Photoredox-Catalyzed Cyclobutane Synthesis by a Deboronative Radical Addition-Polar Cyclization Cascade

The current utility and future potential of multiborylated alkanes

Organoboron chemistry has become a cornerstone of modern synthetic methodology. Most of these reactions use an organoboron starting material that contains just one C(sp2)-B or C(sp3)-B bond; however, there has been a recent and accelerating trend to prepare multiborylated alkanes that possess two or more C(sp3)-B bonds. This is despite a lack of general reactivity, meaning many of these compounds currently offer limited downstream synthetic value. This Review summarizes recent advances in the exploration of multiborylated alkanes, including a discussion on how these products may be elaborated in further synthetic manipulations.

Energy-Transfer Enabled Divergent Synthesis of Polycyclic γ-Sultines

A visible-light-initiated energy-transfer enabled radical cyclization for the divergent synthesis of polycyclic γ-sultine derivatives has been developed. The reaction provides an alternative and expeditious access to benzofused γ-sultine frameworks, the analogues of γ-lactones and γ-sultones, and features good functional group compatibility, mild reaction conditions and excellent diastereoselectivity. The robustness and application potential of this method have also been successfully displayed by two gram-scale reactions and the synthesis of polycyclic sultones. Mechanistic studies indicated the transformations through a possible energy-transfer enabled intramolecular radical homolytic substitution or hydrogen atom transfer process mainly.

Graphitic Carbon Nitride as a Photocatalyst for Decarboxylative C(sp2)-C(sp3) Couplings via Nickel Catalysis

The development of robust and reliable methods for the construction of C(sp2)-C(sp3) bonds is vital for accessing an increased array of structurally diverse scaffolds in drug discovery and development campaigns. While significant advances towards this goal have been achieved using metallaphotoredox chemistry, many of these methods utilise photocatalysts based on precious-metals due to their efficient redox processes and tuneable properties. However, due to the cost, scarcity, and toxicity of these metals, the search for suitable replacements should be a priority. Here, we show the use of commercially available heterogeneous semiconductor graphitic carbon nitride (gCN) as a photocatalyst, combined with nickel catalysis, for the cross-coupling between aryl halide and carboxylic acid coupling partners. gCN has been shown to engage in single-electron-transfer (SET) and energy-transfer (EnT) processes for the formation of C-X bonds, and in this manuscript we overcome previous limitations to furnish C-C over C-O bonds using carboxylic acids. A broad scope of both aryl halides and carboxylic acids is presented, and recycling of the photocatalyst demonstrated. The mechanism of the reaction is also investigated.

Deboronative functionalization of alkylboron species via a radical-transfer strategy

With advances in organoboron chemistry, boron-centered functional groups have become increasingly attractive. In particular, alkylboron species are highly versatile reagents for organic synthesis, but the direct generation of alkyl radicals from commonly used, bench-stable boron species has not been thoroughly investigated. Herein, we describe a method for activating C-B bonds by nitrogen- or oxygen-radical transfer that is applicable to alkylboronic acids and esters and can be used for both Michael addition reactions and Minisci reactions to generate alkyl or arylated products.

Parallel Minisci Reaction of gem-Difluorocycloalkyl Building Blocks

Parallel Minisci reactions of nonfluorinated and gem-difluorinated C4-C7 cycloalkyl building blocks (trifluoroborates and carboxylic acids) with a series of electron-deficient heterocycles were studied. A comparison of the reaction's outcome revealed better product yields in the case of carboxylic acids as the radical precursors in most cases, albeit these reagents were used with three-fold excess under optimized conditions. The nature of the heterocyclic core was found to be important for successful incorporation of the cycloalkyl fragment. The impact of the CF2 moiety on the oxidation potential of fluorinated cycloalkyl trifluoroborates and the reaction outcome, in general, was also evaluated.

A General Three-Component Alkyl Petasis Boron-Mannich Reaction

Aryl amines are one of the most common moieties in biologically active molecules, and approximately 37% of drug candidates contain aromatic amines. Recent advancements in medicinal chemistry, coined "escaping from flatland", have led to a greater focus on accessing highly functionalized C (sp3)-rich amines to improve the physicochemical and pharmacokinetic properties of compounds. This article presents a modular and operationally straightforward three-component alkyl Petasis boron-Mannich (APBM) reaction that utilizes ubiquitous starting materials, including amines, aldehydes, and alkyl boronates. By adaptation of this transformation to high-throughput experimentation (HTE), it offers rapid access to an array of diverse C(sp3)-rich complex amines, amenable for rapid identification of drug candidates.

