Selective [2σ + 2σ] Cycloaddition Enabled by Boronyl Radical Catalysis: Synthesis of Highly Substituted Bicyclo[3.1.1]heptanes

Copper-Catalyzed Cross-Coupling of Bicyclobutanes with Triftosylhydrazone Leading to Skipped Dienes

Here, we report a protocol for the synthesis of skipped dienes through the cross-coupling of bicyclo[1.1.0]butanes with trifluoromethyl triftosylhydrazones. The reaction is run using TpBr3Cu(NCMe) as a catalyst to give access to a library of trifluoromethylated skipped dienes (32 examples, ≤98% yield) with excellent E/Z selectivity under mild and operationally safe conditions. The presented methods proved to be compatible with various functionalized bicyclo[1.1.0]butanes and triftosylhydrazones.

A Radical Precursor Based on the Aromatization of p-Quinol Esters Enabled by Pyridine-Boryl Radical

A class of prearomatic carboxylic acid p-quinol ester radical precursors has been developed successfully, which could undergo homolytic cleavage of the para C-O bond of p-quinol esters via pyridine-boryl radical-induced aromatization in the presence of pyridines and diboron reagents, affording the corresponding alkyl radical via decarboxylation from the carboxyl radical in situ. In addition, the prearomatic radical precursors were further applied in radical substitution with phenylsulfonyl compounds and radical self-coulpings. This method not only provides a new approach to the generation of a radical intermediate but also expands the application of boron radicals.

N-Boryl Pyridyl Anion Chemistry

ConspectusPyridine is a crucial heterocyclic compound in organic chemistry. Typically, the pyridine motif behaves as an N-nucleophile and an electron-deficient aromatic ring. Transforming the pyridine ring into an electron-rich system that exhibits reactivity contrary to classical expectations could unveil new opportunities in pyridine chemistry. This Account describes an approach to the umpolung reactivity of the pyridine ring through the formation of an unprecedented N-boryl pyridyl anion (N-BPA) intermediate that enables new catalysis and transformations.In 2017, we discovered that 4-phenylpyridine acts as an efficient catalyst for the borylation of iodo- and bromoarenes using diboron(4) compounds. Mechanistic studies revealed that the in situ formation of an N-BPA intermediate in the pyridine/diboron(4)/methoxide reaction system is a pivotal step in this transformation. Further investigations showed that N-BPA exhibits dual reactivities as both a strong electron donor and a potent nucleophile. This unique reactivity profile has unveiled novel pathways for redox catalysis, pyridine derivatizations, and umpolung transformations.Based on the electron-donor characteristic of the N-boryl pyridyl anion, we have developed a redox catalytic system mediated by a pyridine catalyst. In the pyridine/diboron(4)/base reaction system, the in situ formation of N-BPA followed by single electron transfer (SET) to a substrate with regeneration of the pyridine molecule establishes a redox catalytic cycle. This approach enables the single-electron reduction of a variety of substrates employing 4-phenylpyridine as a catalyst and diboron(4) as the electron source. Upon visible-light excitation, this intermediate transitions into its excited state, exhibiting significantly enhanced reductivity. This enables the establishment of a modular photoredox system consisting of various pyridine/diboron(4)/base combinations that allow for fine-tuning of its redox property. Using this strategy, we performed a series of challenging single-electron reduction reactions, including the single -electron reduction of nonactivated chloro- and fluoroarenes, and Birch reduction of arenes.The nucleophilic character of the N-boryl pyridyl anion was effectively harnessed to facilitate pyridine derivatization and umpolung transformations. By directly quenching the in situ-generated N-BPA with a proton source, we developed a practical approach to N-H-1,4-dihydropyridines (DHPs). Bimolecular nucleophilic substitution reaction between N-BPA and an alkyl bromide produced a 4-alkyl-1,4-DHP, which subsequently releases an alkyl radical under photoredox conditions. This process enabled a catalytic transformation of alkyl bromides into alkyl radicals. Employing 4-trifluoromethylpyridine in this chemistry, the resulting N-BPA intermediate undergoes elimination of fluoride to yield a 4-pyridyldifluoromethyl nucleophile, which then reacts with electrophiles to realize a defluorinative functionalization reaction to forge pyridyldifluoromethyl compounds. Alternatively, when 4-perfluoroalkylthiopyridine was employed, a similar elimination process occurred to form a perfluoroalkyl anion, demonstrating a novel nucleophilic perfluoroalkylation reagent that offers distinct advantages over traditional reagents.The reactivities of the N-boryl pyridyl anion described in this Account provide new insights into pyridine chemistry. We anticipate that these findings will inspire further exploration of novel reactivities and mechanisms in pyridine and related heterocyclic chemistry.

Facile Synthesis of Housanes by an Unexpected Strategy

Rigid bicyclic hydrocarbons have emerged as important building blocks in the drug discovery industry. Despite progress in this general area, bicyclo[2.1.0]pentanes (housanes) are an understudied class of molecules. Herein we report an unconventional synthesis of borylated housanes. Our method features a broad scope and high diastereoselectivities in the synthesis of versatile intermediates. The route involves a strain-release diboration of bicyclo[1.1.0]butane and intramolecular deborylative alkylation. The versatility of the bridgehead boronic ester was demonstrated in several functionalizations. Lastly, the mechanism of the reaction was investigated, and an unusual stereospecific and diastereoselective ring expansion was uncovered.

