system have been met with only marginal improvements, and a lack of consensus concerning the rate-determining step continues to limit effective reaction development. Here, we analyze the reaction mechanism focusing on a critical, largely overlooked element: the removal of water using a Dean-Stark apparatus. Experimental analysis of the water removal process, coupled with extensive kinetic simulations, demonstrates that the overall rate of the reaction is intimately tied to the rate of water removal. This process can be viewed as a transition between potential energy surfaces and, consequently, subsequent steps of the reaction can progress spontaneously in the absence of water, allowing an explanation of how Le Chatelier's principle, a thermodynamic effect, can have a profound kinetic influence over the rate of the reaction. We identify three bottlenecks in the reaction that inform catalyst design. Additionally, we (a) clarify the ongoing discussion regarding the rate-determining step, (b) provide clear advice concerning future reaction design taking into account the role of water and, (c) discuss the redox-neutral catalytic Mitsunobu reaction in the context of formally endergonic esterification reactions, noting parallels with ratchet mechanisms. Finally, we highlight general principles of catalyst/reaction design that emerge from our analysis and implement our findings to demonstrate a 50% rate acceleration resulting from improved water removal, a substantially greater reaction enhancement than previously obtained from computationally guided catalyst structural changes.

Dehomologative C-C Borylation of Aldehydes and Alcohols via a Rh-Catalyzed Dehydroformylation-Borylation Relay

The dehomologative conversion of linear or α-methyl aldehydes to vinyl boronates is achieved via a one-pot sequence of rhodium-catalyzed transfer dehydroformylation and transfer borylation of the resulting alkenes. Similarly, allylic or aliphatic alcohols are converted to vinyl boronates through a sequence involving, respectively, rhodium-catalyzed isomerization or transfer dehydrogenation to aldehyde intermediates, followed by dehydroformylation-borylation. The vinyl boronates can be further hydrogenated to alkyl boronates using the same rhodium precatalyst, enabling all five catalytic steps with a single catalyst system.

Direct organocatalytic esterification of carboxylic acids and alcohols by redox neutral sulfur(IV) catalysis via intramolecularly interrupted Pummerrer intermediates

Design, synthesis, and catalytic activity of new sulfur(IV) based organocatalysts for the direct esterification of carboxylic acids and alcohols is unveiled. The polar nature of the sulfoxide in the phenol-tethered catalyst accelerates the formation of an intramolecularly interrupted Pummerrer intermediate that further facilitates the catalytic esterification reaction via the activation of carboxylic acids.

Reaction development: a student's checklist

So you've discovered a reaction. But how do you turn this new discovery into a fully-fledged program that maximises the potential of your novel transformation? Herein, we provide a student's checklist to serve as a helpful guide for synthesis development, allowing you to thoroughly investigate the chemistry in question while ensuring that no key aspect of the project is overlooked. A wide variety of the most illuminating synthetic and spectroscopic techniques will be summarised, in conjunction with literature examples and our own insights, to provide sound justifications for their implementation towards the goal of developing new reactions.

Universal Reagent for Mild and Stereospecific Nucleophilic Substitution of Alcohols with Amines

A user-friendly reagent for mild and general activation of alcohols towards bimolecular nucleophilic substitution (SN2) leveraging diverse nucleophiles, including primary and secondary amines is reported herein. The new ion-paired reagent discovery was based upon the putative zwitterionic betaine intermediate of the Mitsunobu reaction and enabled the one-step conversion of enantioenriched alcohols to valuable chiral C-X bonds (where X=N, C, S, O or halide). The described activating reagent has also been applied to a one-step methylation reaction using methanol and to an intermolecular amination/intramolecular cyclization sequence that generates heterocycles, such as tetrahydroisoquinolines. This work provides the first evidence by X-ray crystallography of a protonated betaine as intermediate in the Mitsunobu reaction.

Recent Advances in the Mitsunobu and Related Reactions: A Review from 2010 to 2024

This review discusses recent progress in the most significant synthetic approaches involving transformations under the Mitsunobu reaction. The Mitsunobu reaction entails the "redox" condensation of an acidic pronucleophile 'Nu-H' and an electrophilic primary or secondary alcohol, facilitated by stoichiometric amounts of phosphines and azodicarboxylate reagents. Widely utilized for dehydrative oxidation-reduction condensation, this reaction shows synthetic utility through its tolerance of a broad range of acidic pronucleophiles, including carboxylic acids, pro-imides, hydroxamates, phenols, thiols, fluorinated alcohols, oximes, thioamides, pyridinium and imidazolium salts, pyrimidine bases, α-ketoesters, and trimethylmethane tricarboxylate, thereby yielding a variety of functional and potentially biologically active compounds. The purpose of this review is to focus on recent advances and applications of Mitsunobu reaction chemistry, particularly from 2010 to 2024. In addition to discussing newer reagents that facilitate purification, we will describe contemporary applications of this chemistry, especially concerning the synthesis of potential biological compounds and their precursors. This focus review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications. We aim for this work to serve as a useful resource for scientists working in this research domain.

