Enantioselective synthesis of cyclobutanes via sequential Rh-catalyzed bicyclobutanation/Cu-catalyzed homoconjugate addition

Rhodium(I)-Catalyzed Annulation of Bicyclo[1.1.0]butyl-Substituted Dihydroquinolines and Dihydropyridines

Bicyclo[1.1.0]butane-containing compounds feature a unique chemical reactivity, trigger "strain-release" reaction cascades, and provide novel scaffolds with considerable utility in the drug discovery field. We report the synthesis of new bicyclo[1.1.0]butane-linked heterocycles by a nucleophilic addition of bicyclo[1.1.0]butyl anions to 8-isocyanatoquinoline, or, alternatively, iminium cations derived from quinolines and pyridines. The resulting bicyclo[1.1.0]butanes are converted with high regioselectivity to unprecedented bridged heterocycles in a rhodium(I)-catalyzed annulative rearrangement. The addition/rearrangement process tolerates a surprisingly large range of functional groups. Subsequent chemo- and stereoselective synthetic transformations of urea, alkene, cyclopropane, and aniline moieties of the 1-methylene-5-azacyclopropa[cd]indene scaffolds provide several additional new heterocyclic building blocks. X-ray structure-validated quantum mechanical DFT calculations of the reaction pathway indicate the intermediacy of rhodium carbenoid and metallocyclobutane species.

Synthesis of Stereodefined Polysubstituted Bicyclo[1.1.0]butanes

We report a highly diastereoselective synthesis of polysubstituted bicyclobutanes possessing up to three stereodefined quaternary centers and five substituents. Our strategy involves a diastereoselective carbometalation of cyclopropenes followed by a cyclization to furnish the bicyclobutane ring system. This straightforward approach allows for the incorporation of a diverse range of substituents and functional groups, notably without the need for electron-withdrawing functionalities.

Umpolung reactivity of strained C-C σ-bonds without transition-metal catalysis

Umpolung is an old and important concept in organic chemistry, which significantly expands the chemical space and provides unique structures. While, previous research focused on carbonyls or imine derivatives, the umpolung reactivity of polarized C-C σ-bonds still needs to explore. Herein, we report an umpolung reaction of bicyclo[1.1.0]butanes (BCBs) with electron-deficient alkenes to construct the C(sp3)-C(sp3) bond at the electrophilic position of C-C σ-bonds in BCBs without any transition-metal catalysis. Specifically, this transformation relies on the strain-release driven bridging σ-bonds in bicyclo[1.1.0]butanes (BCBs), which are emerged as ene components, providing an efficient and straightforward synthesis route of various functionalized cyclobutenes and conjugated dienes, respectively. The synthetic utilities of this protocol are performed by several transformations. Preliminary mechanistic studies including density functional theory (DFT) calculation support the concerted Alder-ene type process of C-C σ-bond cleavage with hydrogen transfer. This work extends the umpolung reaction to C-C σ-bonds and provides high-value structural motifs.

Photoredox-Catalyzed Alkylarylation of N-Aryl Bicyclobutyl Amides with α-Carbonyl Alkyl Bromides: Access to 3-Spirocyclobutyl Oxindoles

A visible-light-induced radical alkylarylation of N-aryl bicyclobutyl amides with α-carbonyl alkyl bromides for the synthesis of functionalized 3-spirocyclobutyl oxindoles is described in which β-selective radical addition of the alkyl radical to N-aryl bicyclobutyl amides forms a key radical intermediate followed by interception with intrinsic arene functional group. This approach can be applicable to a wide range of α-carbonyl alkyl bromides, including primary, secondary, and tertiary α-bromoalkyl esters, ketones, nitriles, and nitro compounds.