Photoredox-Catalyzed Amino-Radical-Transfer-Mediated Three-Component Alkylarylation of Alkenes

We herein reported a novel photoredox-catalyzed three-component alkylarylation of vinyl arenes with alkylboronic pinacol esters (APEs) and cyanoarenes via radical addition/cross-coupling to construct 1,1-diarylalkanes. In this transformation, alkyl radicals were easily available by visible-light-induced oxidative N-H cleavage of morpholine, which used APEs as a radical precursor. Furthermore, this protocol exhibited a broad substrate scope, enabling various styrenes, APEs, and cyanoarenes, as well as bioactive molecule derivatives.

Synthesis of constrained bicycloalkanes through bibase-promoted brook rearrangement/radical-polar crossover cyclization

Highly constrained bicyclic scaffolds are ubiquitous and attracting increasing interest in pharmaceutical and biotechnology discoveries owing to the enhanced activities. Herein, we report a protocol to access highly substituted constrained bicycloalkanes from readily accessible α-silyl alcohols and olefins through a bibase-promoted Brook rearrangement/radical-polar crossover cyclization (RPCC) process. Of note, the practical procedure features broad substrate scope and good group tolerance under mild and operationally simple conditions, using an inexpensive organic photocatalyst. Gram-scale preparation and diverse synthetic transformations demonstrate opportunities to rapidly construct molecular complexity. Mechanistic studies have indicated that the transformation involves a bibase-promoted radical transfer rearrangement addition/radical-polar crossover cyclization relay sequence, which differs from traditional solitary RPCC reactions.

Cu-Catalyzed Coupling of Aliphatic Amines with Alkylboronic Esters

We report a Cu-catalyzed oxidative coupling of aliphatic amines with benzylic and aliphatic boronic esters to give high value alkyl amines, products found widely in applications from medicinal chemistry to materials science. This operationally simple reaction, which can be performed on gram scale, runs under mild conditions and exhibits broad functional group tolerance. The terminal oxidant of the reaction is O2 from the air, avoiding the need for additional chemical oxidants. Investigation into the reaction mechanism suggests that the boronic ester is activated by an aminyl radical, formed through oxidation of the amine by the Cu catalyst, to give a key alkyl radical intermediate. To demonstrate its utility and potential for late-stage functionalization, we showcase the method as the final step in the total synthesis of a TRPV1 antagonist.

Electroreductive Radical Addition-Polar Cyclization Cascade to Access Cycloalkanes

Compared with flat aromatic scaffolds, three-dimensional aliphatic ring systems feature high structural complexity and topological diversity and, thus, have received increasing attention in drug discovery. Herein, we describe a mild and general electrochemical method for the modular synthesis of structurally distinct cyclic compounds, including monocyclic alkanes, benzo-fused ring systems, and spirocycles, from readily available alkenes and alkyl halides via a radical-polar crossover mechanism.

Photoredox radical/polar crossover enables C-H gem-difunctionalization of 1,3-benzodioxoles for the synthesis of monofluorocyclohexenes

A photocatalytic C-H gem-difunctionalization of 1,3-benzodioxoles with two different alkenes for the synthesis of highly functionalized monofluorocyclohexenes is described. Using 4CzIPN as the photocatalyst, the direct single electron oxidation of 1,3-benzodioxoles allows their defluorinative coupling with α-trifluoromethyl alkenes to produce gem-difluoroalkenes in a redox-neutral radical polar crossover manifold. The C-H bond of the resultant γ,γ-difluoroallylated 1,3-benzodioxoles was further functionalized via radical addition to electron-deficient alkenes using a more oxidizing iridium photocatalyst. The capture of in situ generated carbanions by an electrophilic gem-difluoromethylene carbon and consecutive β-fluoride elimination afford monofluorocyclohexenes. The synergistic combination of multiple termination pathways of carbanions enables rapid incorporation of molecular complexity via stitching simple and readily accessible starting materials together.

