Site-Selective Electrochemical Arene C-H Amination.

Here we present the discovery and development of a highly selective aromatic C-H amination reaction. This electrochemical strategy involves a cathodic reduction process that generates highly electrophilic dicationic N-centered radicals that can efficiently engage in aromatic C-H functionalization and channel the regioselectivity of the aromatic substitution. The nitrogen-radical cation-pi interaction with arenes used throughout nature leads to a charge transfer mechanism, with subsequent aromatic C-N bond formation. This electrochemical process generates aryl DABCOnium salts in excellent yields and regioselectivities (single regioisomer in most cases). The scope of the reaction on arene is broad where various functionalities such as aryl halides (bromides, chlorides, fluorides), carbonyls (ketones, esters, imides), sulfonamides, and heteroarenes (pyridines, bipyridines, and terpyridines) are well tolerated. Moreover, we disclose the synthetic utility of the aryl DABCOnium salt adducts leading to the direct access of diverse aryl piperazines and the chemoselective cleavage of the exocyclic aryl C(sp2)-N bond over electrophilic C(sp3)-N+ bonds via photoredox catalysis to afford synthetically useful aryl radicals that can engage in aryl C-C and C-P bond formation.

Cobalt-electrocatalytic HAT for functionalization of unsaturated C-C bonds.

The study and application of transition metal hydrides (TMHs) has been an active area of chemical research since the early 1960s1, for energy storage, through the reduction of protons to generate hydrogen2,3, and for organic synthesis, for the functionalization of unsaturated C-C, C-O and C-N bonds4,5. In the former instance, electrochemical means for driving such reactivity has been common place since the 1950s6 but the use of stoichiometric exogenous organic- and metal-based reductants to harness the power of TMHs in synthetic chemistry remains the norm. In particular, cobalt-based TMHs have found widespread use for the derivatization of olefins and alkynes in complex molecule construction, often by a net hydrogen atom transfer (HAT)7. Here we show how an electrocatalytic approach inspired by decades of energy storage research can be made use of in the context of modern organic synthesis. This strategy not only offers benefits in terms of sustainability and efficiency but also enables enhanced chemoselectivity and distinct, tunable reactivity. Ten different reaction manifolds across dozens of substrates are exemplified, along with detailed mechanistic insights into this scalable electrochemical entry into Co-H generation that takes place through a low-valent intermediate.

One-Pot Bioelectrocatalytic Conversion of Chemically Inert Hydrocarbons to Imines.

Petroleum hydrocarbons are our major energy source and an important feedstock for the chemical industry. With the exception of combustion, the deep conversion of chemically inert hydrocarbons to more valuable chemicals is of considerable interest. However, two challenges hinder this conversion. One is the regioselective activation of inert carbon-hydrogen (C-H) bonds. The other is designing a pathway to realize this complicated conversion. In response to the two challenges, a multistep bioelectrocatalytic system was developed to realize the one-pot deep conversion from heptane to N-heptylhepan-1-imine under mild conditions. First, in this enzymatic cascade, a bioelectrocatalytic C-H bond oxyfunctionalization step based on alkane hydroxylase (alkB) was applied to regioselectively convert heptane to 1-heptanol. By integrating subsequent alcohol oxidation and bioelectrocatalytic reductive amination steps based on an engineered choline oxidase (AcCO6) and a reductive aminase (NfRedAm), the generated 1-heptanol was successfully converted to N-heptylhepan-1-imine. The electrochemical architecture provided sufficient electrons to drive the bioelectrocatalytic C-H bond oxyfunctionalization and reductive amination steps with neutral red (NR) as electron mediator. The highest concentration of N-heptylhepan-1-imine achieved was 0.67 mM with a Faradaic efficiency of 45% for C-H bond oxyfunctionalization and 70% for reductive amination. Hexane, octane, and ethylbenzene were also successfully converted to the corresponding imines. Via regioselective C-H bond oxyfunctionalization, intermediate oxidation, and reductive amination, the bioelectrocatalytic hydrocarbon deep conversion system successfully realized the challenging conversion from inert hydrocarbons to imines that would have been impossible by using organic synthesis methods and provided a new methodology for the comprehensive conversion and utilization of inert hydrocarbons.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis.

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Decarbonylative Fluoroalkylation at Palladium(II): From Fundamental Organometallic Studies to Catalysis.

