Fragment Coupling with Tertiary Radicals Generated by Visible-Light Photocatalysis

Photo-Driven Regiodivergent Arylation/Cyclization and Arylation/Hydroxylation of N-Aryl Methacrylamides with Aryltriazenes: Access to Functionalized 3,3-Disubstituted Oxindoles and α-Hydroxylamides

A metal-free, light-induced regiodivergent functionalization of α,β-unsaturated amides with aryltriazenes under ambient conditions was developed. The visible light and B(C6F5)3 cocatalyzed radical cascade arylation/cyclization of N-alkyl-N-arylmethacrylamides can obtain functionalized 3,3-disubstituted oxindoles with the assistance of photocatalyst eosin Y-Na2. In the absence of any catalyst, with purple light irradiation and electron-donor-acceptor (EDA) complex initiation, the radical cascade arylation/hydroxylation of N-arylmethacrylamides affords α-hydroxylamides. This methodology highlights the arts in accessing different regioisomers by altering the substrates and photocatalytic strategies.

Modular synthesis of aryl amines from 3-alkynyl-2-pyrones

The synthesis of aryl amines from 3-alkynyl-2-pyrones and various amines is described. Mechanistically, the aryl amines are proposed to arise from the 3-alkynyl-2-pyrone substrates through their selective opening in a 1,6-fashion by secondary amines followed by decarboxylation and an unexpected rearrangement. The proposed mechanism is supported by quantum chemical transition-state calculations, which are consistent with the regiochemical outcome. The scope of this transformation spans a variety of 3-alkynyl-2-pyrones and a range of secondary amines. The influence of the secondary amine coupling partners on reaction efficiency was elucidated through data-driven modeling as well as scope exploration. These latter studies revealed that the steric bulk of the secondary amine coupling partner under the reaction conditions serves as a strong indicator of overall reaction efficiency.

Mild Approach to Nucleoside Analogues via Photoredox/Cu-Catalyzed Decarboxylative C-N Bond Formation. Total Synthesis of Oxetanocin A

The conventional N-glycosylation methods for nucleoside synthesis usually require strongly acidic or basic conditions. Here we report the decarboxylative C(sp3)-N coupling of glycosyl N-hydroxyphthalimide esters with nucleobases via dual photoredox/Cu catalysis, which offered a mild approach to nucleoside analogues. A total synthesis of oxetanocin A, an antiviral natural product containing an oxetanose moiety, has been achieved by using this method.

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Modular synthesis of congested β2,2-amino acids via the merger of photocatalysis and oxidative functionalisations

A two-step protocol for the modular synthesis of β2- and α-quaternary β2,2-amino acid derivatives is reported. The key steps are a photocatalytic hydroalkylation reaction, followed by an oxidative functionalisation to access N-protected β-amino acids, esters, and amides. This strategy can be effectively scaled up via continuous-flow technology.

Towards a General Access to 1-Azaspirocyclic Systems via Photoinduced, Reductive Decarboxylative Radical Cyclizations

A convenient and versatile approach to important 1-azaspirocyclic systems relevant to medicinal chemistry and natural products is reported herein. The main strategy relies on a reductive decarboxylative cyclization of redox-active esters which can be rapidly assembled from abundant cyclic azaacids and tailored acceptor sidechains, with a focus on alkyne acceptors enabling the generation of useful exo-alkene moieties. Diastereoconvergent variants were studied and could be achieved either through remote stereocontrol or conformational restriction in bicyclic carbamate substrates. Two sets of metal-free photocatalytic conditions employing inexpensive eosin Y were disclosed and studied experimentally to highlight key mechanistic divergences.

A Fenton Approach to Aromatic Radical Cations and Diarylmethane Synthesis

Manipulating carbon-centered radicals to add to electron-deficient systems is a well-precedented process. By coupling the Fe(II)-mediated Fenton reaction with the Fe(III)-mediated single-electron oxidation of anisolic compounds, we demonstrate how electron-rich carbon-centered radicals can react with electron-rich arenes through a radical-polar cascade pathway. This bioinspired approach produces diarylmethane derivatives from simple unfunctionalized precursors.