Beyond symmetric self-assembly and effective molarity: unlocking functional enzyme mimics with robust organic cages

The bespoke environments in enzyme active sites can selectively accelerate chemical reactions by as much as 1019. Macromolecular and supramolecular chemists have been inspired to understand and mimic these accelerations and selectivities for applications in catalysis for sustainable synthesis. Over the past 60+ years, mimicry strategies have evolved with changing interests, understanding, and synthetic advances but, ubiquitously, research has focused on use of a molecular "cavity". The activities of different cavities vary with the subset of features available to a particular cavity type. Unsurprisingly, without synthetic access to mimics able to encompass more/all of the functional features of enzyme active sites, examples of cavity-catalyzed processes demonstrating enzyme-like rate accelerations remain rare. This perspective will briefly highlight some of the key advances in traditional cavity catalysis, by cavity type, in order to contextualize the recent development of robust organic cage catalysts, which can exploit stability, functionality, and reduced symmetry to enable promising catalytic modes.

Direct carbonyl reductive functionalizations by diphenylphosphine oxide

Reductive functionalization of aldehydes and ketones is one of the most challenging but ultimately rewarding areas in synthetic chemistry and related sciences. We report a simple and extremely versatile carbonyl reductive functionalization strategy achieving direct, highly selective, and efficient reductive amination, etherification, esterification, and phosphinylation reactions of (hetero)aryl aldehydes and ketones, which are extremely challenging or unattainable to achieve by traditional strategies, using only diphenylphosphine oxide and an inorganic base. It enables modular synthesis of functionally and structurally diverse tertiary amines, ethers, esters, phosphine oxides, etc., as well as related pesticides, drug intermediates, and pharmaceuticals. Compared to phosphorus-mediated name reactions, this strategy firstly transformed C═O bonds into C-element single bonds. Mechanistically, phosphinates are formed as intermediates, which undergo unconventional nucleophilic substitution at the C atom within their C─O─P unit. Thus, this work provides important strides in the field of reductive functionalization of aldehydes/ketones, phosphorus-mediated transformation, and various fundamental reactions.

Triflic Acid-Catalyzed Dehydrative Amination of 2-Arylethanols with Weak N-Nucleophiles in Hexafluoroisopropanol

The catalytic deoxyamination of readily available 2-arylethanols offers an appealing, simple, and straightforward means of accessing β-(hetero)arylethylamines of biological interest. Yet, it currently represents a great challenge to synthetic chemistry. In most cases, the alcohol has to be either pre-activated in situ or converted into a reactive carbonyl intermediate, limiting the substrate scope for some methods. Examples of direct dehydrative amination of 2-arylethanols are thus still scarce. Here, we describe a catalytic protocol based on the synergy of triflic acid and hexafluoroisopropanol, which enables the direct and stereospecific amination of a broad array of 2-arylethanols, and does not require any pre-activation of the alcohol. This approach yields high value-added products incorporating sulfonamide, amide, urea, and aniline functionalities. In addition, this approach was applied to the sulfidation of 2-arylethanols. Mechanistic experiments and DFT computations indicate the formation of phenonium ions as key intermediates in the reaction.

Scale-Up of Continuous Metallaphotoredox Catalyzed C-O Coupling to a 10 kg-Scale Using Small Footprint Photochemical Taylor Vortex Flow Reactors

We report the development and optimization of a scalable flow process for metallaphotoredox (Ir/Ni) C-O coupling, a mild and efficient approach for forming alkyl-aryl ethers, a common motif in medicinal and process chemistry settings. Time-resolved infrared spectroscopy (TRIR) highlighted the amine as the major quencher of the photocatalyst triplet excited state, along with the formation of an Ir(II) species that, in the presence of the Ni cocatalyst, has its lifetime shortened, suggesting reductive quenching of Ir(III)*, followed by reoxidation facilitated by the Ni cocatalyst. TRIR and batch reaction screening was used to develop conditions transferrable to flow, and many processing benefits of performing the reaction in flow were then demonstrated using a simple to construct/operate, small-footprint FEP coil flow reactor, including short (<10 min) space times and reduced catalyst loadings (down to 0.1 mol % Ir, 1 mol % Ni) while retaining good yield/conversion. Scalability was demonstrated by increasing the length or diameter of the FEP coil flow reactor tubing, however, due to suspected mass transfer/mixing limitations, the yield decreased upon scale-up in some cases. Therefore, we applied a modified version of our previously reported photochemical Taylor Vortex Flow Reactor (PhotoVortex), where Taylor vortices and a short-irradiated path length allow photochemical reactions to be performed efficiently via excellent mixing. In a small PhotoVortex (8 mL irradiated volume), we have demonstrated projected productivities around 1 kg day-1 and >10 kg day-1 in a large PhotoVortex (185 mL irradiated volume) with good product yields (>90%) and low catalyst loadings (0.1 to 0.5 mol % of [Ir{dF(CF3)ppy}2dtbbpy]PF6), enabled by excellent mixing ensuring sufficient mass transfer between short-lived photoexcited and other transient species.