Photoredox-Catalyzed Multicomponent Synthesis of Functionalized γ-Amino Butyric Acids via Reductive Radical Polar Crossover

Multicomponent radical polar crossover (RPC) reactions are useful for leveraging both radical and polar bond-forming steps to rapidly build molecular complexity in a single transformation. However, multicomponent RPC reactions that utilize carbonyl π-bond electrophiles are underrepresented in the literature. Herein, we describe a mild, photoredox-catalyzed decarboxylative multicomponent RPC reaction that couples carboxylic acids, Michael acceptors, and carbonyl electrophiles for the formation of diversely functionalized γ-amino butyric acid derivatives. This transformation also facilitates the synthesis of complex and biologically relevant γ-lactam compounds.

Alkyl Radical Generation from Alkylboronic Pinacol Esters through Substitution with Aminyl Radicals

Alkylboronic pinacol esters (APEs) are highly versatile reagents in organic synthesis. However, the direct generation of alkyl radicals from commonly used, bench-stable APEs has not been well explored. In this communication, alkyl radical generation from APEs through reaction with aminyl radicals is reported. The aminyl radicals are readily generated by visible-light-induced homolytic cleavage of the N-N bond in N-nitrosamines, and C radical generation occurs through nucleohomolytic substitution at boron. As an application, the highly efficient photochemical radical alkyloximation of alkenes with APEs and N-nitrosamines under mild conditions is presented. A wide range of primary, secondary, and tertiary APEs engage in this transformation that is easily scaled up.

Stereoretentive Formation of Cyclobutanes from Pyrrolidines: Lessons Learned from DFT Studies of the Reaction Mechanism

The stereoselective synthesis of cyclobutanes that possess an array of stereocenters in a contiguous fashion has attracted the wide interest of the synthetic community. Cyclobutanes can be generated from the contraction of pyrrolidines through the formation of 1,4-biradical intermediates. Little else is known about the reaction mechanism of this reaction. Here, we unveil the mechanism for this stereospecific synthesis of cyclobutanes by means of density functional theory (DFT) calculations. The rate-determining step of this transformation corresponds to the release of N₂ from the 1,1-diazene intermediate to form an open-shell singlet 1,4-biradical. The formation of the stereoretentive product is explained by the barrierless collapse of this open-shell singlet 1,4-biradical. The knowledge of the reaction mechanism is used to predict that the methodology could be amenable to the synthesis of [2]-ladderanes and bicyclic cyclobutanes.

Visible Light-Induced Diastereoselective Construction of Trifluoromethylated Cyclobutane Scaffolds through [2+2]-Photocycloaddition and Water-Assisted Hydrodebromination

A visible light-induced diastereoselective synthesis of trifluoromethylated cyclobutane derivatives is described, consisting of [2+2]-photocycloaddition and water-assisted hydrodebromination by one pot. Quinolinones, isoquinolinones, and coumarins are able to participate in this one-pot process with 1-bromo-1-trifluoromethylethene. In addition, stereodefined trisubstituted trifluoromethylated cyclobutane alcohols, carboxylic acids, and amines can be obtained in a straightforward manner through the ring opening of lactone or lactam without the loss of original high diastereoselectivity given by the water-tristrimethylsilylsilane coordination. The antineoplastic bioactivities of those compounds are also well studied, which exhibit great antineoplastic potential comparable to cisplatin. In the proposed mechanism, thioxanthone (TX) serves as a dual catalyst and a radical chain pathway may be involved in the hydrodebromination process.

Synthesis of 3-borylated cyclobutanols from epihalohydrins or epoxy alcohol derivatives

There is an increasing interest in cyclobutanes within the medicinal chemistry community. Therefore, methods to prepare cyclobutanes that contain synthetic handles for further elaboration are of interest. Herein, we report a new approach for the synthesis of 3-borylated cyclobutanols via a formal [3 + 1]-cycloaddition using readily accessible 1,1-diborylalkanes and epihalohydrins or epoxy alcohol derivatives. 1-Substituted epibromohydrin starting materials provide access to borylated cyclobutanols containing substituents at three of the four positions on the cyclobutane core, and enantioenriched epibromohydrins lead to enantioenriched cyclobutanols with high levels of enantiospecificity (>98%). Finally, derivatization studies demonstrate the synthetic utility of both the OH and Bpin handles.