Synthesis of (-)-Cotylenol, a 14-3-3 Molecular Glue Component

Small molecules that modulate the 14-3-3 protein-protein interaction (PPI) network represent valuable therapeutics and tool compounds. However, access has been lost to 14-3-3 PPI molecular glues of the cotylenin class, leading to investigations into the practical chemical syntheses of congeners and analogues. Here we report a concise synthesis of (-)-cotylenol via a 10-step asymmetric entry into a diversifiable 5-8-5 core. This route features a mild Liebeskind-Srogl fragment coupling that tolerates unprecedented steric hindrance to produce a highly congested ketone, and a tandem Claisen-ene cascade that establishes the 8-membered ring. Late-stage control of stereochemistry and functionality leads to (-)-cotylenol and sets the stage for focused library synthesis.

Visible-Light-Promoted Decarboxylative Alkylation/Cyclization of Vinylcycloalkanes

An efficient strategy for visible-light-promoted decarboxylative alkylation of vinylcyclopropanes with alkyl N-(acyloxy)phthalimide esters through the dual C-C bond and single N-O bond cleavage, employing triphenylphosphine and lithium iodide as the photoredox system to synthesize 2-alkylated 3,4-dihydronaphthalenes, has been established. This alkylation/cyclization involves a radical process and undergoes a sequence of N-(acyloxy)phthalimide ester single-electron reduction, N-O bond cleavage, decarboxylative, alkyl radical addition, C-C bond cleavage, and intramolecular cyclization. Moreover, using the photocatalyst Na₂-Eosin Y instead of triphenylphosphine and lithium iodide, the vinyl transfer products are acquired when vinylcyclobutanes or vinylcyclopentanes are utilized as alkyl radical receptors.

Radical Decarboxylative Carbon-Nitrogen Bond Formation

The carbon-nitrogen bond is one of the most prevalent chemical bonds in natural and artificial molecules, as many naturally existing organic molecules, pharmaceuticals, agrochemicals, and functional materials contain at least one nitrogen atom. Radical decarboxylative carbon-nitrogen bond formation from readily available carboxylic acids and their derivatives has emerged as an attractive and valuable tool in modern synthetic chemistry. The promising achievements in this research topic have been demonstrated via utilizing this strategy in the synthesis of complex natural products. In this review, we will cover carbon-nitrogen bond formation via radical decarboxylation of carboxylic acids, Barton esters, MPDOC esters, N-hydroxyphthalimide esters (NHP esters), oxime esters, aryliodine(III) dicarboxylates, and others, respectively. This review aims to bring readers a comprehensive survey of the development in this rapidly expanding field. We hope that this review will emphasize the knowledge, highlight the proposed mechanisms, and further disclose the fascinating features in modern synthetic applications.

Alkyl Radical Generation via C–C Bond Cleavage in 2-Substituted Oxazolidines

There is an urgent need to develop uncharged radical precursors to be activated under mild photocatalyzed conditions. 2-Substituted-1,3-oxazolidines (E_ox < 1.3 V vs SCE, smoothly prepared from the corresponding aldehydes) have been herein employed for the successful release of tertiary, α-oxy, and α-amido radicals under photo-organo redox catalysis. The reaction relies on the unprecedented C–C cleavage occurring from the radical cation of these heterocyclic derivatives. Such a protocol is applied to the visible-light-driven conjugate radical addition onto Michael acceptors and vinyl (hetero)arenes under mild metal-free conditions.