Hydrogen-Bonded Di(hydroperoxy)alkane Adducts of the Type Cy3P=O·(HOO)2CHR (R = Alkyl)

Five representatives of a novel type of di(hydroperoxy)alkane adducts of phosphine oxides have been synthesized and fully characterized, including their solubility in organic solvents. The phosphine oxide Cy3PO (1) has been used in combination with the corresponding aldehydes to create the adducts Cy3PO·(HOO)2CHCH3 (2), Cy3PO·(HOO)2CHCH2CH3 (3), Cy3PO·(HOO)2CH(CH2)2CH3 (4), Cy3PO·(HOO)2CH(CH2)3CH3 (5), and Cy3PO·(HOO)2CH(CH2)7CH3 (6). All adducts crystallize easily and contain the peroxide and phosphine oxide hydrogen-bonded in 1:1 ratios. The single crystal X-ray structures of 2-6 and their unique features are discussed. The 31P NMR spectra of the adducts 2-6 show downfield-shifted signals as compared to Cy3PO. In the IR spectra, the ν(P=O) wavenumbers of the adducts have smaller values than the neat phosphine oxide. All spectroscopic results of 2-6 show that the P=O bond is weakened by hydrogen-bonding to the di(hydroperoxy)alkane moieties. Adduct 6 selectively oxidizes PPh3 to OPPh3 within minutes, and nonanal is reformed in the process. The easy synthesis, handling, and administration of these stable, solid, and soluble peroxides with well-defined composition will have a positive impact on synthetic chemistry.

Pnictogen and Chalcogen Salts as Alkylating Agents

Alkylation reactions and their products are considered crucial in various contexts. Synthetically, the alkylation of a nucleophile is usually promoted using hazardous alkyl halides. Here, we aim to highlight the potential of pnictogen (ammonium or phosphonium) and chalcogen salts (sulfonium, selenonium, and telluronium) to function as alkylating agents. These compounds can be considered as non-volatile electrophilic alkyl reservoirs. We will center our discussion on the strategies developed in recent years to expand the synthetic utility of these salts in terms of transferable alkyl groups, substrate scope, and product selectivity.

Polymer-supported strong Lewis acid phosphonium cation catalysis applied to sydnone synthesis

Organochlorophosphonium P(V) species are strong Lewis acids deriving from a low-lying σ* orbital at P opposite to Cl. Herein, applying this strong acidity to heterogenous reactivity, we introduce polymer-supported phosphorus(V)-mediated Lewis acid catalysis. Key to this innovation is the use of a recyclable solid support Merrifield resin P(V) Lewis acid catalyst with demonstrated utility for the one-pot synthesis of sydnones. This concept of pnictogen P(V) Lewis acid catalysis is simple to apply, selective, and benefits from mild conditions with short reaction times. Further, experimental and computational findings reveal the crucial mechanistic role of phosphonium cation P(V) species in these reactions.

Novel Guanidinium-Functionalized Stigmasterol for Bile Salt Binding and Serum Cholesterol Reduction: Synthesis, Interaction Mechanisms, and In Vivo Function

A novel amphiphilic guanidyl-functionalized stigmasterol hydrochloride (GFSH) was designed and synthesized as bile salt sequestrants for cholesterol reduction. GFSH exhibited a considerable in vitro capacity for bile salt binding in gastrointestinal digestion and alleviated hypercholesterolemia in vivo. GFSH spontaneously interacted with sodium cholate via synergistic electrostatic, hydrophobic, and hydrogen-bonding interactions. The effects of GFSH on serum cholesterol reduction in mice fed a high-fat-high-cholesterol diet were explored by measuring the expression of key transcription factors related to bile acid metabolism. GFSH produced a dose-dependent reduction in weight gain, hepatic fat accumulation, and fecal and blood markers. Real-time quantitative polymerase chain reaction (RT-qPCR) and western blot analyses demonstrated GFSH-induced expression of hepatic CYP7A, LXRα, and LDL-R. GFSH exerts the cholesterol-lowering activity by inducing the bile acid metabolism.

SO2F2 mediated click chemistry enables modular disulfide formation in diverse reaction media

The dynamic disulfide linkage plays a vital role in various biological processes as well as drugs and biomaterials. The conversion of thiols to their corresponding disulfides is a hallmark of sulfur chemistry, but notoriously difficult to control. Achieving optimal reactivity and selectivity continues to pose significant challenges. Here, we describe a click chemistry for disulfide formation from thiols in both batch and flow-mode using SO2F2, which display exceptional selectivity toward disulfide formation through an effective nucleophilic substitution cascade. This reaction's unique characteristics satisfy the stringent click-criteria with its high thermodynamic driving force, straightforward conditions, wide scope, quantitative yields, exceptional chemoselectivity, and non-chromatographic purification process. The modular synthesis of symmetrical, unsymmetrical, cyclic and polydisulfides is demonstrated, along with the formation of disulfide cross-linked hydrogels.

Electrochemical Cyanation of Alcohols Enabled by an Iodide-Mediated Phosphine P(V/III) Redox Couple

We report herein a mild electrochemical method to transform alcohols into their corresponding nitriles by using commercially available reagents. This protocol accepts substrates with various functional groups including those that are susceptible to oxidative decomposition. Mechanistic studies revealed a critical iodide-mediated phosphine electrochemical oxidation pathway leading to the alkoxyphosphonium intermediate, followed by nucleophilic substitution by a cyanide nucleophile. This method demonstrates the use of electrochemistry in replacing azo-type reagents in direct nucleophilic substitution and homologation of alcohol substrates.