Access to functionalized alkynylcyclopropanes via reductive radical-polar crossover-based reactions of 1,3-enynes with alkyl radicals

Herein, using a single-electron-transfer reduction-based radical-polar crossover process as a strategy, protocols dealing with the preparation of functionalized alkynylcyclopropanes have been successfully developed via the reactions of 1,3-enynes with alkyl radicals. In addition to redox-neutral photocatalysis, nickel catalysis with zinc as the reductant is also an alternative to enable reactions of 1,3-enynes with redox-active N-hydroxyphthalimide esters. The synthetic application of alkynylcyclopropane has also been demonstrated.

Alkylboronic acids as alkylating agents: photoredox-catalyzed alkylation reactions assisted by K3PO4

Despite the ubiquity of alkylboronic acids in organic synthesis, their utility as alkyl radical precursors in visible-light-induced photocatalytic reactions is limited by their high oxidation potentials. In this study, we demonstrated that an inorganophosphorus compound can modulate the oxidation potentials of alkylboronic acids so that they can act as alkyl radical precursors. We propose a mechanism based on the results of fluorescence quenching experiments, electrochemical experiments, ¹¹B and ³¹P NMR spectroscopy, and other techniques. In addition, we describe a simple and reliable alkylation method that has good functional group tolerance and can be used for direct C-B chlorination, cyanation, vinylation, alkynylation, and allylation, as well as late-stage functionalization of derivatized drug molecules. Notably, alkylboronic acids can be selectively activated in the presence of a boronic pinacol ester.

Photocatalytic Decarboxylative Coupling of Arylacetic Acids with Aromatic Aldehydes

An efficient protocol was proposed for the preparation of secondary alcohols in good to excellent yields via photoredox-catalyzed decarboxylative couplings between readily available arylacetic acids and a variety of less reactive (hetero)aromatic aldehydes. The formation of carbanion is the key intermediate in this reaction. Various substituted arylacetic acids and aldehydes were all compatible with this transformation under mild reaction conditions. Furthermore, the current protocol was successfully applied to the direct alcoholization of several drug acids.

α-Thianthrenium Carbonyl Species: The Equivalent of an α-Carbonyl Carbocation

Here we report an α-thianthrenium carbonyl species, as the equivalent of an α-carbonyl carbocation, which is generated by the radical conjugate addition of a trifluoromethyl thianthrenium salt to Michael acceptors. The reactivity allows for the synthesis of Cα -tetrasubstituted α- and β-amino acid analogues via a Ritter reaction by addition of acetonitrile. Addition of hydroxide, methoxide, and even fluoride can afford α-heteroatom substituted α-phenylpropanoates.

The Impact of Boron Hybridisation on Photocatalytic Processes

Recently the fruitful merger of organoboron chemistry and photocatalysis has come to the forefront of organic synthesis, resulting in the development of new technologies to access complex (non)borylated frameworks. Central to the success of this combination is control of boron hybridisation. Contingent on the photoactivation mode, boron as its neutral planar form or tetrahedral boronate can be used to regulate reactivity. This Minireview highlights the current state of the art in photocatalytic processes utilising organoboron compounds, paying particular attention to the role of boron hybridisation for the target transformation.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts

Site-selective transition-metal-catalyzed mono-deboronative cross-couplings of 1,2-bis-boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross-couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox-catalyzed mono-deboronative arylation of 1,2-bis-boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2-boron shift of primary β-boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β-aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to give trans-substituted products.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

Concise synthesis of piperarborenine B

A concise synthesis of piperarborenine B is reported. Organocatalytic electrophilic amination of pyrrolidines, stereospecific oxidative ring contraction and an original diastereoselective Krapcho dealkoxycarbonylation/transmethylation contribute to a novel synthetic strategy to the preparation of a non-symmetrical cyclobutane core. Being transition-metal-free, directing-group-free and protecting-group-free, a five-step synthesis of piperarborenine B was accomplished.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Selective Coupling of 1,2-Bis-Boronic Esters at the more Substituted Site through Visible-Light Activation of Electron Donor-Acceptor Complexes