Enantioselective photocatalytic synthesis of bicyclo[2.1.1]hexanes as ortho-disubstituted benzene bioisosteres with improved biological activity

1,5-Disubstituted bicyclo[2.1.1]hexanes are bridged scaffolds with well-defined exit vectors that are becoming increasingly popular building blocks in medicinal chemistry because they are saturated bioisosteres of ortho-substituted phenyl rings. Here we have developed a Lewis-acid-catalysed [2 + 2] photocycloaddition to obtain these motifs as enantioenriched scaffolds, providing an efficient approach for their incorporation in a variety of drug analogues. Retention of the biological activity of the bicyclo[2.1.1]hexane-containing analogues in the specific proteins targeted by the original drugs has confirmed the suitability of this moiety to serve as a bioisostere of ortho-substituted phenyl rings. Moreover, we have studied the potential of the different enantiomers of the drug analogues to selectively induce cytotoxicity in a panel of tumour cell lines, observing markedly differential effects for the two enantiomers and a substantial improvement over the corresponding sp2-based drugs. This showcases that the control of the absolute configuration and tridimensionality of the drug analogue has a large impact on its biological properties.

Diverse Synthesis of Arene-Fused [n.1.1]-Bridged Molecules via Catalytic Cycloaddition and Rearrangement Reactions

Although great advancement has been made in synthesis of 3D bridged bicyclic[n.1.1]-bioisosteres, facile construction of 2D/3D merged molecules incorporating bridged rings, as novel chemical space in drug discovery, remains a significant challenge. Herein a collective, selective, and diversity-oriented approach for up to 6 types of 2D/3D polycyclic scaffolds featuring bicyclo[n.1.1] substructure is reported. A boronyl radical-catalyzed [2σ+2π] cycloaddition between bicyclo[1.1.0]butanes and ortho-quinone methides afforded spirocyclic compounds containing a bicyclo[2.1.1]hexanes unit, which were used as intermediates for synthesis of three types of 2D/3D scaffolds via judiciously controlled Lewis acid-catalyzed rearrangements. The reaction and rearrangement of para-quinone methides worked analogously and provided another two polycyclic scaffolds.

Double Strain-Release (3+3)-Cycloaddition: Lewis Acid Catalyzed Reaction of Bicyclobutane Carboxylates and Aziridines

A (3+3)-cycloaddition to afford 2-azabiyclo[3.1.1]heptanes was realized by reacting highly strained aryl bicyclo[1.1.0]butane (BCBs) carboxylates with tosylated aziridines. Under Sc(OTf)3 catalysis the transformation proceeded smoothly under mild conditions to the corresponding bicyclic products bearing at the bridgeheads of the embedded four-membered ring aryl and ester moieties, respectively. This reaction is a rare example of a cycloaddition of two highly strained ring systems under Lewis acid catalysis. Mechanistic experiments provided insights into the reaction mechanism.

Photoredox Activation of Donor-Acceptor Cyclopropanes: Distonic Radical Cation Reactivity in [3+2] Cycloaddition Reactions

Altering the reactivity model of a molecule can potentially eliminate limitations existing in its current paradigm. When it comes to the activation of Donor-Acceptor Cyclopropanes (DACs), Lewis acids have been the state-of-the-art. Although a variety of polarized 2π components have been successfully coupled with DACs for [3+2] cycloaddition, unpolarized alkenes prove to be a roadblock due to an inherent polarity mismatch with the Lewis acid-mediated 1,3-zwitterionic intermediate. Hereby, harnessing the distonic radical cation mode of cleavage by photoredox catalysis overcomes this mismatched reactivity of the zwitterionic intermediate, providing a unique route to highly substituted cyclopentanes and cyclopentenes. Expansion of this strategy to bicyclo[1.1.0]butanes enables access to bicyclo[3.1.1]heptanes (BCHs) through a facile [3σ+2σ] cycloaddition. Detailed mechanistic insights are also provided using dispersion-corrected density functional theory.

Quenching Rate Constants of Lewis Base-Boryl Radical by Substrates: a Laser Flash Photolysis Study

The advanced strategy using Lewis base-boryl radicals (LBRs) has recently been proposed for the addition of alkyl substituents to the full-carbon quaternary center of an organic molecule. However, as the rate-determining step in the whole route, reaction rate constants of LBRs with substrates are extremely lacking. In this paper, 4-dimethylaminopyridine (DMAP)-BH2⋅ was selected as a representative of LBRs, and its reactions with six monochloro-substituted substrates, including three methyl chlorobenzoates and three chlorinated acetanilides were studied in experiments and theoretical calculations. The bimolecular reaction rate constants, kq, were determined using laser flash photolysis approach. By comparing activation energies along the two addition pathways, we have clarified the rate-determining step as the attacking to carbonyl oxygen instead of chlorine atom. Furthermore, noncovalent interaction (NCI) analyses on these substrates indicate that weak interactions, such as hydrogen-bonding and van der Waals interactions, have significant influence on the reactivity of these substrates. Our study provides concrete clues to extend this synthetic strategy.

Enantioselective Synthesis of Tetrahydro-1H-1,3-methanocarbazoles by Formal (3 + 3)-Cycloaddition Using Bicyclo[1.1.0]butanes

Asymmetric synthesis presents many challenges, with the selective formation of chiral bridged polyheterocycles being a notable example. Cycloadditions using bicyclo[1.1.0]butanes (BCB) offer a promising solution along those lines, yet, despite significant advances in that emerging area, asymmetric control has remained limited thus far. Here, we describe an organocatalytic, enantioselective formal (3 + 3)-cycloaddition of BCBs with 1H-indol-3-yl((hetero)aryl)methanol derivatives. This approach enables the rapid and efficient synthesis of chiral tetrahydro-1H-1,3-methanocarbazole derivatives (34 examples) from readily available starting materials, with very good stereochemical control (up to 98:2 er). Successful scale-up experiments and product modification demonstrated the potential of this methodology. Control experiments and DFT calculations provide insights into the mechanistic pathway.