Base-Promoted Nucleophilic Phosphorylation of Benzyl Fluorides via C(sp3)-F Cleavage

Herein, a transition-metal-free phosphorylation of benzyl fluorides with P(O)-H compounds is disclosed. In the presence of tBuOK, various benzyl fluorides react with P(O)-H compounds to produce the corresponding benzyl phosphine oxides, phosphinates, and phosphonates in good to high yields. This base-promoted phosphorylation reaction offers a facile and general strategy for the construction of a C(sp3)-P bond.

Asymmetric total synthesis of polycyclic xanthenes and discovery of a WalK activator active against MRSA

The development of new antibiotics continues to pose challenges, particularly considering the growing threat of multidrug-resistant Staphylococcus aureus. Structurally diverse natural products provide a promising source of antibiotics. Herein, we outline a concise approach for the collective asymmetric total synthesis of polycyclic xanthene myrtucommulone D and five related congeners. The strategy involves rapid assembly of the challenging benzopyrano[2,3-a]xanthene core, highly diastereoselective establishment of three contiguous stereocenters through a retro-hemiketalization/double Michael cascade reaction, and a Mitsunobu-mediated chiral resolution approach with high optical purity and broad substrate scope. Quantum mechanical calculations provide insight into stereoselective construction mechanism of the three contiguous stereocenters. Additionally, this work leads to the discovery of an antibacterial agent against both drug-sensitive and drug-resistant S. aureus. This compound operates through a unique mechanism that promotes bacterial autolysis by activating the two-component sensory histidine kinase WalK. Our research holds potential for future antibacterial drug development.

Mechanism of the Mitsunobu Reaction: An Ongoing Mystery

The Mitsunobu reaction is one the most widely known reactions in the organic chemistry canon. Despite its fame, some aspects of the mechanism remain poorly understood, 55 years after its initial discovery. This short review collates the findings of several publications focused on the mechanism of the Mitsunobu reaction, highlighting both the current state of knowledge and the remaining missing pieces.

Surface-Assisted Selective Air Oxidation of Phosphines Adsorbed on Activated Carbon

Trialkyl- and triarylphosphines readily adsorb onto the surface of porous activated carbon (AC) even in the absence of solvents through van der Waals interactions between the lone electron pair and the AC surface. This process has been proven by solid-state NMR techniques. Subsequently, it is demonstrated that the AC enables the fast and selective oxidation of adsorbed phosphines to phosphine oxides at ambient temperature in air. In solution, trialkylphosphines are oxidized to a variety of P(V) species when exposed to the atmosphere, while neat or dissolved triarylphosphines cannot be oxidized with air. When the trialkyl- and triarylphosphines PnBu3 (1), PEt3, (2), PnOct3 (3), PMetBu2 (4), PCy3 (5), and PPh3 (6) are adsorbed in a mono- or submonolayer on the surface of AC, in the absence of a solvent and at ambient temperature, they are quantitatively oxidized to the adsorbed phosphine oxides, 1ox-6ox, once air is admitted. No formation of any unwanted P(V) side products or water adducts is observed. The phosphine oxides can then be recovered in good yields by washing them off of the AC. The oxidation is likely facilitated by a radical activation of molecular oxygen due to delocalized electrons on the aromatic surface coating of AC, as proven by ESR. This easy and inexpensive oxidation method renders hydrogen peroxide or other oxidizers unnecessary and is broadly applicable to sterically hindered and even to air-stable triarylphosphines. Phosphines adsorbed at lower surface coverages on AC oxidize at a faster rate. All oxidation reactions were monitored by solution- and solid-state NMR spectroscopy.

Electrochemical Azo-free Mitsunobu-type Reaction

The classic chemical Mitsunobu reaction suffers from the need of excess alcohol activation reagents and the generation of significant by-products. Efforts to overcome these limitations have resulted in numerous creative solutions, but the substrate scope of these catalytic processes remains limited. Here we report an electrochemical Mitsunobu-type reaction, which features azo-free alcohol activation and broad substrate scope. This user-friendly technology allows a vast collection of heterocycles as the nucleophile, which can couple with a series of chiral cyclic and acyclic alcohols in moderate to high yields and excellent ee's. This practical reaction is scalable, chemoselective, uses simple Electrasyn setup with inexpensive electrodes and requires no precaution to exclude air and moisture. The synthetic utility is further demonstrated on the structural modification of diverse bioactive natural products and pharmaceutical derivatives and its straightforward application in a multiple-step synthesis of a drug candidate.

Skeletal rearrangement of 6,8-dioxabicyclo[3.2.1]octan-4-ols promoted by thionyl chloride or Appel conditions

A skeletal rearrangement of a series of 6,8-dioxabicyclo[3.2.1]octan-4-ols has been developed using SOCl2 in the presence of pyridine. An oxygen migration from C5 to C4 was observed when the C4 alcohols were treated with SOCl2/pyridine, giving a 2-chloro-3,8-dioxabicyclo[3.2.1]octane ring-system via the chlorosulfite intermediate. Analogous allylic alcohols with endocyclic and exocyclic unsaturations underwent chlorination without rearrangement due to formation of allylic cations. The rearrangement was also demonstrated using Appel conditions, which gave similar results via the alkoxytriphenylphosphonium intermediate. Several reactions of the products were investigated to show the utility of the rearrangement.