1,2-Bis-boronic esters are useful synthetic intermediates particularly as the two boronic esters can be selectively functionalized. Usually, the less hindered primary boronic ester reacts, but herein, we report a coupling reaction that enables the reversal of this selectivity. This is achieved through the formation of a boronate complex with an electron-rich aryllithium which, in the presence of an electron-deficient aryl nitrile, leads to the formation of an electron donor-acceptor complex. Following visible-light photoinduced electron transfer, a primary radical is generated which isomerizes to the more stable secondary radical before radical-radical coupling with the arene radical-anion, giving β-aryl primary boronic ester products. The reactions proceed under catalyst-free conditions. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to provide trans-substituted products, complementing the selectivity observed in the Suzuki-Miyaura reaction.

Photo-induced trifunctionalization of bromostyrenes via remote radical migration reactions of tetracoordinate boron species

Tetracoordinate boron species have emerged as radical precursors via deboronation by photo-induced single electron transfer (SET) pathway. These reactions usually produce an alkyl radical and boron-bound species, and the valuable boron species are always discarded as a by-product. Given the importance of boron species, it will be very attractive if the two parts could be incorporated into the eventual products. Herein we report a photo-catalyzed strategy in which in situ generated tetracoordinated boron species decomposed into both alkyl radicals and boron species under visible light irradiation, due to the pre-installation of a vinyl group on the aromatic ring, the newly generated alkyl radical attacks the vinyl group while leaving the boron species on ipso-position, then both radical part and boron moiety are safely incorporated into the final product. Tertiary borons, secondary borons, gem-diborons as well as 1,2-diborons, and versatile electrophiles are all well tolerated under this transformation, of note, ortho-, meta- and para-bromostyrenes all demonstrated good capabilities. The reaction portraits high atom economy, broad substrate scope, and diversified valuable products with tertiary or quaternary carbon center generated, with diborons as substrates, Csp²-B and Csp³-B are established simultaneously, which are precious synthetic building blocks in chemical synthesis.

Tetracoordinate boron species are common radical precursors in organic synthesis, but the boron species are discarded as by-products. Herein the authors report a strategy to incorporate both the alkyl moiety and boron species into the eventual products, yielding organoboron compounds.

Boryl Radical Activation of Benzylic C-OH Bond: Cross-Electrophile Coupling of Free Alcohols and CO2 via Photoredox Catalysis

A new strategy for the direct cleavage of the C(sp³)-OH bond has been developed via activation of free alcohols with neutral diphenyl boryl radical generated from sodium tetraphenylborate under mild visible light photoredox conditions. This strategy has been verified by cross-electrophile coupling of free alcohols and carbon dioxide for the synthesis of carboxylic acids. Direct transformation of a range of primary, secondary, and tertiary benzyl alcohols to acids has been achieved. Control experiments and computational studies indicate that activation of alcohols with neutral boryl radical undergoes homolysis of the C(sp³)-OH bond, generating alkyl radicals. After reducing the alkyl radical into carbon anion under photoredox conditions, the following carboxylation with CO₂ affords the coupling product.

Boron, silicon, nitrogen and sulfur-based contemporary precursors for the generation of alkyl radicals by single electron transfer and their synthetic utilization

Recent developments in the use of boron, silicon, nitrogen and sulfur derivatives in single-electron transfer reactions for the generation of alkyl radicals are described. Photoredox catalyzed, electrochemistry promoted or thermally-induced oxidative and reductive processes are discussed highlighting their synthetic scope and discussing their mechanistic pathways.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Mechanistic Dichotomy in Solvent Dependent Access to E vs Z-allylic Amines via Decarboxylative Vinylation of Amino Acids

The solvent plays an important role in the photophysical properties of donor–acceptor based photocatalysts. The solvent-dependent access to E vs. Z-allylic amines was achieved via decarboxylative vinylation of amino acids with vinyl sulfones. Detailed experimental studies have been conducted to understand the role of the solvent in the reactivity and stereoselectivity of the vinylation reactions.

A solvent-dependent access to E vs. Z-allylic amines was achieved via decarboxylative vinylation of amino acids. Detailed experimental studies have been conducted to understand the role of the solvent in the reactivity and stereoselectivity of the vinylation reactions.