Alcohols as Alkylating Agents: Photoredox-Catalyzed Conjugate Alkylation via In Situ Deoxygenation

The rapid exploration of sp3 -enriched chemical space is facilitated by fragment-coupling technologies that utilize simple and abundant alkyl precursors, among which alcohols are a highly desirable, commercially accessible, and synthetically versatile class of substrate. Herein, we describe an operationally convenient, N-heterocyclic carbene (NHC)-mediated deoxygenative Giese-type addition of alcohol-derived alkyl radicals to electron-deficient alkenes under mild photocatalytic conditions. The fragment coupling accommodates a broad range of primary, secondary, and tertiary alcohol partners, as well as structurally varied Michael acceptors containing traditionally reactive sites, such as electrophilic or oxidizable moieties. We demonstrate the late-stage diversification of densely functionalized molecular architectures, including drugs and biomolecules, and we further telescope our protocol with metallaphotoredox cross-coupling for step-economic access to sp3 -rich complexity.

Rapid Access to Aliphatic Sulfonamides

Herein, we report a mild, rapid, straightforward method for visible-light-mediated sulfonamide ethylation to afford a diverse array of compounds with C(sp³)-sulfonamide skeletons. The method relies on inexpensive, abundant, commercially available primary, secondary, and tertiary alkyl carboxylic acids and alkyl iodides as substrates. The method has a broad substrate scope and potential utility for late-stage functionalization of natural products and synthetic medicines and can be expected to facilitate rapid structural diversification of bioactive molecules.

Iridium-Catalyzed Enantioselective and Chemodivergent Allenylic Alkylation of Vinyl Azides for the Synthesis of α-Allenylic Amides and Ketones

The first enantioselective synthesis of α-allenylic amides and ketones through allenylic alkylation of vinyl azides is reported. In these chemodivergent reactions, cooperatively catalyzed by a Ir^I /(phosphoramidite,olefin) complex and Sc(OTf)₃ , vinyl azides act as the surrogate for both amide enolates and ketone enolates. The desiccant (molecular sieves) plays a crucial role in controlling the chemodivergency of this enantioconvergent and regioselective reaction: Under otherwise identical reaction conditions, the presence of the desiccant led to α-allenylic amides, while its absence resulted in α-allenylic ketones. Utilizing racemic allenylic alcohols as the alkylating agent, the overall process represents a dynamic kinetic asymmetric transformation (DyKAT), where both the products are formed with the same absolute configuration. To the best of our knowledge, this is the first example of the use of vinyl azide as the ketone enolate surrogate in an enantioselective transformation.

Photoredox-Catalyzed Oxidation of Anions for the Atom-Economical Hydro-, Amido-, and Dialkylation of Alkenes

Photoredox catalysis has become a powerful method to generate free radical intermediates in organic synthesis. This report describes the use of photoredox catalysis to directly oxidize common nucleophilic anions to access electrophilic 1,3-dicarbonyl and amidyl radical intermediates. First, conjugate bases of 1,3-dicarbonyls were oxidized to neutral radical species for intramolecular hydro- and dialkylation of alkenes. This overall redox-neutral process provided cyclopentanone products in excellent yields (up to 96%). The scope included a variety of styrene radical acceptors and products with newly formed vicinal quaternary carbons. This process was then extended to the synthesis of pyrrolidinones by alkene amidoalkylation that proceeded via N-aryl amidyl radical intermediates in good yield (up to 85%). These reactions were characterized by their mild conditions, high atom economy, and the absence of stoichiometric byproducts. Mechanistic and computational studies supported a stepwise proton-coupled electron transfer mechanism, where an "electron borrowing" photocatalyst oxidizes an anion and reduces a benzylic radical after bond formation.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

Total Synthesis of Resiniferatoxin Enabled by Photocatalytic Decarboxylative Radical Cyclization

Resiniferatoxin (1) is a complex daphnane diterpenoid with a highly oxygenated 5/7/6-membered ABC-ring system. Here we report a new synthetic route to 1 that requires 27 steps from a starting d-ribose derivative. The carbon spacer and A-ring are sequentially attached to the C-ring by radical allylation and Stille coupling reactions, respectively. An Ir(III)-catalyzed photoinduced decarboxylative radical reaction then forged the sterically hindered bond between the tetra- and trisubstituted carbons to cyclize the central seven-membered B-ring.