Stepwise Operation of a Molecular Rotary Motor Driven by an Appel Reaction

To date, only a small number of chemistries and chemical fueling strategies have been successfully used to operate artificial molecular motors. Here, we report the 360° directionally biased rotation of phenyl groups about a C-C bond, driven by a stepwise Appel reaction sequence. The motor molecule consists of a biaryl-embedded phosphine oxide and phenol, in which full rotation around the biaryl bond is blocked by the P-O oxygen atom on the rotor being too bulky to pass the oxygen atom on the stator. Treatment with SOCl2 forms a cyclic oxyphosphonium salt (removing the oxygen atom of the phosphine oxide), temporarily linking the rotor with the stator. Conformational exchange via ring flipping then allows the rotor and stator to twist back and forth past the previous limit of rotation. Subsequently, the ring opening of the tethered intermediate with a chiral alcohol occurs preferentially through a nucleophilic attack on one face. Thus, the original phosphine oxide is reformed with net directional rotation about the biaryl bond over the course of the two-step reaction sequence. Each repetition of SOCl2-chiral alcohol additions generates another directionally biased rotation. Using the same reaction sequence on a derivative of the motor molecule that forms atropisomers rather than fully rotating 360° results in enantioenrichment, suggesting that, on average, the motor molecule rotates in the "wrong" direction once every three fueling cycles. The interconversion of phosphine oxides and cyclic oxyphosphonium groups to form temporary tethers that enable a rotational barrier to be overcome directionally adds to the strategies available for generating chemically fueled kinetic asymmetry in molecular systems.

Sulfination of Unactivated Allylic Alcohols via Sulfinate-Sulfone Rearrangement

A dehydrative cross-coupling of unactivated allylic alcohols with sulfinic acids was achieved under catalyst-free conditions. This reaction proceeded via allyl sulfination and concomitant allyl sulfinate-sulfone rearrangement. Various allylic sulfones could be obtained in good to excellent yields with water as the only byproduct. This study expands the synthetic toolbox for constructing allylic sulfone molecules.

Ionic-Liquid Hydrogen-Bonding Promoted Alcohols Amination over Cobalt Catalyst via Dihydrogen Autotransfer Mechanism

Higher amines are important high-valuable chemicals with wide applications, and amination of alcohols is a green route to them, which however generally suffers from harsh reaction conditions and use of equivalent base. Herein, we report an ionic-liquid (IL) hydrogen-bonding promoted dihydrogen autotransfer strategy for amination of alcohols to higher amines over cobalt catalyst under base-free conditions. Co(BF4 )2  ⋅ 6 H2 O complexed with triphos and IL (e. g., tetrabutylphosphonium tetrafluoroborate, [P4444 ][BF4 ]) shows high performances for the reaction and is tolerant of a wide scope of amines and alcohols, affording higher amines in good to excellent yields. Mechanism investigation indicates that the [BF4 ]- anion activates the alcohol via hydrogen bonding, promoting transfer of both hydroxyl H and α-H atoms of alcohol to the cobalt catalyst to form an aldehyde intermediate and cobalt dihydride complex, which are involved in the subsequent reductive amination. This strategy provides a green and effective route for alcohol amination, which may have promising applications in alcohol-involved alkylation reactions.

Enantioconvergent transformations of secondary alcohols through borrowing hydrogen catalysis

Direct substitution of readily available alcohols is recognized as a key research area in green chemical synthesis. Starting from simple racemic secondary alcohols, the achievement of catalytic enantioconvergent transformations of the substrates will be highly desirable for efficient access to valuable enantiopure compounds. To accomplish such attractive yet challenging transformations, the strategy of the enantioconvergent borrowing hydrogen methodology has proven to be uniquely effective and versatile. This review aims to provide an overview of the impressive progress made on this topic of research that has only thrived in the past decade. In particular, the conversion of racemic secondary alcohols to enantioenriched chiral amines, N-heterocycles, higher-order alcohols and ketones will be discussed in detail.

Electrochemically Driven Deoxygenative Borylation of Alcohols and Carbonyl Compounds

We present a new, unified approach for the transformation of benzylic and allylic alcohols, aldehydes, and ketones into boronic esters under electroreductive conditions. Key to our strategy is the use of readily available pinacolborane, which serves both as an activator and an electrophile by first generating a redox-active trialkylborate species and then delivering the desired deoxygenatively borylated product. This strategy is applicable to a variety of substrates and can be employed for the late-stage functionalization of complex molecules.

An Electrochemical Design for Catalytic Dehydration: Direct, Room-Temperature Esterification without Acid or Base Additives

An electrochemical approach has been leveraged to underpin a new conceptual platform for dehydration reactions, which has been demonstrated in the context of esterification. Esters were prepared from the corresponding acid and alcohol partners at room temperature without acid or base additives and without consuming stoichiometric reagents. This methodology therefore addresses key complications that plague esterification and dehydration reactions more broadly and that represent a leading challenge in synthetic chemistry.