Bridge Cross-Coupling of Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) have gained growing popularity in "strain release" chemistry for the synthesis of four-membered-ring systems and para- and meta-disubstituted arene bioisosteres as well as applications in chemoselective bioconjugation. However, functionalization of the bridge position of BCBs can be challenging due to the inherent strain of the ring system and reactivity of the central C-C bond. Here we report the first late-stage bridge cross-coupling of BCBs, mediated by directed metalation/palladium catalysis.

Stereoselective Alder-Ene Reactions of Bicyclo[1.1.0]butanes: Facile Synthesis of Cyclopropyl- and Aryl-Substituted Cyclobutenes

Bicyclo[1.1.0]butanes (BCBs), strained carbocycles comprising two fused cyclopropane rings, have become well-established building blocks in organic synthesis, medicinal chemistry, and chemical biology due to their diverse reactivity profile with radicals, nucleophiles, cations, and carbenes. The constraints of the bicyclic ring system confer high p-character on the interbridgehead C-C bond, leading to this broad reaction profile; however, the use of BCBs in pericyclic processes has to date been largely overlooked in favor of such stepwise, non-concerted additions. Here, we describe the use of BCBs as substrates for ene-like reactions with strained alkenes and alkynes, which give rise to cyclobutenes decorated with highly substituted cyclopropanes and arenes. The former products are obtained from highly stereoselective reactions with cyclopropenes, generated in situ from vinyl diazoacetates under blue light irradiation (440 nm). Cyclobutenes featuring a quaternary aryl-bearing carbon atom are prepared from equivalent reactions with arynes, which proceed in high yields under mild conditions. Mechanistic studies highlight the importance of electronic effects in this chemistry, while computational investigations support a concerted pathway and rationalize the excellent stereoselectivity of reactions with cyclopropenes.

Divergent Synthesis of 1,2,3,4-Tetrasubstituted Cyclobutenes from a Common Scaffold: Enantioselective Desymmetrization by Dual-Catalyzed Photoredox Cross-Coupling

Four-membered carbocycles are important structural motifs found in several natural products and drugs. Amongst those, cyclobutenes are attractive intermediates because the residual olefin can be manipulated selectively into various saturated and unsaturated analogs. Few methods exist to access chiral tri- and tetra-C-substituted cyclobutenes and they are generally limited in terms of diversification. Herein, a divergent synthetic strategy was developed where a single optically enriched scaffold is diversified into a variety of derivatives with different substitution patterns. To this end, the enantioselective desymmetrization of prochiral 1,2-dibromocyclobutene imides was enabled by a dual Ir/Ni-catalyzed photoredox C(sp2 )-C(sp3 ) cross-coupling with an alkyltrifluoroborate salt to install a convertible carbon fragment in good yields and >90 % enantiomeric excess. Exceptional mono-coupling selectivity is observed and the resulting chiral bromocyclobutene serves as a common scaffold that can be transformed in a divergent manner into several valuable 1,2,3,4-tetra-C-substituted cyclobutane products while maintaining optical purity.

Silver-Catalyzed Dearomative [2π+2σ] Cycloadditions of Indoles with Bicyclobutanes: Access to Indoline Fused Bicyclo[2.1.1]hexanes

Bicyclo[2.1.1]hexanes (BCHs) are becoming ever more important in drug design and development as bridged scaffolds that provide underexplored chemical space, but are difficult to access. Here a silver-catalyzed dearomative [2π+2σ] cycloaddition strategy for the synthesis of indoline fused BCHs from N-unprotected indoles and bicyclobutane precursors is described. The strain-release dearomative cycloaddition operates under mild conditions, tolerating a wide range of functional groups. It is capable of forming BCHs with up to four contiguous quaternary carbon centers, achieving yields of up to 99 %. In addition, a scale-up experiment and the synthetic transformations of the cycloadducts further highlighted the synthetic utility.