Nickel(ii)/TPMPP catalyzed reductive coupling of oxalates and tetrasulfides: synthesis of unsymmetric disulfides

A Ni(ii)/TPMPP-catalyzed reductive cross-coupling reaction of benzyl oxalates and tetrasulfides to synthesize unsymmetric disulfides is reported.

Efficient and practical synthesis of monoalkyl oxalates under green conditions

Monoalkyl oxalates are among the most important building blocks being applied to the synthesis of a variety of significant classes of compounds or applied to various cutting-edge reactions. However, their commercial availability is limited. Their synthetic methods are also limited because of the difficulty to synthesize them, and those hitherto reported are carried out in organic solvents often with the use of toxic reagents with mostly low to modest yields. Here we have developed practical synthesis of monoalkyl oxalates in aqueous media by applying the highly efficient selective monohydrolysis reactions of symmetric diesters which we reported previously. The best conditions apply an aqueous NaOH solution with relatively nontoxic THF or acetonitrile as a co-solvent at around 0–5 °C. The procedures are simple and environmentally friendly without requiring toxic or expensive reagents, yet yielding the corresponding half-esters in high yields with high purities. All the half-esters prepared here are stable over a long period of time. Therefore, our studies are expected to offer practical green methods for the synthesis of monoalkyl oxalates.

Simple and environmentally-friendly methods have been developed for the synthesis of monoalkyl oxalates without requiring toxic or expensive reagents.

Recent advances in visible-light mediated functionalization of olefins and alkynes using copper catalysts

Over the past decade, visible-light photoredox catalysis has blossomed as a powerful strategy and offers a discrete activation mode complementary to thermal controlled reactions. Visible-light-mediated photoredox catalysis also offers exciting...

Nickel-Catalyzed Decarboxylative Coupling of Redox-Active Esters with Aliphatic Aldehydes

The addition of alkyl fragments to aliphatic aldehydes is a highly desirable transformation for fragment couplings, yet existing methods come with operational challenges related to the basicity and instability of the nucleophilic reagents commonly employed. We report herein that nickel catalysis using a readily available bioxazoline (BiOx) ligand can catalyze the reductive coupling of redox-active esters with aliphatic aldehydes using zinc metal as the reducing agent to deliver silyl-protected secondary alcohols. This protocol is operationally simple, proceeds under mild conditions, and tolerates a variety of functional groups. Initial mechanistic studies suggest a radical chain pathway. Additionally, alkyl tosylates and epoxides are suitable alkyl precursors to this transformation providing a versatile suite of catalytic reactions for the functionalization of aliphatic aldehydes.

Photoinduced Decarboxylative Radical Coupling Reaction of Multiply Oxygenated Structures by Catalysis of Pt-Doped TiO2

A new reaction system was devised for decarboxylative radical coupling reactions by heterogeneous semiconductor photoredox catalysis. When an α-alkoxy carboxylic acid and Pt-doped TiO₂ in EtOAc were irradiated with a violet light-emitting diode at room temperature, the photogenerated electron hole of TiO₂ oxidatively induced the ejection of CO₂ via the formation of a carboxyl radical to produce the corresponding α-alkoxy radical. C(sp³)-C(sp³) bond formation between the radicals led to dimers with reductive conversion of protons to H₂ by the photogenerated electron. Alternatively, in the presence of an electron-deficient olefin, an intermolecular radical addition reaction occurred, resulting in the formation of a 1,4-adduct via single-electron reduction and subsequent protonation. These operationally simple and mild transformations are amenable to the one-step assembly of densely oxygenated linear and branched carbon chains.

tert-Butyl Nitrite as a Twofold Hydrogen Abstractor for Dehydrogenative Coupling of Aldehydes with N-Hydroxyimides

A synthetically practical transition metal/catalyst/halogen-free dehydrogenative coupling of aldehydes with N-hydroxyimides promoted solely by tert-butyl nitrite under mild conditions was developed. tert-Butyl nitrite generates two radicals (^tBuO and NO) and thus works as a twofold hydrogen abstractor. A diverse array of N-hydroxyimide esters were prepared from either aliphatic or aromatic aldehydes. Benzoyl-substituted aldehydes such as 2-oxo-2-phenylacetaldehyde are also suitable.

Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

Organic catalysts have the potential to carry out a wide range of otherwise thermally inaccessible reactions via photoredox routes. Early demonstrated successes of organic photoredox catalysts include one-electron CO₂ reduction and H₂ generation via water splitting. Photoredox systems are challenging to study and design owing to the sheer number and diversity of phenomena involved, including light absorption, emission, intersystem crossing, partial or complete charge transfer, and bond breaking or formation. Designing a viable photoredox route therefore requires consideration of a host of factors such as absorption wavelength, solvent, choice of electron donor or acceptor, and so on. Quantum chemistry methods can play a critical role in demystifying photoredox phenomena. Using one-electron CO₂ reduction with phenylene-based chromophores as an illustrative example, this perspective highlights recent developments in quantum chemistry that can advance our understanding of photoredox processes and proposes a way forward for driving the design and discovery of organic catalysts.

Electrochemical Deoxygenative Thiolation of Preactivated Alcohols and Ketones

This work describes an electrochemically promoted nickel-catalyzed deoxygenative thiolation of alcohols and ketones under mild conditions. Excellent substrate tolerance and good chemical yields can be achieved by graphene/nickel foam electrodes in an undivided cell. Further study to gain mechanistic insight into this electrochemical cross-coupling has been carried out.

Combined Experimental and Computational Study of Ruthenium N-Hydroxyphthalimidoyl Carbenes in Alkene Cyclopropanation Reactions

A combined experimental-computational approach has been used to study the cyclopropanation reaction of N-hydroxyphthalimide diazoacetate (NHPI-DA) with various olefins, catalyzed by a ruthenium-phenyloxazoline (Ru-Pheox) complex. Kinetic studies show that the better selectivity of the employed redox-active NHPI diazoacetate is a result of a much slower dimerization reaction compared to aliphatic diazoacetates. Density functional theory calculations reveal that several reactions can take place with similar energy barriers, namely, dimerization of the NHPI diazoacetate, cyclopropanation (inner-sphere and outer-sphere), and a previously unrecognized migratory insertion of the carbene into the phenyloxazoline ligand. The calculations show that the migratory insertion reaction yields an unconsidered ruthenium complex that is catalytically competent for both the dimerization and cyclopropanation, and its relevance is assessed experimentally. The stereoselectivity of the reaction is argued to stem from an intricate balance between the various mechanistic scenarios.

Asymmetric Total Synthesis of Clionastatins A and B

Herein we report the first total synthesis of polychlorinated steroids clionastatins A and B, which was accomplished asymmetrically by means of a convergent, radical fragment coupling approach. Key features of the synthesis include an Ireland-Claisen rearrangement to introduce the C5 stereocenter (which was ultimately transferred to the C10 quaternary stereocenter of the clionastatins via a traceless stereochemical relay), a regioselective acyl radical conjugate addition to join the two fragments, an intramolecular Heck reaction to install the C10 quaternary stereocenter, and a diastereoselective olefin dichlorination to establish the synthetically challenging pseudoequatorial dichlorides. This work also enabled us to determine that the true structures of clionastatins A and B are in fact C14 epimers of the originally proposed structures.

Electrochemical Nozaki-Hiyama-Kishi Coupling: Scope, Applications, and Mechanism

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process, those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process.