Organophosphorus-Catalyzed "Dual-Substrate Deoxygenation" Strategy for C-S Bond Formation from Sulfonyl Chlorides and Alcohols/Acids

A green method to construct C-S bonds using sulfonyl chlorides and alcohols/acids via a P^III/P^V═O catalytic system is reported. The organophosphorus-catalyzed umpolung reaction promotes us to propose the "dual-substrate deoxygenation" strategy. Herein, we adopt the "dual-substrate deoxygenation" strategy, which achieves the deoxygenation of sulfonyl chlorides and alcohols/acids to synthesize thioethers/thioesters driven by P^III/P^V═O redox cycling. The catalytic method represents an operationally simple approach using stable phosphine oxide as a precatalyst and shows broad functional group tolerance. The potential application of this protocol is demonstrated by the late-stage diversification of drug analogues.

Expanding Reaction Profile of Allyl Carboxylates via 1,2-Radical Migration (RaM): Visible-Light-Induced Phosphine-Catalyzed 1,3-Carbobromination of Allyl Carboxylates

Allyl carboxylates are useful synthetic intermediates in a variety of organic transformations, including catalytic nucleophilic/electrophilic allylic substitution reactions and 1,2-difunctionalization reactions. However, the catalytic 1,3-difunctionalization of allyl carboxylates remains elusive. Herein, we report the first photoinduced, phosphine-catalyzed 1,3-carbobromination of allyl carboxylates, affording a range of valuable substituted isopropyl carboxylates (sIPC). The transformation has broad functional group tolerance, is amenable to the late-stage modification of complex molecules and gram-scale synthesis, and expands the reaction profiles of allyl carboxylates and phosphine catalysis. Preliminary experimental and computational studies suggest a non-chain-radical mechanism involving the formation of an electron donor-acceptor complex, 1,2-radical migration (RaM), and Br-atom transfer processes. We anticipate that the 1,2-RaM reactivity of allyl carboxylates and the phosphine-catalyzed radical reaction will both serve as a platform for the development of new transformations in organic synthesis.

Recent advances in electrochemical deoxygenation reactions of organic compounds

As naturally abundant and recyclable industrial feedstock, alcohols and carboxylic acids have drawn tremendous attention in medicinal chemistry and polymer chemistry. The selective C-O cleavage of the hydroxyl group represents an appealing strategy to deliver alkyl and carbonyl moieties into organic molecules. Classical examples of hydroxyl activation include the Appel reaction, Mitsunobu reaction, and Barton-McCombie deoxygenation. However, these early approaches still require large amounts of oxidants or reductants, and suffer from harsh conditions and low atom economy. Electrosynthesis has proven to be an effective and mild way of the modern chemical industry, avoiding the use of chemical oxidants/reductants through the action of an electric current. In this review, we have summarized the recent advances in electrochemical deoxygenation reactions and categorized the deoxygenation methods by different functionalities.

Alkoxyphosphorane/Borane Cooperative Alkylations: A Frustrated Lewis Pair Version of the Mitsunobu Reaction

The combination of the alkoxyphosphoranes, Ph₂ P(OR)(O₂ C₆ Cl₄ ) and the borane B(C₆ F₅ )₃ generates the zwitterions 3 which act as FLP to effect the alkylation of several nucleophiles affording C-C, C-N, C-H and C-Cl coupling products. A DFT study shows the reaction proceeds via an FLP activation pathway generating an alkoxyphosphonium intermediate which effects the alkylation of the nucleophiles, akin to the Mitsunobu reaction.

Electrosynthetic C-O Bond Activation in Alcohols and Alcohol Derivatives

Alcohols and their derivatives are ubiquitous and versatile motifs in organic synthesis. Deoxygenative transformations of these compounds are often challenging due to the thermodynamic penalty associated with the cleavage of the C-O bond. However, electrochemically driven redox events have been shown to facilitate the C-O bond cleavage in alcohols and their derivatives either through direct electron transfer or through the use of electron transfer mediators and electroactive catalysts. Herein, a comprehensive overview of preparative electrochemically mediated protocols for C-O bond activation and functionalization is detailed, including direct and indirect electrosynthetic methods, as well as photoelectrochemical strategies.

Electrostatically tuned phenols: a scalable organocatalyst for transfer hydrogenation and tandem reductive alkylation of N-heteroarenes

One of the fundamental aims in catalysis research is to understand what makes a certain scaffold perform better as a catalyst than another. For instance, in nature enzymes act as versatile catalysts, providing a starting point for researchers to understand how to achieve superior performance by positioning the substrate close to the catalyst using non-covalent interactions. However, translating this information to a non-biological catalyst is a challenging task. Here, we report a simple and scalable electrostatically tuned phenol (ETP) as an organocatalyst for transfer hydrogenation of N-arenes using the Hantzsch ester as a hydride source. The biomimetic catalyst (1-5 mol%) displays potential catalytic activity to prepare diverse tetrahydroquinoline derivatives with good to excellent conversion under ambient reaction conditions. Kinetic studies reveal that the ETP is 130-fold faster than the uncharged counterpart, towards completion of the reaction. Control experiments and NMR spectroscopic investigations elucidate the role of the charged environment in the catalytic transformation.