Nickel-Catalyzed and Tetrahydroxydiboron-Mediated Allylation of Aldehydes with Allyl Alcohols

The allylation of carbonyl compounds to form homoallylic alcohols represents one of the significant synthetic transformations. In this letter, we describe a new method for the allylation of aldehydes with allyl alcohols facilitated by nickel chloride and tetrahydroxydiboron. This approach offers a mild and straightforward procedure for the synthesis of homoallylic alcohols from aldehydes and ketones.

Solvent-Dependent Divergent Cyclization of Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) have recently garnered significant research interest as versatile precursors for synthesizing potential [n.1.1] bioisosteres and multi-functionalized cyclobutanes in a straightforward and atom-economical manner. Here, we report a solvent-dependent divergent cyclization of BCBs that provides highly diastereospecific decorated cyclobutanes and oxygen-containing bicyclo[3.1.1]heptanes (BCHeps), which serve as bioisosteres of meta-substituted arenes. Additionally, an unprecedented 1,2-difunctionalization reaction mode for BCBs was explored, thus expanding the chemical space of arene bioisosteres and highly functionalized cyclobutanes.

Lewis Acid-Catalyzed 1,3-Dipolar Cycloaddition of Bicyclobutanes with Isatogens: Access to Tetracyclic 2-Oxa-3-azabicyclo[3.1.1]heptanes

The 'escape from flatland' concept has gained significant traction in modern drug discovery, emphasizing the importance of three-dimensional molecular architectures, which serve as saturated bioisosteres of benzenoids. Bicyclo[1.1.0]butanes (BCBs), known for their high ring strain and numerous reactivities, offer a simple yet effective method for synthesizing these bicyclic frameworks. Although (3 + 2) annulations involving BCBs have been extensively studied, the 1,3-dipolar cycloaddition of BCBs leading to (3 + 3) annulation has received limited attention. Herein, we report the Lewis acid-catalyzed 1,3-dipolar cycloaddition of BCBs with isatogens allowing the synthesis of biologically relevant tetracyclic 2-oxa-3-azabicyclo[3.1.1]heptanes. Moreover, the reaction can be performed in a one-pot process by the in situ generation of isatogens from 2-alkynylated nitrobenzenes. Additionally, preliminary mechanistic and photophysical studies of the (3 + 3) annulated products and experiments toward the asymmetric version of this reaction are also provided.

Ring Expansion toward Fused Diazabicyclo[3.1.1]heptanes through Lewis Acid Catalyzed Highly Selective C-C/C-N Bond Cross-Exchange Reaction between Bicyclobutanes and Diaziridines

The synthesis of bicyclic scaffolds has garnered considerable interest in drug discovery because of their ability to mimic benzene bioisosteres. Herein, we introduce a new approach that utilizes a Lewis acid (Sc(OTf)3)-catalyzed σ-bond cross-exchange reaction between the C-C bond of bicyclobutanes and the C-N bond of diaziridines to produce multifunctionalized and medicinally interesting azabicyclo[3.1.1]heptane derivatives. The reaction proceeds well with different bicyclobutanes and a broad range of aryl- as well as alkenyl-, but also alkyl-substituted diaziridines (up to 98 % yield). Conducting a scale-up experiment and exploring the synthetic transformations of the cycloadducts emphasized the practical application of the synthesis. Furthermore, a zinc-based chiral Lewis acid catalytic system was developed for the enantioselective version of this reaction (up to 96 % ee).

Synthesis of fluorine-containing bicyclo[4.1.1]octenes via photocatalyzed defluorinative (4 + 3) annulation of bicyclo[1.1.0]butanes with gem-difluoroalkenes

Although bicyclo[4.1.1] systems are privileged scaffolds in many natural products and drug molecules, efficient synthetic approaches to these systems remain underdeveloped. In this work, we disclose a photoredox-catalyzed defluorinative (4 + 3) annulation of bicyclo[1.1.0]butanes with gem-difluoroalkenes, which provides practical and straightforward access to the fluorine-containing bicyclo[4.1.1]octenes. Our protocol is characterized by mild conditions, broad substrate scope, excellent functional group tolerance and good to excellent yields. Notably, the ease and variety of product derivatizations further enrich the diversity and complexity of the fluorine-containing bicyclo[4.1.1] systems.

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Palladium-Catalyzed Strain-Enabled [2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with Methyleneindolinones

A palladium-catalyzed [2π + 2σ] cycloaddition of vinyl bicyclo[1.1.0]butanes with methyleneindolinones has been developed. The reaction enables the construction of spirobicyclo[2.1.1]hexanes bearing an all-carbon quaternary center in moderate to good yields with excellent diastereoselectivities. This method features a broad substrate scope with good functional group compatibility. The practical utility of this protocol was further demonstrated by gram-scale synthesis and postsynthetic transformations of desired product.