Nickel-Catalyzed Reductive Vinylation of Chloro-hexahydropyrroloindoline Derivatives with Vinyl Triflates

This work emphasizes facile construction of C-3a vinyl substituted hexahydropyrrolidinoindolines based upon Ni-catalyzed reductive coupling of chloro-hexahydropyrroloindoline derivatives with a wide range of alkyl-decorated vinyl triflates. The remarkable compatibility of sterically hindered branched vinyl groups is highlighted.

New Methods and Strategies in the Synthesis of Terpenoid Natural Products

Indoloterpenoids of the paxilline type belong to a large family of secondary metabolites that exhibit unique molecular architectures and a diverse set of biological activities. More than 100 congeners identified to date share a common structural motif that contains an indole moiety fused to a rearranged diterpenoid fragment. The representative physiological and cellular effects attributed to this family of natural products include neurological and insecticidal activities, modulation of lipid balance, and inhibition of mitosis. The uniting polycyclic motif combined with the diversity of individual structural features of paxilline indoloterpenoids and the broad scope of their biological activities have fascinated organic chemists for the past four decades and have led to the development of numerous syntheses. In this Account, we describe our contributions to this field and how they in turn shape new directions that are developing in our laboratory.We begin with the discussion of our strategy for the synthesis of the shared indoloterpenoid core. To address stereochemical challenges encountered in earlier reports, we planned to leverage a suitably substituted cyclopentanone in a polycyclization to form the desired trans-decalin motif. This polycyclization relied on a radical-polar crossover cascade initiated by hydrogen atom transfer. The original process exhibited poor diastereoselectivity, but we discovered an efficient solution to this problem that took advantage of intramolecular tethering effects, culminating in short synthesis of emindole SB. During these studies, we also identified indium-mediated alkenylation of silyl enol ethers with alkynes as a suitable method for the synthesis of highly substituted β,γ-unsaturated ketones that was critical to achieving brevity of our route. We subsequently developed a catalytic version of this transformation that allowed for a formal bimolecular ene reaction that exhibited unusual and potentially useful selectivity in construction of quaternary centers.To test the scope and limitations of our approach to paxilline indoloterpenoids and identify potential improvements, we developed a synthesis of the more complex congener nodulisporic acid C. The convergent assembly of this natural product was enabled by identification of new elements of stereocontrol in the radical-polar crossover polycyclization en route to the polycyclic terpenoid motif and development of a highly diastereoselective enyne cycloisomerization to access the indenopyran motif and a ketone arylation protocol to unite the two complex fragments.In subsequent studies, we expanded the radical-polar crossover cascade underlying our approach to paxilline indoloterpenoids to a bimolecular setting, which allowed for annulation of two unsaturated carbonyl components to produce functionalized cyclohexanes. This transformation is particularly well suited for installation of fully substituted carbons and can be complementary to the venerable Diels-Alder reaction. The utility of the new annulation was tested in the synthesis of forskolin, allowing for rapid construction of the complex polycyclic motif in this densely functionalized labdane diterpenoid.Over the past five years, our initial forays into the synthesis of paxilline indoloterpenoids have grown into a program that incorporates development of new synthetic methods and pursues artificial assembly of terpenoid natural products from several different families. We are encouraged by the increasing diversity of structural motifs made accessible by application of this chemistry and continue to discover new aspects of the underlying reactivity.

Nickel-catalyzed reductive benzylation of tertiary alkyl halides with benzyl chlorides and chloroformates

We report a Ni-catalyzed cross-electrophile coupling of benzyl chlorides and chloroformates with unactivated tertiary alkyl halides to forge the challenging benzylated all C(sp³)-quaternary carbon centers.

Nickel-catalyzed formation of quaternary carbon centers using tertiary alkyl electrophiles

This review provides a comprehensive summary of recent advances in nickel-catalyzed reactions employing tertiary alkyl electrophiles for the construction of quaternary carbon centers.