Photochemical α-selective radical ring-opening reactions of 1,3-disubstituted acyl bicyclobutanes with alkyl halides: modular access to functionalized cyclobutenes

Although ring-opening reactions of bicyclobutanes bearing electron-withdrawing groups, typically with β-selectivity, have evolved as a powerful platform for synthesis of cyclobutanes, their application in the synthesis of cyclobutenes remains underdeveloped. Here, a novel visible light induced α-selective radical ring-opening reaction of 1,3-disubstituted acyl bicyclobutanes with alkyl radical precursors for the synthesis of functionalized cyclobutenes is described. In particular, primary, secondary, and tertiary alkyl halides are all suitable substrates for this photocatalytic transformation, providing ready access to cyclobutenes with a single all-carbon quaternary center, or with two contiguous centers under mild reaction conditions.

Enolate addition to bicyclobutanes enables expedient access to 2-oxo-bicyclohexane scaffolds

We report the synthesis of 2-oxo-bicyclo[2.1.1]hexanes (2-oxo-BCHs) from bicyclobutanes (BCBs) and readily available enolate precursors. Glycine-derived enolates directly give protected 2-oxo-3-amino-BCH derivatives that can be further functionalized. Arylacetate derivatives are also suitable enolate precursors, giving 2-oxo-3-aryl-BCH scaffolds from readily available starting materials.

syn-Selective Difunctionalization of Bicyclobutanes Enabled by Photoredox-Mediated C-S σ-Bond Scission

Given the importance of cyclic frameworks in molecular scaffolds and drug discovery, it is intriguing to precisely forge and manipulate ring systems in synthetic chemistry. In this field, the intermolecular synthesis of densely substituted cyclobutanes with precise diastereocontrol under simple reaction conditions remains a challenge. Herein, a photoredox strategy for the difunctionalization of bicyclo[1.1.0]butanes (BCBs) under high regio- and syn-selectivity is disclosed. C-S σ-bond cleavage of partially unsaturated sulfur-containing bifunctional reagents in an overall strain-release-driven process enables the thio-alkynylation, -alkenylation, and -allylation of BCBs under mild conditions and demonstrates the generality of this protocol. Mechanistic studies suggest that the intermediacy of cyclic distonic radical cations might be key for the efficient scission of C-S σ-bonds and the origin of diastereoselectivity.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

C(sp2)-H cyclobutylation of hydroxyarenes enabled by silver-π-acid catalysis: diastereocontrolled synthesis of 1,3-difunctionalized cyclobutanes

Ring-opening of bicyclo[1.1.0]butanes (BCBs) is emerging as a powerful strategy for 1,3-difunctionalized cyclobutane synthesis. However, reported radical strain-release reactions are typically plagued with diastereoselectivity issues. Herein, an atom-economic protocol for the highly chemo- and diastereoselective polar strain-release ring-opening of BCBs with hydroxyarenes catalyzed by a π-acid catalyst AgBF4 has been developed. The use of readily available starting materials, low catalyst loading, high selectivity (up to >98 : 2 d.r.), a broad substrate scope, ease of scale-up, and versatile functionalizations of the cyclobutane products make this approach very attractive for the synthesis of 1,1,3-trisubstituted cyclobutanes. Moreover, control experiments and theoretical calculations were performed to illustrate the reaction mechanism and selectivity.

Lewis acid-catalyzed diastereoselective carbofunctionalization of bicyclobutanes employing naphthols

Traditional radical-mediated ring-opening of bicyclo[1.1.0]butanes (BCBs) for cyclobutane synthesis suffers from poor diastereoselectivity. Although few reports on BCB ring-opening via polar mechanisms are available, the Lewis acid-catalyzed diastereoselective ring-opening of BCBs using carbon nucleophiles is still underdeveloped. Herein, we report a mild and diastereoselective Bi(OTf)₃-catalyzed ring-opening of BCBs employing 2-naphthols. The anticipated carbofunctionalized trisubstituted cyclobutanes were obtained via a bicoordinated bismuth complex and the products are formed in good to excellent yields with high regio- and diastereoselectivity. The scope of the reaction was further extended using electron-rich phenols and naphthylamine. The functionalization of the synthesized trisubstituted cyclobutanes shows the synthetic utility of the present method.

Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics

The use of metalated (aza)bicyclo[1.1.0]butanes in synthesis is currently experiencing a renaissance, as evidenced by the numerous reports in the last 5 years that have relied on such intermediates to undergo unique transformations or generate novel fragments. Since their discovery, these species have been demonstrated to participate in a wide range of reactions with carbon and heteroatom electrophiles, as well as metal complexes, to facilitate the rapid diversification of (aza)bicyclo[1.1.0]butane-containing compounds. Key to this is the relative acidity of the bridgehead C-H bonds which promotes facile deprotonation and subsequent functionalization of an unsubstituted position on the carbon framework via the intermediacy of a metalated (aza)bicyclo[1.1.0]butane. Additionally, the late-stage incorporation of deuterium atoms in strained fragments has led to the elucidation of numerous reaction mechanisms that involve strained bicycles. The continued investigation into the inimitable reactivity of metalated bicycles will cement their importance within the field of organometallic chemistry.

Enolates ambushed - asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

Metal enolates are useful intermediates and building blocks indispensable in many organic synthetic transformations. Chiral metal enolates obtained by asymmetric conjugate additions of organometallic reagents are structurally complex intermediates that can be employed in many transformations. In this review, we describe this burgeoning field that is reaching maturity after more than 25 years of development. The effort of our group to broaden possibilities to engage metal enolates in reactions with new electrophiles is described. The material is divided according to the organometallic reagent employed in the conjugate addition step, and thus to the particular metal enolate formed. Short information on applications in total synthesis is also given.

Bicyclobutanes as unusual building blocks for complexity generation in organic synthesis

Bicyclobutanes are among the most highly strained isolable organic compounds and their associated low activation barriers to reactivity make them intriguing building-blocks in organic chemistry. In recent years, numerous creative synthetic strategies exploiting their heightened reactivity have been presented and these discoveries have often gone hand-in-hand with the development of more practical routes for their synthesis. Their proclivity as strain-release reagents through their weak central C-C bond has been harnessed in a variety of addition, rearrangement and insertion reactions, providing rapid access to a rich tapestry of complex molecular scaffolds. This review will provide an overview of the different options available for bicyclobutane synthesis, the main classes of compounds that can be prepared from bicyclobutanes, and the associated modes of reactivity used.

How mono- and diphosphine ligands alter regioselectivity of the Rh-catalyzed annulative cleavage of bicyclo[1.1.0]butanes

Rh(I)-catalyzed cycloisomerizations of bicyclo[1.1.0]butanes provide a fruitful approach to cyclopropane-fused heterocycles. Products and stereochemical outcome are highly dependent on catalyst. The triphenylphosphine (PPh3) ligand provides pyrrolidines, placing substituents anti to the cyclopropyl group. The 1,2-bis(diphenylphosphino)ethane (dppe) ligand yields azepanes with substituents syn to the cyclopropyl group. In this work, quantum mechanical DFT calculations pinpoint a reversal of regio- and diastereoselectivity, suggesting a concerted (double) C−C bond cleavage and rhodium carbenoid formation, driven by strain-release. The ligand-influenced cleavage step determines the regioselectivity of carbometalation and product formation, and suggests new applications of bicyclobutanes.

Palladium Hydride-Enabled Hydroalkenylation of Strained Molecules

We report the first palladium hydride enabled hydroalkenylation of strained molecules. This new mild protocol proceeds via a regio- and chemoselective hydropalladation step, followed by a photoinduced radical alkyl Heck reaction. This methodology represents a new reactivity mode for strained molecules and opens new avenues for photoinduced palladium catalysis. The reaction is compatible with a wide range of functional groups and can be applied to complex structures, delivering a diverse array of highly valuable and modifiable alkenylated cyclobutanes and cyclopropanes. A hydroalkenylation/diastereoselective rearrangement cascade toward a cyclopentene scaffold has also been demonstrated.