This Article describes the development of a decarbonylative Pd-catalyzed aryl-fluoroalkyl bond-forming reaction that couples fluoroalkylcarboxylic acid-derived electrophiles [RFC(O)X] with aryl organometallics (Ar-M'). This reaction was optimized by interrogating the individual steps of the catalytic cycle (oxidative addition, carbonyl de-insertion, transmetalation, and reductive elimination) to identify a compatible pair of coupling partners and an appropriate Pd catalyst. These stoichiometric organometallic studies revealed several critical elements for reaction design. First, uncatalyzed background reactions between RFC(O)X and Ar-M' can be avoided by using M' = boronate ester. Second, carbonyl de-insertion and Ar-RF reductive elimination are the two slowest steps of the catalytic cycle when RF = CF3. Both steps are dramatically accelerated upon changing to RF = CHF2. Computational studies reveal that a favorable F2C-H---X interaction contributes to accelerating carbonyl de-insertion in this system. Finally, transmetalation is slow with X = difluoroacetate but fast with X = F. Ultimately, these studies enabled the development of an (SPhos)Pd-catalyzed decarbonylative difluoromethylation of aryl neopentylglycol boronate esters with difluoroacetyl fluoride.

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.

Electroreductive Olefin-Ketone Coupling.

A user-friendly approach is presented to sidestep the venerable Grignard addition to unactivated ketones to access tertiary alcohols by reversing the polarity of the disconnection. In this work a ketone instead acts as a nucleophile when adding to simple unactivated olefins to accomplish the same overall transformation. The scope of this coupling is broad as enabled using an electrochemical approach, and the reaction is scalable, chemoselective, and requires no precaution to exclude air or water. Multiple applications demonstrate the simplifying nature of the reaction on multistep synthesis, and mechanistic studies point to an intuitive mechanism reminiscent of other chemical reductants such as SmI2 (which cannot accomplish the same reaction).

Biphasic Bioelectrocatalytic Synthesis of Chiral ß-Hydroxy Nitriles.

Two obstacles limit the application of oxidoreductase-based asymmetric synthesis. One is the consumption of high stoichiometric amounts of reduced cofactor. The other is the low solubility of organic substrates, intermediates, and products in the aqueous phase. In order to address these two obstacles to oxidoreductase-based asymmetric synthesis, a biphasic bioelectrocatalytic system was constructed and applied. In this study, the preparation of chiral ß-hydroxy nitriles catalyzed by alcohol dehydrogenase (AdhS) and halohydrin dehalogenase (HHDH) was investigated as a model bioelectrosynthesis, since they are high-value intermediates in statin synthesis. Diaphorase (DH) was immobilized by a cobaltocene-modified poly(allylamine) redox polymer on the electrode surface (DH/Cc-PAA bioelectrode) to achieve effective bioelectrocatalytic NADH regeneration. Since AdhS is a NAD-dependent dehydrogenase, the diaphorase-modified biocathode was used to regenerate NADH to support the conversion from ethyl 4-chloroacetoacetate (COBE) to ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE) catalyzed by AdhS. The addition of methyl tert-butyl ether (MTBE) as an organic phase not only increased the uploading of COBE but also prevented the spontaneous hydrolysis of COBE, extended the lifetime of DH/Cc-PAA bioelectrode, and increased the Faradaic efficiency and the concentration of generated (R)-ethyl-4-cyano-3-hydroxybutyrate ((R)-CHCN). After 10 h of reaction, the highest concentration of (R)-CHCN in the biphasic bioelectrocatalytic system was 25.5 mM with 81.2% enantiomeric excess (eep). The conversion ratio of COBE achieved 85%, which was 8.8 times higher than that achieved with the single-phase system. Besides COBE, two other substrates with aromatic ring structures were also used in this biphasic bioelectrocatalytic system to prepare the corresponding chiral ß-hydroxy nitriles. The results indicate that the biphasic bioelectrocatalytic system has the potential to produce a variety of ß-hydroxy nitriles with different structures.

Nickel-Catalyzed Decarbonylative Amination of Carboxylic Acid Esters.