A binuclear aluminium complex as a single competent catalyst for efficient synthesis of urea, biuret, isourea, isothiourea, phosphorylguanidine, and quinazolinones

The synthesis and characterisation of two mononuclear aluminium alkyl complexes with the general composition [Al(Me)2{Ph2P(E)N(CH2)2N(CH2CH2)2O}] (E = Se (2a); S (2b)), and two binuclear aluminium complexes, [Al(Me)2{Ph2P-(E)N(CH2)2N(CH2CH2)2O}(AlMe3)] (E = Se (3a) and S (3b)), are described. The binuclear aluminium alkyl complex 3a proved to be a proficient catalyst for the addition of simple nucleophiles to heterocumulenes, leading to the synthesis of a variety of products such as urea, biuret, isourea, isothiourea, phosphorylguanidine, and quinazolinone derivatives, in contrast to its mononuclear analogues. Complex 3a is the first example of a single competent catalyst, which is also low-cost and eco-friendly and derived from a main-group metal, under solvent-free conditions either at room temperature or mild temperatures. Complex 3a possessed a wide functional group tolerance including heteroatoms, yielding the corresponding insertion products in good quantities and with high selectivity.

One-Pot and Unsymmetrical Bis-Allylation of Malononitrile with Conjugated Dienes and Allylic Alcohols

A Pd/Ca catalytic system to promote the unsymmetrical bis-allylation of malononitrile was developed by selecting conjugated dienes and allylic alcohols as allylic reagents. This catalytic system suppressed the competitive symmetrical bis-allylation process and guaranteed the desired unsymmetrical bis-allylation with high chemoselectivity. A wide range of conjugated dienes and allylic alcohols were tolerated well in this transformation, and diverse 1,6-dienes were obtained with high efficiency.

Catalytic Vilsmeier-Haack Reactions for C1-Deuterated Formylation of Indoles

The Vilsmeier-Haack reaction is a powerful tool to introduce formyl groups into electron-rich arenes, but its wide application is significantly restricted by stoichiometric employment of caustic POCl₃. Herein, we reported a catalytic version of the Vilsmeier-Haack reaction enabled by a P(III)/P(V)═O cycle. This catalytic reaction provides a facile and efficient route for the direct construction of C1-deuterated indol-3-carboxaldehyde under mild conditions with stoichiometric DMF-d7 as the deuterium source. The products feature a remarkably higher deuteration level (>99%) than previously reported ones and are not contaminated by the likely unselective deuteration at other sites. The present transformation can also be used to transfer other carbonyl groups. Further downstream derivatizations of these deuterated products manifested their potential applications in the synthesis of deuterated bioactive molecules. Mechanistic insight was disclosed from studies of kinetics and intermediates.

Cyclopropenium Ions in Catalysis

Cyclopropenium ions are the smallest class of aromatic compounds, satisfying Hückel's rules of aromaticity with two π electrons within a three-membered ring. First prepared by Breslow in 1957, cyclopropenium ions have been found to possess extraordinary stability despite being both cationic and highly strained. In the 65 years since their first preparation, cyclopropenium ions have been the subject of innumerable studies concerning their synthesis, physical properties, and reactivity. However, prior to our work, the reactivity of these unique carbocations had not been exploited for reaction promotion or catalysis.Over the past 13 years, we have been exploring aromatic ions as unique and versatile building blocks for the development of catalysts for organic chemistry. A major portion of this work has been focused on leveraging the remarkable properties of the smallest of the aromatic ions─cyclopropeniums─as a design element in the invention of highly reactive catalysts. Indeed, because of its unique profile of hydrolytic stability, compact geometry, and relatively easy oxidizability, the cyclopropenium ring has proven to be a highly advantageous construction module for catalyst invention.In this Account, we describe some of our work using cyclopropenium ions as a key element in the design of novel catalysts. First, we discuss our early work aimed at promoting dehydrative reactions, starting with Appel-type chlorodehydrations of alcohols and carboxylic acids, cyclic ether formations, and Beckmann rearrangements and culminating in the realization of catalytic chlorodehydrations of alcohols and a catalytic Mitsunobu-type reaction. Next, we describe the development of cyclopropenimines as strong, neutral organic Brønsted bases and, in particular, the use of chiral cyclopropenimines for enantioselective Brønsted catalysis. We also describe the development of higher-order cyclopropenimine superbases. The use of tris(amino)cyclopropenium (TAC) ions as a novel class of phase-transfer catalysts is discussed for the reaction of epoxides with carbon dioxide. Next, we describe the formation of a cyclopropenone radical cation that has a portion of its spin density on the oxygen atom, leading to some peculiar metal ligand behavior. Finally, we discuss recent work that employs TAC electrophotocatalysts for oxidation reactions. The key intermediate for this chemistry is a TAC radical dication, which as an open-shell photocatalyst has remarkably strong excited-state oxidizing power. We describe the application of this strategy to transformations ranging from the oxidative functionalization of unactivated arenes to the regioselective derivatization of ethers, C-H aminations, vicinal C-H diaminations, and finally aryl olefin dioxygenations. Collectively, these catalytic platforms demonstrate the utility of charged aromatic rings, and cyclopropenium ions in particular, to enable unique advances in catalysis.