Radical Transformations towards the Synthesis of Quinoline: A Review

Quinoline is considered one of the most ubiquitous heterocycles due to its engaging biological activities and synthetic utility over organic transformations. Over the past few decades, numerous reports have been documented in the synthesis of quinolines. The classical methods including, Skraup, Friedlander, Doebner-von-Miller, Conrad-Limpach, Pfitzinger quinoline synthesis, and so forth, these are the well-known methods to construct principal quinoline scaffold with several advantages and limitations. Recently, radical insertion or catalyzed reactions have emerged as a powerful and efficient tool to construct heterocycles with high atom efficiency and step economy. In this concern, this minireview mainly focused on the developments of Quinoline synthesis via radical reactions. In addition, a brief description of the preparation procedure, reactivity, and mechanisms is also included, where as possible. Respectively, the synthesis of quinolines is classified and summarized based on its reactivity, so it will help the researchers to grab the information in this exploration area, as Quinolines are promising pharmacophores.

Tertiary Amines Acting as Alkyl Radical Equivalents Enabled by a P/N Heteroleptic Cu(I) Photosensitizer

An unprecedented exploration of tertiary amines as alkyl radical equivalents for cross-coupling with aromatic alkynes to access allylarenes has been achieved by a P/N heteroleptic Cu(I)-based photosensitizer under photoredox catalysis conditions. Mechanistic studies reveal that the reaction might undergo radical addition of in situ-generated α-amino radical intermediates to alkynes followed by 1,5-hydrogen transfer, C-N bond cleavage, and concomitant isomerization of the resulting allyl radical species.

Decarboxylative Alkyl Coupling Promoted by NADH and Blue Light

Photoexcited dihydronicotinamides like NADH and analogues have been found to generate alkyl radicals upon reductive decarboxylation of redox-active esters without auxiliary photocatalysts. This principle allowed aliphatic photocoupling between redox-active carboxylate derivatives and electron-poor olefins, displaying surprising water and air-tolerance and unusually high coupling rates in dilute conditions. The orthogonality of the reaction in the presence of other carboxylic acids and its utility in the functionalization of DNA is presented, notably using visible light in combination with NADH, the ubiquitous reductant of life.

Manganese-mediated reductive functionalization of activated aliphatic acids and primary amines

Alkyl carboxylic acids as well as primary amines are ubiquitous in all facets of biological science, pharmaceutical science, chemical science and materials science. By chemical conversion to redox-active esters (RAE) and Katritzky's N-alkylpyridinium salts, respectively, alkyl carboxylic acids and primary amines serve as ideal starting materials to forge new connections. In this work, a Mn-mediated reductive decarboxylative/deaminative functionalization of activated aliphatic acids and primary amines is disclosed. A series of C-X (X = S, Se, Te, H, P) and C-C bonds are efficiently constructed under simple and mild reaction conditions. The protocol is applicable to the late-stage modification of some structurally complex natural products or drugs. Preliminary mechanistic studies suggest the involvement of radicals in the reaction pathway.

Acridine Photocatalysis: Insights into the Mechanism and Development of a Dual-Catalytic Direct Decarboxylative Conjugate Addition

Conjugate addition is one of the most synthetically useful carbon‒carbon bond-forming reactions, however, reactive carbon nucleophiles are typically required to effect the addition. Radical conjugate addition provides an avenue for replacing reactive nucleophiles with convenient radical precursors. Carboxylic acids can serve as simple and stable radical precursors by way of decarboxylation, but activation to reactive esters is typically necessary to facilitate the challenging decarboxylation. Here, we report a direct, dual-catalytic decarboxylative radical conjugate addition of a wide range of carboxylic acids that does not require acid preactivation and is enabled by the visible light-driven acridine photocatalysis interfaced with an efficient copper catalytic cycle. Mechanistic and computational studies provide insights into the roles of the ligands and metal species in the dual catalytic process and the photocatalytic activity of substituted acridines.