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

The unique chemistry of small, strained carbocyclic systems has long captivated organic chemists from a theoretical and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as laboratory curiosities. However, new approaches for preparing, functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.

Catalytic Synthesis of Cyclopropenium Cations with Rh-Carbynoids

Herein, we report the first catalytic one-step synthesis of cyclopropenium cations (CPCs) with readily available alkynes and hypervalent iodine reagents as carbyne sources. Key to the process is the catalytic generation of a novel Rh-carbynoid that formally transfers monovalent cationic carbynes (:⁺C-R) to alkynes via an oxidative [2+1] cycloaddition. Our process is able to synthesize a new type of CPC substituted with an ester group that underpins the regioselective attack of a broad range of carbon and heteroatomic nucleophiles, thus providing a new platform for the synthesis of valuable cyclopropenes difficult or not possible to make by current methodologies.

How Strain-Release Determines Chemoselectivity: A Mechanistic Study of Rhodium-Catalyzed Bicyclo[1.1.0]butane Activation

Bicyclo[1.1.0]butane (BCB) derivatives are versatile coupling partners, and various reaction modes for their activation and transformation have been proposed. In this work, three BCB-activation modes in Rh-catalyzed BCB transformations that construct diastereoselective α-quaternary β-lactones were investigated by density functional theory calculations. Our results show that, compared with C1-C3 insertion and C-C3 oxidative addition, C2-C3 oxidative addition is more favorable. The whole catalytic cycle involves five main steps: C-H activation, oxidative addition, β-C elimination/reductive elimination, Rh walking, and aldehyde insertion/protonation. Independent gradient model, intrinsic reaction coordinate, distortion-interaction energy, and Laplacian electron-density analyses were carried out to investigate the mode of BCB activation. Our calculation also showed that aldehyde-insertion is the diastereoselectivity determining step, which is controlled by the steric effect between the ligand, methyl group, and aldehyde.

Strain-Release [2π + 2σ] Cycloadditions for the Synthesis of Bicyclo[2.1.1]hexanes Initiated by Energy Transfer

Saturated bicycles are becoming ever more important in the design and development of new pharmaceuticals. Here a new strategy for the synthesis of bicyclo[2.1.1]hexanes is described. These bicycles are significant because they have defined exit vectors, yet many substitution patterns are underexplored as building blocks. The process involves sensitization of a bicyclo[1.1.0]butane followed by cycloaddition with an alkene. The scope and mechanistic details of the method are discussed.

Investigating Bicyclobutane-Triazolinedione Cycloadditions as a Tool for Peptide Modification

Acyl bicyclobutanes are shown to engage in strain-promoted cycloaddition reactions with a diverse array of triazolinedione reagents. The synthesis of an orthogonally protected urazole building block enabled the facile preparation of amino acid- and peptide-derived triazolinediones that undergo cycloaddition reactions to afford novel peptide conjugates. The additive-free and fully atom-economical nature of the transformation is a promising starting point for the generalization of this cycloaddition reaction for the functionalization of biomolecules.

Diastereoselective synthesis of 1,1,3,3-tetrasubstituted cyclobutanes enabled by cycloaddition of bicyclo[1.1.0]butanes

A diastereoselective cycloaddition of bicyclo[1.1.0]butanes (BCBs) with triazolinedione or nitrosoarenes was developed to access multi-substituted cyclobutanes.

Synthesis and Applications of Polysubstituted Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) are valuable substrates in the "strain release" synthesis of polysubstituted four-membered ring systems, with applications including bioconjugation agents. The introduction of substituents onto the BCB bridges is challenging due to limitations in current methods for the preparation of this bicyclic scaffold, typically necessitating linear syntheses with limited functional group tolerance and/or substituent scope. Here, we report the synthesis of tri- and tetrasubstituted BCBs via directed metalation of readily accessed BCB amides; this straightforward "late stage" approach generates a wide variety of bridge-substituted BCBs that can be easily converted into other useful small ring building blocks. Access to a monodeuterated BCB afforded unprecedented insight into the mechanism of dihalocarbene insertion into BCBs to afford bicyclo[1.1.1]pentanes (BCPs).