Catalytic Intermolecular Asymmetric [2π + 2σ] Cycloadditions of Bicyclo[1.1.0]butanes: Practical Synthesis of Enantioenriched Highly Substituted Bicyclo[2.1.1]hexanes

The high percentage of sp3-hybridized carbons and the presence of chiral carbon centers could contribute to increased molecular complexity, enhancing the likelihood of clinical success of drug candidates. Three-dimensional (3D) bridged motifs have recently garnered significant interest in medicinal chemistry. Bicyclo[2.1.1]hexanes (BCHs) are emerging 3D benzene bioisosteres, but the synthesis of chiral, highly substituted BCHs has been underexplored. Herein, we disclose the Lewis acid-catalyzed asymmetric intermolecular [2π + 2σ] cycloaddition of bicyclo[1.1.0]butanes with coumarins, 2-pyrone, or chromenes to access diverse enantioenriched 1,2,3,4-tetrasubstituted BCHs bearing vicinal tertiary-quaternary stereocenters. The key to success is the introduction of chiral bisoxazoline ligands to effectively suppress the side reactions, inhibit significant racemic background reactions, and fine-tune the reactivity and regio-, enantio-, and diastereoselectivities of the reactions. The resulting BCHs hold significant potential as benzene bioisosteres in the synthesis of chiral BCHex-Sonidegib and BCHex-BMS-202, mimicking the anticancer drug Sonidegib and the PD-1/PD-L1 inhibitor BMS-202, respectively. The outcome highlights the positive impact of bioisosteric replacement on physicochemical properties, while maintaining comparable antitumor activity to their aryl-containing counterparts.

Structure-Dependent, Switchable Alder-Ene/[2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with α-Ketoesters Enabled by Palladium Catalysis

A structure-dependent, palladium-catalyzed switchable alder-ene/[2π + 2σ] cycloaddition of VBCBs with α-ketoesters has been reported. A variety of cyclobutenes and 2-oxabicyclo[2.1.1]hexanes have been efficiently achieved in good to excellent yields through strain-release-driven alder-ene reactions and [2π + 2σ] cycloadditions, respectively. The potential of this method is illustrated by the scale-up reaction and diverse postsynthetic transformations of the obtained cyclic scaffolds. Additionally, the reaction mechanism and origins of the chemoselectivity have been probed by computational studies.

Visible Light Promoted de Mayo Type Reaction of Bicyclo[1.1.0]butanes

We reported herein a visible light mediated de Mayo-type reaction between 1,3-diketones and BCB. The reaction proceeds through a [2π+2σ] cycloaddition and retro-aldol sequence, producing cis-difunctionalized cyclobutanes in high yields with good regio- and diastereoselectivity.

Enantioselective dearomative formal (3+3) cycloadditions of bicyclobutanes with aromatic azomethine imines: access to fused 2,3-diazabicyclo[3.1.1]heptanes

Although cycloadditions of bicyclobutanes (BCBs) have emerged as a reliable approach for producing bicyclo[n.1.1]alkanes such as azabicyclo[3.1.1]heptanes (aza-BCHeps), serving as saturated bioisosteres of arenes, the catalytic asymmetric variant remains underdeveloped and presents challenges. Herein, we developed several Lewis acid-catalyzed systems for the challenging dearomative (3+3) cycloaddition of BCBs and aromatic azomethine imines. This resulted in fused 2,3-diazabicyclo[3.1.1]heptanes, introducing a novel chemical space for the caged hydrocarbons. Moreover, an asymmetric Lewis acid catalysis strategy was devised for the (3+3) cycloadditions of BCBs and N-iminoisoquinolinium ylides, forming chiral diaza-BCHeps with up to 99% yield and 97% ee. This study showcases a unique instance of asymmetric (3+3) cycloaddition facilitated by the creation of a chiral environment via the activation of BCBs.

Zinc-Catalyzed Enantioselective Formal (3+2) Cycloadditions of Bicyclobutanes with Imines: Catalytic Asymmetric Synthesis of Azabicyclo[2.1.1]hexanes

The cycloaddition reaction involving bicyclo[1.1.0]butanes (BCBs) offers a versatile and efficient synthetic platform for producing C(sp3)-rich rigid bridged ring scaffolds, which act as phenyl bioisosteres. However, there is a scarcity of catalytic asymmetric cycloadditions of BCBs to fulfill the need for enantioenriched saturated bicycles in drug design and development. In this study, an efficient synthesis of valuable azabicyclo[2.1.1]hexanes (aza-BCHs) by an enantioselective zinc-catalyzed (3+2) cycloadditions of BCBs with imines is reported. The reaction proceeds effectively with a novel type of BCB that incorporates a 2-acyl imidazole group and a diverse array of alkynyl- and aryl-substituted imines. The target aza-BCHs, which consist of α-chiral amine fragments and two quaternary carbon centers, are efficiently synthesized with up to 94 % and 96.5:3.5 er under mild conditions. Experimental and computational studies reveal that the reaction follows a concerted nucleophilic ring-opening mechanism of BCBs with imines. This mechanism is distinct from previous studies on Lewis acid-catalyzed cycloadditions of BCBs.

Lewis Acid-Catalyzed Unusual (4+3) Annulation of para-Quinone Methides with Bicyclobutanes: Access to Oxabicyclo[4.1.1]Octanes

Over the past few years, there has been a surge of interest in the chemistry of bicyclobutanes (BCBs). Although BCBs have been used to synthesize bicyclo[2.1.1]hexanes and bicyclo[3.1.1]heptanes, the synthesis of bicyclo[4.1.1]octanes has remained elusive. Herein, we report the first Lewis acid-catalyzed unexpected (4+3) annulation of para-quinonemethides (p-QMs) with BCBs allowing the synthesis of oxabicyclo[4.1.1]octanes proceeding under mild conditions. With 5 mol % of Bi(OTf)3, the reaction afforded the (4+3) annulated product in high regioselectivity and good functional group compatibility via a simultaneous Lewis acid activation of BCBs and p-QMs. The reaction is likely initiated by the 1,6-addition of Lewis acid activated BCBs to p-QMs followed by the C2-selective intramolecular addition of the phenol moiety to the generated cyclobutyl cation intermediate. Moreover, detailed mechanistic studies provided insight into the mechanism of the reaction.

gem-Difluorobicyclo[2.1.1]hexanes via Photochemical [2π + 2σ] Cycloaddition Initiated by Oxidative Activation of gem-Difluorodienes