Three-component 1,2-carboamination of vinyl boronic esters via amidyl radical induced 1,2-migration

Three-component 1,2-carboamination of vinyl boronic esters with alkyl/aryl lithium reagents and N-chloro-carbamates/carboxamides is presented. Vinylboron ate complexes generated in situ from the boronic ester and an organo lithium reagent are shown to react with readily available N-chloro-carbamates/carboxamides to give valuable 1,2-aminoboronic esters. These cascades proceed in the absence of any catalyst upon simple visible light irradiation. Amidyl radicals add to the vinylboron ate complexes followed by oxidation and 1,2-alkyl/aryl migration from boron to carbon to give the corresponding carboamination products. These practical cascades show high functional group tolerance and accordingly exhibit broad substrate scope. Gram-scale reaction and diverse follow-up transformations convincingly demonstrate the synthetic potential of this method.

Stereoselective Synthesis of Cyclobutanes by Contraction of Pyrrolidines

Here we report a contractive synthesis of multisubstituted cyclobutanes containing multiple stereocenters from readily accessible pyrrolidines using iodonitrene chemistry. Mediated by a nitrogen extrusion process, the stereospecific synthesis of cyclobutanes involves a radical pathway. Unprecedented unsymmetrical spirocyclobutanes were prepared successfully, and a concise, formal synthesis of the cytotoxic natural product piperarborenine B is reported.

A Modular Access to 1,2- and 1,3-Disubstituted Cyclobutylboronic Esters by Consecutive Radical Additions

A modular approach to substituted cyclobutylboronic esters is described. It proceeds by successive intermolecular radical additions of xanthates to pinacolato 1-cyclobutenylboronate and to pinacolato bicyclo[1.1.0]but-1-ylboronate. Success hinges on tuning the stability of the α-boryl radical by exploiting the stabilizing influence of the trivalent boronic ester and the slightly destabilizing cyclobutane, which increases the σ-character of the radical. Reductive removal of the xanthate group finally provides a range of 1,2- and 1,3-disubstituted cyclobutylboronic esters. The contrast with cyclopropylboronic esters is striking, since the strong destabilization by the highly strained cyclopropane ring allows the first radical addition to take place but not the second. Furthermore, the first adducts are geminal xanthyl boronic esters that can be converted into cyclobutanones. This chemistry furnishes cyclobutylboronic esters that would be quite difficult to obtain otherwise and thus complements existing methods.

Construction of Complex Cyclobutane Building Blocks by Photosensitized [2 + 2] Cycloaddition of Vinyl Boronate Esters

Cyclobutyl moieties in drug molecules are rare, and in general, they are minimally substituted and stereochemically simple. Methods to assemble structurally complex cyclobutane building blocks suitable for rapid diversification are thus highly desirable. We report herein a photosensitized [2 + 2] cycloaddition with vinyl boronate esters affording straightforward access to complex, densely functionalized cyclobutane scaffolds. Mechanistic studies suggest an activation mode involving energy transfer to the styrenyl alkene rather than the vinyl boronate ester.

Photoredox-enabled 1,2-dialkylation of α-substituted acrylates via Ireland-Claisen rearrangement

Herein, we report the 1,2-dialkylation of simple feedstock acrylates for the synthesis of valuable tertiary carboxylic acids by merging Giese-type radical addition with an Ireland-Claisen rearrangement. Key to success is the utilization of the reductive radical-polar crossover concept under photocatalytic reaction conditions to force the [3,3]-sigmatropic rearrangement after alkyl radical addition to allyl acrylates. Using readily available alkyl boronic acids as radical progenitors, this redox-neutral, transition-metal-free protocol allows the mild formation of two C(sp³)-C(sp³) bonds, thus providing rapid access to complex tertiary carboxylic acids in a single step. Moreover, this strategy enables the efficient synthesis of highly attractive α,α-dialkylated γ-amino butyric acids (GABAs) when α-silyl amines are used as radical precursors - a structural motif that was still inaccessible in related transformations. Depending on the nature of the radical precursors and their inherent oxidation potentials, either a photoredox-induced radical chain or a solely photoredox mechanism is proposed to be operative.