Multicomponent Reactions Involving Diazo Reagents: A 5-Year Update

This review summarizes recent developments in multicomponent reactions of diazo compounds. The role of diazo reagent and the type of interaction between components was analyzed to structure the discussion. In contrast to previous reviews on related topics mostly focused on metal catalyzed transformations, a substantial amount of organocatalytic or catalyst-free methodologies is covered in this work.

Directed Evolution: Methodologies and Applications

Directed evolution aims to expedite the natural evolution process of biological molecules and systems in a test tube through iterative rounds of gene diversifications and library screening/selection. It has become one of the most powerful and widespread tools for engineering improved or novel functions in proteins, metabolic pathways, and even whole genomes. This review describes the commonly used gene diversification strategies, screening/selection methods, and recently developed continuous evolution strategies for directed evolution. Moreover, we highlight some representative applications of directed evolution in engineering nucleic acids, proteins, pathways, genetic circuits, viruses, and whole cells. Finally, we discuss the challenges and future perspectives in directed evolution.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Use of Strain-Release for the Diastereoselective Construction of Quaternary Carbon Centers

Herein, we describe the formation of quaternary carbon centers with excellent diastereoselectivity via a strain-release protocol. An organometallic species is generated by Cp*Rh(III)-catalyzed C-H activation, which is then coupled with strained bicyclobutanes (BCBs) and a prochiral carbon electrophile in a three-component reaction. This work illustrates a rare example of BCBs in transition metal catalysis and demonstrates their broad potential to access novel reaction pathways. The method developed exhibits ample functional group tolerance, and the products can be further transformed into valuable α-quaternary β-lactones. Preliminary mechanistic investigations suggest a twofold C-C bond cleavage sequence involving σ-bond insertion and an ensuing β-carbon elimination event.

Semipinacol-Type Rearrangements of [3-(Arylsulfonyl)bicyclo[1.1.0]butan-1-yl]alkanols

Selective lithiation of arylsulfonylbicyclo[1.1.0]butanes at the bridgehead methine and addition to carbonyl compounds yield tertiary bicyclobutyl alcohols that form spiro[3.4]octanes and related heteroatom-containing spirocycles via an acid- or halogen-mediated semipinacol rearrangement. Further synthetic transformations at the carbonyl or arylsulfone positions, in general in high yield and good chemoselectivity, allow access to acetals, difluorides, amides, and methylenecyclobutene building blocks.

Synthesis of 1,3-disubstituted bicyclo[1.1.0]butanes via directed bridgehead functionalization

Bicyclo[1.1.0]butanes (BCBs) are increasingly valued as intermediates in ‘strain release’ chemistry for the synthesis of substituted four membered rings and bicyclo[1.1.1]pentanes, with applications including bioconjugation processes. Variation of the BCB bridgehead substituents can be challenging due to the inherent strain of the bicyclic scaffold, often necessitating linear syntheses of specific BCB targets. Here we report the first palladium catalyzed cross-coupling on pre-formed BCBs which enables a ‘late stage’ diversification of the bridgehead position, and the conversion of the resultant products into a range of useful small ring building blocks.

Bicyclo[1.1.0]butanes (BCBs) are valuable precursors to four-membered rings and bicyclo[1.1.1]pentanes, and useful bioconjugation agents. We describe a versatile approach to access 1,3-disubstituted BCBs, which are otherwise challenging to prepare.