The incorporation of fluorine atoms into three-dimensional sp3-rich scaffolds represents an attractive tactic during bioisosteric evolution campaigns by endowing bioisosteric candidates with improved pharmacokinetic properties. Photo- or Lewis acid-mediated bicyclo[1.1.0]butane cycloaddition has offered an efficient approach for the construction of numerous regular bicyclo[n.1.1] scaffolds (n = 1-5) but remains a significant challenge to the synthesis of related 3D fluorinated scaffolds. Herein, we unveiled a photochemical single-electron oxidative strategy for gem-difluorodiene activation and subsequent [2π + 2σ] cycloaddition with bicyclo[1.1.0]butanes to provide a broad range of gem-difluorobicyclo[2.1.1]hexane scaffolds containing several post-transformable handles. A combination of experimental and computational mechanistic studies suggested that the conjugated π system of gem-difluorodiene plays important dual roles in promoting its preferential single-electron oxidation and stabilizing various radical-involved intermediates during the cyclization.

Lewis-Acid-Catalyzed Dearomative [4π + 2σ] Cycloaddition of Bicyclobutanes with Isoquinolinium Methylides for the Synthesis of Ring-Fused Azabicyclo[3.1.1]heptanes

Dearomative cycloadditions are valuable for efficiently generating three-dimensional molecular complexity. However, despite recent reports of cycloadditions of bicyclobutanes (BCBs) for the synthesis of aza-bicyclo[3.1.1]heptanes (aza-BCHeps), which are bioisosteres of meta-substituted aza-arenes, dearomative cycloaddition of BCBs with N-heteroarenes for the synthesis of ring-fused aza-BCHeps has yet to be achieved. Herein, we disclose a method for Lewis acid-catalyzed [4π + 2σ] cycloaddition of isoquinolinium methylides with BCBs, which furnished a diverse array of previously inaccessible ring-fused 3-aza-BCHeps. We demonstrated the synthetic utility of the method by carrying out scaled-up reactions and transforming the products.

Copper-Catalyzed Enantioselective [4π + 2σ] Cycloaddition of Bicyclobutanes with Nitrones

The selective construction of bridged bicyclic scaffolds has garnered increasing attention because of their extensive use as saturated bioisosteres of arene in pharmaceutical industry. However, in sharp contrast to their racemic counterparts, assembling chiral bridged bicyclic structures in an enantioselective and regioselective manner remains challenging. Herein, we describe our protocol for constructing chiral 2-oxa-3-azabicyclo[3.1.1]heptanes (BCHeps) by enantioselective [4π + 2σ] cycloadditions of bicyclo[1.1.0]butanes (BCBs) and nitrones taking advantage of a chiral copper(II) complex as a Lewis acid catalyst. This method features mild conditions, good functional group tolerance, high yield (up to 99%), and excellent enantioselectivity (up to 99% ee). Density functional theory (DFT) calculation elucidates the origin of the reaction's enantioselectivity and the mechanism of BCB activation by Cu(II) complex.

Formal [2σ + 2σ]-Cycloaddition of Aziridines with Bicyclo[1.1.0]butanes: Access to Enantiopure 2-Azabicyclo[3.1.1]heptane Derivatives

Saturated nitrogen heterocycles are among the most significant structural components in small-molecule pharmaceuticals. Herein, a protocol for the construction of enantiopure 2-azabicyclo[3.1.1]heptane derivatives by a stereospecific intermolecular formal cycloaddition of aziridines with bicyclo[1.1.0]butanes is described. The reaction is run by using B(C6F5)3 as a catalytic additive to give access to a library of enantiopure 2-azabicyclo[3.1.1]heptane derivatives (37 examples) under mild and operationally simple conditions. Successful scale-up reactions, mechanistic experiments, density functional theory (DFT) calculations and synthetic applications are presented.

Divergent Synthesis of Sulfur-Containing Bridged Cyclobutanes by Lewis Acid Catalyzed Formal Cycloadditions of Pyridinium 1,4-Zwitterionic Thiolates and Bicyclobutanes

Bridged cyclobutanes and sulfur heterocycles are currently under intense investigation as building blocks for pharmaceutical drug design. Two formal cycloaddition modes involving bicyclobutanes (BCBs) and pyridinium 1,4-zwitterionic thiolate derivatives were described to rapidly expand the chemical space of sulfur-containing bridged cyclobutanes. By using Ni(ClO4)2 as the catalyst, an uncommon higher-order (5+3) cycloaddition of BCBs with quinolinium 1,4-zwitterionic thiolate was achieved with broad substrate scope under mild reaction conditions. Furthermore, the first Lewis acid-catalyzed asymmetric polar (5+3) cycloaddition of BCB with pyridazinium 1,4-zwitterionic thiolate was accomplished. In contrast, pyridinium 1,4-zwitterionic thiolates undergo an Sc(OTf)3-catalyzed formal (3+3) reaction with BCBs to generate thia-norpinene products, which represent the initial instance of synthesizing 2-thiabicyclo[3.1.1]heptanes (thia-BCHeps) from BCBs. Moreover, we have successfully used this (3+3) protocol to rapidly prepare thia-BCHeps-substituted analogues of the bioactive molecule Pitofenone. Density functional theory (DFT) computations imply that kinetic factors govern the (5+3) cycloaddition reaction between BCB and quinolinium 1,4-zwitterionic thiolate, whereas the (3+3) reaction involving pyridinium 1,4-zwitterionic thiolates is under thermodynamic control.