Cross-Electrophile Couplings of Activated and Sterically Hindered Halides and Alcohol Derivatives

Transition metal catalyzed cross-electrophile coupling of alkyl electrophiles has evolved into a privileged strategy that permits the facile construction of valuable C(sp³)-C bonds. Numerous elegant Ni-catalyzed coupling methods, for example, arylation, allylation, acylation, and vinylation of primary and secondary alkyl halides have been developed. This prior work has provided important mechanistic insights into the selectivity and reactivity of the coupling partners, which are largely dictated by both the catalysts and the reactants. In spite of the advances made to date, a number of challenging issues remain, including (1) achieving stereoselective syntheses of C-C bonds that rely primarily on functionalized or activated alkyl precursors, (2) diversifying the electrophiles, and (3) gaining insights into the underlying reaction mechanisms.In this Account, we summarize a number of Ni- and Fe-catalyzed reductive C-C bond forming methods developed in our laboratory, which have allowed us to couple activated, sterically hindered tertiary alkyl and C(sp³)-O bond electrophiles and to access methylated and trifluoromethylated products, esters, C-glycosides, and quaternary carbon centers. We will begin with a brief discussion of Ni-catalyzed chemoselective construction of unactivated alkyl-alkyl bonds, with focus on the effects of ligands and reductants, along with leaving group-directed reactivities of alkyl halides, and the role they play in promoting the reductive coupling of activated electrophiles, including methyl, trifluoromethyl, and glycosyl electrophiles, and chloroformates. Matching the reactivities of these electrophiles with suitable coupling partners is considered essential for success; this is something that can be tuned by means of appropriate Ni catalysts. Second, we will detail how tuning the steric and electronic effects of nickel catalysts with labile pyridine-type ligands and additives (primarily MgCl₂) permits effective creation of arylated all-carbon quaternary centers through the coupling of aryl halides with sterically encumbered tertiary alkyl halides. In contrast, the use of bulkier bipyridine and terpyridine ligands permits the incorporation of relative small-sized acyl and allyl groups into acylated and allylated all-carbon quaternary centers. Finally, we will show how the knowledge gained with halide electrophiles enabled us to develop methods that permit the coupling of tertiary alkyl oxalates with allyl, aryl, and vinyl electrophiles, wherein Barton C-O bond radical fragmentation is mediated by Zn and MgCl₂ and promoted by Ni catalysts. The same protocol is applicable to the arylation of secondary alkyl oxalates derived from α-hydroxyl carbonyl substrates, which involves the formation of relatively stable α-carbonyl carbon centered radicals. Thus, this Account not only summarizes synthetic methods that allow formation of valuable C-C bonds using challenging electrophiles but also provides insight into the relationship between the structure and reactivity of the substrates and catalysts, as well as the effects of additives.

Luminescent tungsten(vi) complexes as photocatalysts for light-driven C-C and C-B bond formation reactions

The realization of photocatalysis for practical synthetic application hinges on the development of inexpensive photocatalysts which can be prepared on a large scale. Herein an air-stable, visible-light-absorbing photoluminescent tungsten(vi) complex which can be conveniently prepared at the gram-scale is described. This complex could catalyse photochemical organic transformation reactions including borylation of aryl halides, such as aryl chloride, reductive coupling of benzyl bromides for C-C bond formation, reductive coupling of phenacyl bromides, and decarboxylative coupling of redox-active esters of alkyl carboxylic acid with high product yields and broad functional group tolerance.

Deoxygenative α-alkylation and α-arylation of 1,2-dicarbonyls

Construction of C-C bonds at the α-carbon is a challenging but synthetically indispensable approach to α-branched carbonyl motifs that are widely represented among drugs, natural products, and synthetic intermediates. Here, we describe a simple approach to generation of boron enolates in the absence of strong bases that allows for introduction of both α-alkyl and α-aryl groups in a reaction of readily accessible 1,2-dicarbonyls and organoboranes. Obviation of unselective, strongly basic and nucleophilic reagents permits carrying out the reaction in the presence of electrophiles that intercept the intermediate boron enolates, resulting in two new α-C-C bonds in a tricomponent process.