A Theoretical Stereoselectivity Model of Photochemical Denitrogenations of Diazoalkanes Toward Strained 1,3-Dihalogenated Bicyclobutanes

Photochemical reactions exemplify "green" chemistry and are an essential tool for synthesizing highly strained molecules under mild conditions with light. The light-promoted denitrogenation of bicyclic azoalkanes affords functionalized, stereoenriched bicyclo[1.1.0]butanes. These reactions were revisited with multireference calculations and non-adiabatic molecular dynamics (NAMD) simulations to provide a detailed analysis of the photophysics, reactivities, and unexplained stereoselectivity of a series of diazabicyclo[2.1.1]hexenes. We used complete active space self-consistent field (CASSCF) calculations with an (8,8) active space and ANO-S-VDZP basis set; the CASSCF energies were corrected with CASPT2 (8,8)/ANO-S-VDZP. The nature of the electronic excitation is n → π* and ranges from 3.77 to 3.91 eV for the diazabicyclo[2.1.1]hexenes reported here. Minimum energy path calculations showed stepwise C-N bond breaking and led directly to a minimum energy crossing point, corresponding to a stereochemical "double inversion" product. Wigner sampling of diazabicyclo[2.1.1]hexene provided initial conditions for 692 NAMD trajectories. We identified competing complete stereoselective and stereochemical scrambling pathways. The stereoselective pathways feature concerted bicyclobutane inversion and N₂ extrusion. The stereochemical scrambling pathways involve N₂ extrusion followed by bicyclobutane planarization, leading to stereochemical scrambling. The predicted diastereomeric excess (d.e.) almost exactly matches the experiment (calc.d.e. = 46% vs exp.d.e. = 47%). Our NAMD simulations with 672, 568, and 596 trajectories for 1-F, 1-Cl, and 1-Br predicted a d.e. of 94-97% for the double inversion products. Halogenation significantly perturbs the potential energy surface (PES) toward the retention products due to hyperconjugative interactions. The n_C → σ*_{C-X, X = F, Cl, Br} hyperconjugative effect leads to a broader shoulder region on the PES for double inversion.

Cooperation of Cis Vicinal Acceptors for Donor-Acceptor Cyclopropane Activation: TfOH-Promoted Ring-Opening/Aryl Shift Rearrangement to 3- and 5-Ylidenebutenolides

A convenient route to 3- and 5-ylidenebutenolides from readily available cis-2-acylcyclopropane-1-carboxylates is described. Upon exposure to TfOH, synergistic activation of the vicinal acceptors in cis-2-acylcyclopropane-1-carboxylates generates highly strained bicyclic oxocarbenium ion intermediates, which undergo the ring-opening/aryl shift/deprotonation cascade process to form the 3- or 5-ylidenebutenolides depending on the acyl group. On the other hand, the corresponding trans isomers, from which it is difficult to form such oxocarbenium ions, are inactive under the same conditions.

Difunctionalization of C-C σ-Bonds Enabled by the Reaction of Bicyclo[1.1.0]butyl Boronate Complexes with Electrophiles: Reaction Development, Scope, and Stereochemical Origins

Difunctionalization reactions of C-C σ-bonds have the potential to streamline access to molecules that would otherwise be difficult to prepare. However, the development of such reactions is challenging because C-C σ-bonds are typically unreactive. Exploiting the high ring-strain energy of polycyclic carbocycles is a common strategy to weaken and facilitate the reaction of C-C σ-bonds, but there are limited examples of highly strained C-C σ-bonds being used in difunctionalization reactions. We demonstrate that highly strained bicyclo[1.1.0]butyl boronate complexes (strain energy ca. 65 kcal/mol), which were prepared by reacting boronic esters with bicyclo[1.1.0]butyl lithium, react with electrophiles to achieve the diastereoselective difunctionalization of the strained central C-C σ-bond of the bicyclo[1.1.0]butyl unit. The reaction shows broad substrate scope, with a range of different electrophiles and boronic esters being successfully employed to form a diverse set of 1,1,3-trisubstituted cyclobutanes (>50 examples) with high diastereoselectivity. The high diastereoselectivity observed has been rationalized based on a combination of experimental data and DFT calculations, which suggests that separate concerted and stepwise reaction mechanisms are operating, depending upon the migrating substituent and electrophile used.