Regio- and Diastereoselective Synthesis of Polysubstituted Piperidines Enabled by Boronyl Radical-Catalyzed (4+2) Cycloaddition

Piperidines are widely present in small molecule drugs and natural products. Despite many methods have been developed for their synthesis, new approaches to polysubstituted piperidines are highly desirable. This work presents a radical (4+2) cycloaddition reaction for synthesis of piperidines featuring dense substituents at 3,4,5-positions that are not readily accessible by known methods. Using commercially available diboron(4) compounds and 4-phenylpyridine as the catalyst precursors, the boronyl radical-catalyzed cycloaddition between 3-aroyl azetidines and various alkenes, including previously unreactive 1,2-di-, tri-, and tetrasubstituted alkenes, has delivered the polysubstituted piperidines in generally high yield and diastereoselectivity. The reaction also features high modularity, atom economy, broad substrate scope, metal-free conditions, simple catalysts and operation. The utilization of the products has been demonstrated by selective transformations. A plausible mechanism, with the ring-opening of azetidine as the rate-limiting step, has been proposed based on the experimental and computational results.

Enantioselective formal (3 + 3) cycloaddition of bicyclobutanes with nitrones enabled by asymmetric Lewis acid catalysis

The absence of catalytic asymmetric methods for synthesizing chiral (hetero)bicyclo[n.1.1]alkanes has hindered their application in new drug discovery. Here we demonstrate the achievability of an asymmetric polar cycloaddition of bicyclo[1.1.0]butane using a chiral Lewis acid catalyst and a bidentate chelating bicyclo[1.1.0]butane substrate, as exemplified by the current enantioselective formal (3 + 3) cycloaddition of bicyclo[1.1.0]butanes with nitrones. In addition to the diverse bicyclo[1.1.0]butanes incorporating an acyl imidazole group or an acyl pyrazole moiety, a wide array of nitrones are compatible with this Lewis acid catalysis, successfully assembling two congested quaternary carbon centers and a chiral aza-trisubstituted carbon center in the pharmaceutically important hetero-bicyclo[3.1.1]heptane product with up to 99% yield and >99% ee.

Broad-Scope (3 + 2) Cycloaddition of Cyclopropanes and Alkynes Enabled by Boronyl Radical Catalysis

Cyclopentene skeletons are ubiquitous in natural products and small molecule drugs. The (3 + 2) cycloaddition of cyclopropanes and alkynes represents an efficient and atom-economic strategy for synthesizing these structures. However, the types of substituents on cyclopropane and alkyne used in previous works show evident limitations, restricting the application of this type of reaction to some extent. Herein, we report a broad-scope (3 + 2) cycloaddition of cyclopropanes and alkynes catalyzed by boronyl radicals. In this method, various substrates, such as mono-, di-, tri-, and tetrasubstituted cyclopropanes, as well as mono- and disubstituted alkynes, were compatible with up to 98% isolated yield.

Palladium-catalyzed decarboxylative (4 + 3) cycloadditions of bicyclobutanes with 2-alkylidenetrimethylene carbonates for the synthesis of 2-oxabicyclo[4.1.1]octanes

While cycloaddition reactions of bicyclobutanes (BCBs) have emerged as a potent method for synthesizing (hetero-)bicyclo[n.1.1]alkanes (usually n ≤ 3), their utilization in the synthesis of bicyclo[4.1.1]octane derivatives (BCOs) is still underdeveloped. Here, a palladium-catalyzed formal (4 + 3) reaction of BCBs with 1,4-O/C dipole precursors for the synthesis of oxa-BCOs is described. Unlike previous catalytic polar (3 + X) cycloadditions of BCBs, which are typically achieved through the activation of BCB substrates, the current reaction represents a novel strategy for realizing the cycloaddition of BCBs through the activation of the "X" cycloaddition partner. Moreover, the obtained functionalized oxa-BCOs products can be readily modified through various synthetic transformations.

Lewis Acid Catalyzed Cycloaddition of Bicyclobutanes with Ynamides for the Synthesis of Polysubstituted 2-Amino-bicyclo[2.1.1]hexenes

Synthesis of bicyclic scaffolds has gained significant attention in drug discovery due to their potential to mimic benzene bioisosteres. Here, we present a mild and scalable Sc(OTf)3-catalyzed [3+2] cycloaddition of bicyclo[1.1.0]butanes (BCBs) with ynamides, yielding a diverse array of polysubstituted 2-amino-bicyclo[2.1.1]hexenes in good to excellent yields. These products offer valuable starting materials for the construction of novel functionalized bicyclo[1.1.0]butanes. Preliminary mechanistic studies indicate that the reaction involves a nucleophilic addition of ynamides to bicyclo[1.1.0]butanes, followed by an intramolecular cyclization of in situ generated enolate and keteniminium ion. We expect that these findings will encourage utilization of complex bioisosteres and foster further investigation into BCB-based cycloaddition chemistry.

Intermolecular Formal [2π + 2σ] Cycloaddition of Enol Silyl Ethers with Bicyclo[1.1.0]butanes Promoted by Lewis Acids

Silyl enol ethers react with bicyclo[1.1.0]butanes (BCBs) through Yb(OTf)3-promoted formal [2π + 2σ] cycloaddition reactions to furnish bicyclo[2.1.1]hexanes (BCHs). This new reaction tolerated a wide range of enol silyl ethers and BCBs. Furthermore, the amplification experiments and synthetic transformations of the cycloaddition compounds further highlighted their practicality.