The reaction of carboxylic acid derivatives with amines to form amide bonds has been the most widely used transformation in organic synthesis over the past century. Its utility is driven by the broad availability of the starting materials as well as the kinetic and thermodynamic driving force for amide bond formation. As such, the invention of new reactions between carboxylic acid derivatives and amines that strategically deviate from amide bond formation remains both a challenge and an opportunity for synthetic chemists. This report describes the development of a nickel-catalyzed decarbonylative reaction that couples (hetero)aromatic esters with a broad scope of amines to form (hetero)aryl amine products. The successful realization of this transformation was predicated on strategic design of the cross-coupling partners (phenol esters and silyl amines) to preclude conventional reactivity that forms inert amide byproducts.

Selective Electroenzymatic Oxyfunctionalization by Alkane Monooxygenase in a Biofuel Cell.

Aliphatic synthetic intermediates with high added value are generally produced from alkane sources (e.g., petroleum) by inert carbon-hydrogen (C-H) bond activation using classical chemical methods (i.e. high temperature, rare metals). As an alternative approach for these reactions, alkane monooxygenase from Pseudomonas putida (alkB) is able to catalyze the difficult terminal oxyfunctionalization of alkanes selectively and under mild conditions. Herein, we report an electrosynthetic system using an alkB biocathode which produces alcohols, epoxides, and sulfoxides through bioelectrochemical hydroxylation, epoxidation, sulfoxidation, and demethylation. The capacity of the alkB binding pocket to protect internal functional groups is also demonstrated. By coupling our alkB biocathode with a hydrogenase bioanode and using H2 as a clean fuel source, we have developed and characterized a series of enzymatic fuel cells capable of oxyfunctionalization while simultaneously producing electricity.

Nickel-Catalyzed Decarbonylative Synthesis of Fluoroalkyl Thioethers.

This report describes the development of a nickel-catalyzed decarbonylative reaction for the synthesis of fluoroalkyl thioethers (RFSR) from the corresponding thioesters. Readily available, inexpensive, and stable fluoroalkyl carboxylic acids (RFCO2H) serve as the fluoroalkyl (RF) source in this transformation. Stoichiometric organometallic studies reveal that RF-S bond-forming reductive elimination is a challenging step in the catalytic cycle. This led to the identification of diphenylphosphinoferrocene as the optimal ligand for this transformation. Ultimately, this method was applied to the construction of diverse fluoroalkyl thioethers (RFSR), with R = both aryl and alkyl.

Mechanism and Scope of Nickel-Catalyzed Decarbonylative Borylation of Carboxylic Acid Fluorides.

This Article describes the development of a base-free, nickel-catalyzed decarbonylative coupling of carboxylic acid fluorides with diboron reagents to selectively afford aryl boronate ester products. Detailed studies were conducted to assess the relative rates of direct transmetalation between aryl boronate esters and diboron reagents and a bisphosphine nickel(aryl)(fluoride) intermediate. These investigations revealed that diboron reagents undergo transmetalation with this Ni(aryl)(fluoride) intermediate at rates significantly faster than their aryl boronate ester congeners. Furthermore, the reactivity of both boron reagents toward transmetalation is enhanced with increasing electrophilicity of the boron center. These mechanistic insights were leveraged to develop a catalytic decarbonylative borylation of acid fluorides that proved applicable to a variety of (hetero)aryl carboxylic acid fluorides as well as diverse diboron reagents. The acid fluorides can be generated in situ directly from carboxylic acids. Furthermore, the mechanistic studies directed the identification of various air-stable Ni pre-catalysts for this transformation.

Palladium-Catalyzed Difluoromethylation of Aryl Chlorides and Bromides with TMSCF2H.

A palladium-catalyzed cross-coupling of aryl chlorides/bromides with TMSCF2H is described. Two different catalysts, Pd(dba)2/BrettPhos and Pd(P tBu3)2, are demonstrated and provide a variety of difluoromethylated arenes in good yields.

Base-free nickel-catalysed decarbonylative Suzuki-Miyaura coupling of acid fluorides.