Alcohol Synthesis by Cobalt-Catalyzed Visible-Light-Driven Reductive Hydroformylation

A cobalt-catalyzed reductive hydroformylation of terminal and 1,1-disubstituted alkenes is described. One-carbon homologated alcohols were synthesized directly from CO and H₂, affording anti-Markovnikov products (34-87% yield) with exclusive regiocontrol (linear/branch >99:1) for minimally functionalized alkenes. Irradiation of the air-stable cobalt hydride, (dcype)Co(CO)₂H (dcype = dicyclohexylphosphinoethane) with blue light generated the active catalyst that mediates alkene hydroformylation and subsequent aldehyde hydrogenation. Mechanistic origins of absolute regiocontrol were investigated by in situ monitoring of the tandem catalytic reaction using multinuclear NMR spectroscopy with syngas mixtures.

Development of a novel chemoenzymatic route to enantiomerically enriched β-adrenolytic agents. A case study toward propranolol, alprenolol, pindolol, carazolol, moprolol, and metoprolol

Efficient chemoenzymatic routes toward the synthesis of both enantiomers of adrenergic β-blockers were accomplished by identifying a central chiral building block, which was first prepared using lipase-catalyzed kinetic resolution (KR, Amano PS-IM) as the asymmetric step at a five gram-scale (209 mM conc.). The enantiopure (R)-chlorohydrin (>99% ee) subsequently obtained was used for the synthesis of a series of model (R)-(+)-β-blockers (i.e., propranolol, alprenolol, pindolol, carazolol, moprolol, and metoprolol), which were produced with enantiomeric excess in the range of 96–99.9%. The pharmaceutically relevant (S)-counterpart, taking propranolol as a model, was synthesized in excellent enantiomeric purity (99% ee) via acetolysis of the respective enantiomerically pure (R)-mesylate by using cesium acetate and a catalytic amount of 18-Crown-6, followed by acidic hydrolysis of the formed (S)-acetate. Alternatively, asymmetric reduction of a prochiral ketone, namely 2-(3-chloro-2-oxopropyl)-1H-isoindole-1,3(2H)-dione, was performed using lyophilized E. coli cells harboring overexpressed recombinant alcohol dehydrogenase from Lactobacillus kefir (E. coli/Lk-ADH-Lica) giving the corresponding chlorohydrin with >99% ee. Setting the stereocenter early in the synthesis and performing a 4-step reaction sequence in a ‘one-pot two-step’ procedure allowed the design of a ‘step-economic’ route with a potential dramatic improvement in process efficiency. The synthetic method can serve for the preparation of a broad scope of enantiomerically enriched β-blockers, the chemical structures of which rely on the common α-hydroxy-N-isopropylamine moiety, and in this sense, might be industrially attractive.

Efficient chemoenzymatic routes toward both enantiomers of β-blockers were accomplished by identifying a central chiral building block prepared using either lipase-catalyzed kinetic resolution methodology or ADH-catalyzed biotranshydrogenation.

Dehydrative Cross-Coupling for C-N Bond Construction under Transition-Metal-Free Conditions

A transition-metal-free catalytic system was designed to address the dehydrative cross-coupling of unactivated primary/secondary alcohols with amines/amides under environmentally benign conditions. Mg²⁺ and counteranion (PF₆^-) worked synergistically to realize C-OH bond cleavage and concomitant C-N bond formation. A wide range of allylic alcohols and amines/amides were tolerated well in this transformation, which allowed C-N bond construction with high efficiency.

Catalytic Electrophilic Halogenation of Arenes with Electron-Withdrawing Substituents

The electrophilic halogenation of arenes is perhaps the simplest method to prepare aryl halides, which are important structural motifs in agrochemicals, materials, and pharmaceuticals. However, the nucleophilicity of arenes is weakened by the electron-withdrawing substituents, whose electrophilic halogenation reactions usually require harsh conditions and lead to limited substrate scopes and applications. Therefore, the halogenation of arenes containing electron-withdrawing groups (EWGs) and complex bioactive compounds under mild conditions has been a long-standing challenge. Herein, we describe Brønsted acid-catalyzed halogenation of arenes with electron-withdrawing substituents under mild conditions, providing an efficient protocol for aryl halides. The hydrogen bonding of Brønsted acid with the protic solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) enables this transformation and thus solves this long-standing problem.

Sulfonyl Chlorides as Thiol Surrogates for Carbon-Sulfur Bond Formation: One-Pot Synthesis of Thioethers and Thioesters

A method to synthesize thioethers and thioesters directly from readily available sulfonyl chlorides is reported. We demonstrate that a transient intermediate formed during phosphine-mediated deoxygenation of sulfonyl chlorides can be trapped in situ by activated alcohols or carboxylic acids to effect carbon-sulfur bond formation. The method is operationally simple and tolerates a broad range of functional groups. Special attention has been focused on the late-stage diversification of densely functionalized natural products and pharmaceuticals.