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

Lewis acid catalyzed [4+2] annulation of bicyclobutanes with dienol ethers for the synthesis of bicyclo[4.1.1]octanes

Bicyclic carbocycles containing a high fraction of Csp3 have become highly attractive synthetic targets because of the multiple applications they have found in medicinal chemistry. The formal cycloaddition of bicyclobutanes (BCBs) with two- or three-atom partners has recently been extensively explored for the construction of bicyclohexanes and bicycloheptanes, but applications to the synthesis of medium-sized bridged carbocycles remained more limited. We report herein the formal [4+2] cycloaddition of BCB ketones with silyl dienol ethers. The reaction occurred in the presence of 5 mol% aluminium triflate as a Lewis acid catalyst. Upon acidic hydrolysis of the enol ether intermediates, rigid bicyclo[4.1.1]octane (BCO) diketones could be accessed in up to quantitative yields. This procedure tolerated a range of both aromatic and aliphatic substituents on both the BCB substrates and the dienes. The obtained BCO products could be functionalized through reduction and cross-coupling reactions.

Catalytic Asymmetric Construction of Chiral Polysubstituted 3-Azabicyclo[3.1.1]heptanes by Copper-Catalyzed Stereoselective Formal [4π+2σ] Cycloaddition

The direct construction of bioisosteric compounds enriched in Csp3 content represents an attractive and dependable approach to imbuing biologically active molecules with enhanced three-dimensional characteristics, finding wide utility across the synthetic and medicinal chemistry community. Despite recent advancements in the synthesis of (aza)-bicyclo[3.1.1]heptanes (BCHeps and aza-BCHeps), which serve as meta-substituted (aza)-arene bioisosteres, the enantioselective assembly of chiral 3-aza-BCHeps remains a coveted goal yet to be achieved. Here, we disclose an unprecedented copper-catalyzed asymmetric formal [4π+2σ] cycloaddition of bicyclo[1.1.0]butanes (BCBs) and azomethine ylides, furnishing a diverse array of enantioenriched 3-aza-BCHeps with exceptional levels of diastereo- and enantioselectivites (51 examples, all >20:1 dr, mostly 97-99% ee). Both mono- and disubstituted BCBs are well compatible with this protocol, offering an enticing route for the efficient synthesis of challenging tetrasubstituted bicyclic products bearing two quaternary centers. The synthetic significance of this methodology is further demonstrated by the successful preparation of several piperidine drug analogues.

Switching between the [2π+2σ] and Hetero-[4π+2σ] Cycloaddition Reactivity of Bicyclobutanes with Lewis Acid Catalysts Enables the Synthesis of Spirocycles and Bridged Heterocycles

The exploration of the complex chemical diversity of bicyclo[n.1.1]alkanes and their use as benzene bioisosteres has garnered significant attention over the past two decades. Regiodivergent syntheses of thiabicyclo[4.1.1]octanes (S-BCOs) and highly substituted bicyclo[2.1.1]hexanes (BCHs) using a Lewis acid-catalyzed formal cycloaddition of bicyclobutanes (BCBs) and 3-benzylideneindoline-2-thione derivatives have been established. The first hetero-(4+3) cycloaddition of BCBs, catalyzed by Zn(OTf)2, was achieved with a broad substrate scope under mild conditions. In contrast, the less electrophilic BCB ester undergoes a Sc(OTf)3-catalyzed [2π+2σ] reaction with 1,1,2-trisubstituted alkenes, yielding BCHs with a spirocyclic quaternary carbon center. Control experiments and preliminary theoretical calculations suggest that the diastereoselective [2π+2σ] product formation may involve a concerted cycloaddition between a zwitterionic intermediate and E-1,1,2-trisubstituted alkenes. Additionally, the hetero-(4+3) cycloaddition may involve a concerted nucleophilic ring-opening mechanism.

Synthesis of Azabicyclo[3.1.1]heptenes Enabled by Catalyst-Controlled Annulations of Bicyclo[1.1.0]butanes with Vinyl Azides

Bridged bicyclic scaffolds are emerging bioisosteres of planar aromatic rings under the concept of "escape from flatland". However, adopting this concept into the exploration of bioisosteres of pyridines remains elusive due to the challenge of incorporating a N atom into such bridged bicyclic structures. Herein, we report practical routes for the divergent synthesis of 2- and 3-azabicyclo[3.1.1]heptenes (aza-BCHepes) as potential bioisosteres of pyridines from the readily accessible vinyl azides and bicyclo[1.1.0]butanes (BCBs) via two distinct catalytic annulations. The reactivity of vinyl azides tailored with BCBs is the key to achieving divergent transformations. TiIII-catalyzed single-electron reductive generation of C-radicals from BCBs allows a concise (3 + 3) annulation with vinyl azides, affording novel 2-aza-BCHepe scaffolds. In contrast, scandium catalysis enables an efficient dipolar (3 + 2) annulation with vinyl azides to generate 2-azidobicyclo[2.1.1]hexanes, which subsequently undergo a chemoselective rearrangement to construct 3-aza-BCHepes. Both approaches efficiently deliver unique azabicyclo[3.1.1]heptene scaffolds with a high functional group tolerance. The synthetic utility has been further demonstrated by scale-up reactions and diverse postcatalytic transformations, providing valuable azabicyclics including 2- and 3-azabicyclo[3.1.1]heptanes and rigid bicyclic amino esters. In addition, the related sp2-hybridized nitrogen atom and the similar geometric property between pyridines and corresponding aza-BCHepes indicate that they are promising bioisosteres of pyridines.

Lewis Acid-Catalyzed Formal [2π+2σ] Cycloaddition of Bicyclobutanes with Quinoxalin-2(1H)-ones: Access to Quinoxaline-Fused Aza-Bicyclo[2.1.1]hexanes

An efficient synthesis of quinoxaline-fused aza-bicyclo[2.1.1]hexanes bearing multiple quaternary carbon centers via the intermolecular [2π+2σ] cycloaddition of bicyclo[1.1.0]butanes and quinoxalin-2(1H)-ones, facilitated by Lewis acid catalysis, is presented. This reaction is carried out under mild conditions and exhibits a broad substrate scope and excellent functional group tolerance.