The Suzuki-Miyaura cross-coupling of organoboron nucleophiles with aryl halide electrophiles is one of the most widely used carbon-carbon bond-forming reactions in organic and medicinal chemistry1,2. A key challenge associated with these transformations is that they generally require the addition of an exogenous base, the role of which is to enable transmetallation between the organoboron nucleophile and the metal catalyst3. This requirement limits the substrate scope of the reaction because the added base promotes competitive decomposition of many organoboron substrates3-5. As such, considerable research has focused on strategies for mitigating base-mediated side reactions6-12. Previous efforts have primarily focused either on designing strategically masked organoboron reagents (to slow base-mediated decomposition)6-8 or on developing highly active palladium precatalysts (to accelerate cross-coupling relative to base-mediated decomposition pathways)10-12. An attractive alternative approach involves identifying combinations of catalyst and electrophile that enable Suzuki-Miyaura-type reactions to proceed without an exogenous base12-14. Here we use this approach to develop a nickel-catalysed coupling of aryl boronic acids with acid fluorides15-17, which are formed in situ from readily available carboxylic acids18-22. This combination of catalyst and electrophile enables a mechanistic manifold in which a 'transmetallation-active' aryl nickel fluoride intermediate is generated directly in the catalytic cycle13,16. As such, this transformation does not require an exogenous base and is applicable to a wide range of base-sensitive boronic acids and biologically active carboxylic acids.

Palladium- and Nickel-Catalyzed Decarbonylative C-S Coupling to Convert Thioesters to Thioethers.

This Letter describes the development of a catalytic decarbonylative C-S coupling reaction that transforms thioesters into thioethers. Both Pd- and Ni-based catalysts are developed and applied to the construction of diaryl, aryl alkyl, and heterocycle-containing thioethers.

Rh-Catalyzed Conjugate Addition of Aryl and Alkenyl Boronic Acids to α-Methylene-ß-lactones: Stereoselective Synthesis of trans-3,4-Disubstituted ß-Lactones.

A one-step preparation of 3,4-disubstituted ß-lactones through Rh-catalyzed conjugate addition of aryl or alkenyl boronic acids to α-methylene-ß-lactones is described. The operationally simple, stereoselective transformation provides a broad range of ß-lactones from individual α-methylene-ß-lactone templates. This methodology allowed for a direct, final-step C-3 diversification of nocardiolactone, an antimicrobial natural product.

Pd-Catalyzed Decarbonylative Cross-Couplings of Aroyl Chlorides.

This report describes a method for Pd-catalyzed decarbonylative cross-coupling that enables the conversion of carboxylic acid derivatives to biaryls, aryl amines, aryl ethers, aryl sulfides, aryl boronate esters, and trifluoromethylated arenes. The success of this transformation leverages the Pd0/Brettphos-catalyzed decarbonylative chlorination of aroyl chlorides, which can then participate in diverse cross-coupling reactions in situ using the same Pd catalyst.

Rhodium-Catalyzed Addition of Aryl Boronic Acids to 2,2-Disubstituted Malononitriles.

ß-Ketonitriles bearing a quaternary carbon at the 2-position were prepared through Rh-catalyzed addition of aryl boronic acids to 2,2-disubstituted malononitriles. In contrast to the previously described transnitrilative cyanation of aryl boronic acids with dialkylmalononitriles, the present reaction avoids retro-Thorpe collapse of the intermediate addition product through the use of a milder base. The reaction was amenable to a variety of aryl boronic acids and disubstituted malononitriles, providing a diverse array of ß-ketonitriles. The products could be further derivatized to valuable chiral α,α-disubstituted-ß-aminonitriles through addition reactions to the corresponding N-tert-butanesulfinyl imines.

1,4-Dicarbofunctionalization of 4-Fluoroaryl Grignard and Lithium Reagents with Disubstituted Malononitriles.

An efficient one-pot 1,4-dicarbofunctionalization of 4-fluoroaryl Grignard or lithium reagents with 2,2-disubstituted malononitriles is described. The reaction proceeds by sequential transnitrilation and SNAr reactions. Commercial Grignard solutions, Grignard reagents prepared in situ by halogen/magnesium exchange with i-PrMgCl, or aryllithium reagents prepared in situ by bromine/lithium exchange with n-BuLi are compatible with the reaction conditions. Moreover, 2,2-disubstituted malononitriles of diverse structures are accommodated. The reaction provides a unique approach to 1,4-dicarbofunctionalization of activated arenes in a tandem, one-pot transformation.

Pd-Catalyzed Acyl C-O Bond Activation for Selective Ring-Opening of α-Methylene-ß-lactones with Amines.

A Pd-catalyzed ring-opening of ß-lactones with various types of amines (primary, secondary, and aryl) to provide ß-hydroxy amides with excellent selectivity toward acyl C-O bond cleavage is reported. The utility of this protocol is demonstrated in an asymmetric kinetic resolution providing enantioenriched α-methylene-ß-lactones.