PdII -Catalyzed Enantioselective C(sp3 )-H Arylation of Cyclobutyl Ketones Using a Chiral Transient Directing Group

The use of chiral transient directing groups (TDGs) is a promising approach for developing PdII -catalyzed enantioselective C(sp3 )-H activation reactions. However, this strategy is challenging because the stereogenic center on the TDG is often far from the C-H bond, and both TDG covalently attached to the substrate and free TDG are capable of coordinating to PdII centers, which can result in a mixture of reactive complexes. We report a PdII -catalyzed enantioselective β-C(sp3 )-H arylation reaction of aliphatic ketones using a chiral TDG. A chiral trisubstituted cyclobutane was efficiently synthesized from a mono-substituted cyclobutane through sequential C-H arylation reactions, thus demonstrating the utility of this method for accessing structurally complex products from simple starting materials. The use of an electron-deficient pyridone ligand is crucial for the observed enantioselectivity. Interestingly, employing different silver salts can reverse the enantioselectivity.

Recent advances in the chemistry of bicyclo- and 1-azabicyclo[1.1.0]butanes

Bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes are structurally unique compounds that exhibit diverse chemistry. Bicyclo[1.1.0]butane is a four-membered carbocycle with a bridging C(1)-C(3) bond and 1-azabicyclo[1.1.0]butane is an analog of bicyclo[1.1.0]butane featuring a nitrogen atom at one bridgehead. These structures are highly strained, allowing them to participate in a range of strain-releasing reactions which typically cleave the central, strained bond to deliver cyclobutanes or azetidines. However, despite these molecules being discovered in the 1950s and 1960s, and possessing a myriad of alluring chemical features, the chemistry and applications of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes remain underexplored. In the past 5 years, there has been a resurgent interest in their chemistry driven by the pharmaceutical industry’s increasing desire for new methods to access cyclobutanes and azetidines. This short review intends to provide a timely summary of the most recent developments in the chemistry of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butane to highlight the diverse chemistry they can access, their value as synthetic precursors to cyclobutanes and azetidines, and to identify areas for future research.

Structurally Diverse Acyl Bicyclobutanes: Valuable Strained Electrophiles

Bicyclo[1.1.0]butanes (BCBs) are highly strained carbocycles that have emerged as versatile synthetic tools, particularly for the construction of functionalized small molecules. This work reports two efficient pathways for the rapid preparation of over 20 structurally diverse BCB ketones, encompassing simple alkyl and aryl derivatives, as well as unprecedented amino acid, dipeptide, bioisostere, and bifunctional linchpin reagents currently inaccessible using literature methods. Analogues are readily forged in two steps and in high yields from simple carboxylic acids or through unsymmetrical ketone synthesis beginning with a convenient carbonyl dication equivalent. The utility of this novel toolbox of strained electrophiles for the selective modification of proteinogenic nucleophiles is highlighted.

Advances in the catalytic asymmetric synthesis of quaternary carbon containing cyclobutanes

The advances in the catalytic asymmetric synthesis of quaternary carbon containing cyclobutanes are described.

Strain release – an old tool for new transformations

This Feature Article provides an overview of research advances in the chemistry of spring-loaded molecules, focusing mainly on strain-release transformations.

Radical α-C-H Cyclobutylation of Aniline Derivatives

A catalytic system has been developed for the direct alkylation of α-C-H bonds of aniline derivatives with strained C-C σ-bonds. This method operates through a photoredox mechanism in which oxidative formation of aminoalkyl radical intermediates enables addition to a bicyclobutane derivative, giving rise to α-cyclobutyl N-alkylaniline products. This mild system proceeds through a redox- and proton-neutral mechanism and is operational for a range of substituted arylamine derivatives.