Vinylcyclopropane-Cyclopentene (VCP-CP) Rearrangement Enabled by Pyridine-Assisted Boronyl Radical Catalysis

An unprecedented VCP-CP (vinylcyclopropane-cyclopentene) rearrangement approach has been established herein by virtue of the pyridine-boronyl radical catalyzed intramolecular ring expansions. This metal-free radical pathway harnesses readily available catalysts and unactivated vinylcyclopropane starting materials, providing an array of cyclopentene derivatives chemoselectively under relatively mild conditions. Mechanistic studies support the idea that the boronyl radical engages in the generation of allylic/ketyl radical species, thus inducing the ring opening of cyclopropanes and the following intramolecular cyclization processes.

Synthesis of (2R,4S)-4-Amino-5-hydroxybicyclo[3.1.1]heptane-2-carboxylic Acid via an Asymmetric Intramolecular Mannich Reaction

Inhibition of human ornithine aminotransferase interferes with glutamine and proline metabolism in hepatocellular carcinoma, depriving tumors of essential nutrients. A proposed mechanism-based inhibitor containing a bicyclo[3.1.1]heptanol warhead is reported herein. The proposed inactivation mechanism involves a novel α-iminol rearrangement. The synthesis of the proposed inhibitor features an asymmetric intramolecular Mannich reaction, utilizing a chiral sulfinamide. This study presents a novel approach toward the synthesis of functionalized bicyclo[3.1.1]heptanes and highlights an underutilized method to access enantiopure exocyclic amines.

Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Titanium catalyzed [2σ + 2π] cycloaddition of bicyclo[1.1.0]-butanes with 1,3-dienes for efficient synthesis of stilbene bioisosteres

Natural stilbenes have shown significant potential in the prevention and treatment of diseases due to their diverse pharmacological activities. Here we present a mild and effective Ti-catalyzed intermolecular radical-relay [2σ + 2π] cycloaddition of bicyclo[1.1.0]-butanes and 1,3-dienes. This transformation enables the synthesis of bicyclo[2.1.1]hexane (BCH) scaffolds containing aryl vinyl groups with excellent regio- and trans-selectivity and broad functional group tolerance, thus offering rapid access to structurally diverse stilbene bioisosteres.

Silver-Enabled Cycloaddition of Bicyclobutanes with Isocyanides for the Synthesis of Polysubstituted 3-Azabicyclo[3.1.1]heptanes

Synthesis of bicyclic scaffolds has emerged as an important research topic in modern drug development because they can serve as saturated bioisosters to enhance the physicochemical properties and metabolic profiles of drug candidates. Here we report a remarkably simple silver-enabled strategy to access polysubstituted 3-azabicyclo[3.1.1]heptanes in a single operation from readily accessible bicyclobutanes (BCBs) and isocyanides. The process is proposed to involve a formal (3+3)/(3+2)/retro-(3+2) cycloaddition sequence. This novel protocol allows for rapid generation of molecular complexity from simple starting materials, and the products can be easily derivatized, further enriching the BCB cycloaddition chemistry and the growing set of valuable sp3-rich bicyclic building blocks.

B(C6F5)3-Catalyzed Formal (n + 3) (n = 5 and 6) Cycloaddition of Bicyclo[1.1.0]butanes to Medium Bicyclo[n.1.1]alkanes

Herein, a B(C6F5)3-catalyzed formal (n + 3) (n = 5 and 6) cycloaddition of bicyclo[1.1.0]butanes (BCBs) with imidazolidines/hexahydropyrimidines is described. The reaction provides a modular, atom-economical, and efficient strategy to two libraries of synthetically challenging medium-bridged rings, 2,5-diazabicyclo[5.1.1]nonanes and 2,6-diazabicyclo[6.1.1]decanes, in moderate to excellent yields. This reaction also features simple operation, mild reaction conditions, and broad substrate scope. A scale-up experiment and various synthetic transformations of products further highlight the synthetic utility.

DFT Insight into a Strain-Release Mechanism in Bicyclo[1.1.0]butanes via Concerted Activation of Central and Lateral C-C Bonds with Rh(III) Catalysis

Transition-metal-catalyzed, strain-release-driven transformations of "spring-loaded" bicyclo[1.1.0]butanes (BCBs) are considered potent tools in synthetic organic chemistry. Previously proposed strain-release mechanisms involve either the insertion of the central C-C bond of BCBs into a metal-carbon bond, followed by β-C elimination, or the oxidative addition of the central or lateral C-C bond on the transition metal center, followed by reductive elimination. This study, employing DFT calculations on a Rh(III)-catalyzed model system in a three-component protocol involving oxime ether, BCB ester, and ethyl glyoxylate for constructing diastereoselective quaternary carbon centers, introduces an unusual strain-release mechanism for BCBs. In this mechanism, the catalytic reaction is initiated by the simultaneous cleavage of two C-C bonds (the central and lateral C-C bonds), resulting in the formation of a Rh-carbene intermediate. The new mechanism exhibits a barrier of 21.0 kcal/mol, making it energetically more favorable by 11.1 kcal/mol compared to the previously suggested most favorable pathway. This unusual reaction mode rationalizes experimental observation of the construction of quaternary carbon centers, including the excellent E-selectivity and diastereoselectivity. The newly proposed strain-release mechanism holds promise in advancing our understanding of transition-metal-catalyzed C-C bond activation mechanisms and facilitating the synthesis of transition metal carbene complexes.