Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

Merging Two Strained Carbocycles: Lewis Acid Catalyzed Remote Site-Selective Friedel-Crafts Alkylation of in Situ Generated β-Naphthol

Lewis acid catalyzed tandem activation of the two smallest carbocycles, 3-ethoxy cyclobutanones, and donor-acceptor cyclopropanes has been demonstrated. The diphenyl-substituted 3-ethoxy cyclobutanone rearranges itself by intramolecular cyclization for the in situ generation of 1-phenyl 2-naphthol, which further undergoes remote site-selective Friedel-Crafts alkylation with donor-acceptor cyclopropane to synthesize a series of γ-naphthyl butyric acid derivatives. Further control experiments for mechanistic investigations and synthetic applications have also been carried out.

Organophotocatalytic [2+2] Cycloaddition of Electron-Deficient Styrenes

A visible-light organophotocatalytic [2+2] cycloaddition of electron-deficient styrenes is described. Photocatalytic [2+2] cycloadditions are typically performed with electron-rich styrene derivatives or α,β-unsaturated carbonyl compounds, and with transition-metal-based catalysts. We have discovered that an organic cyanoarene photocatalyst is able to deliver high-value cyclobutane products bearing electron-deficient aryl substituents in good yields. A range of electron-deficient substituents are tolerated, and both homodimerisations and intramolecular [2+2] cycloadditions to fused bicyclic systems are available by using this methodology.

Rhodium-Catalyzed Diastereo- and Enantioselective Divergent Annulations between Cyclobutanones and 1,5-Enynes: Rapid Construction of Complex C(sp3)-Rich Scaffolds

Given the emerging demand to "escape from flatland" for drug discovery, synthetic methods that can efficiently construct complex three-dimensional structures with multi-stereocenters become increasingly valuable. Here, we describe the development of Rh(I)-catalyzed intramolecular annulations between cyclobutanones and 1,5-enyne groups to construct complex C(sp3)-rich scaffolds. Divergent reactivities are realized with different catalysts, and excellent diastereo- and enantioselectivity have been achieved. The use of (R)-H8-binap as the ligand favors forming the bis-bicyclic scaffolds with multiple quaternary stereocenters, while the (R)-segphos ligand prefers to generate the tetrahydro-azapinone products. Owing to the versatile reactivity of ketone moieties, these C(sp3)-rich scaffolds can be further functionalized. Experimental and computational mechanistic studies support a reaction pathway involving enyne-cyclometallation, 1,2-carbonyl addition, and then β-carbon elimination; the divergent reactivities are dictated by a product-determining Rh-alkyl migratory insertion step.

Investigations of an Unexpected [2+2] Photocycloaddition in the Synthesis of (-)-Scabrolide A from Quantum Mechanics Calculations

We utilize ab initio quantum mechanics calculations to evaluate a range of plausible mechanistic pathways for the unexpected formation of a [6-4-4] ring system from an enone-olefin photocycloaddition in the synthesis of (-)-scabrolide A, previously reported by our group. We present a mechanistic analysis that is consistent with all current experimental observations, including the photoexcitation, the C-C bond formation, and the associated chemo- and diastereoselectivity.

Development of a C-C Bond Cleavage/Vinylation/Mizoroki-Heck Cascade Reaction: Application to the Total Synthesis of 14- and 15-Hydroxypatchoulol

A C-C bond cleavage/vinylation/Mizoroki-Heck cascade reaction has been developed to provide access to densely functionalized bicyclo[2.2.2]octane frameworks. The sequence proceeds through the coupling of dihydroxylated pinene derivatives, prepared from carvone, with gem-dichloroalkenes. The method was applied to 12-step total syntheses of both 14- and 15-hydroxypatchoulol, which provided unambiguous support for the structure of the natural products and corrects a misassignment in the isolation report.

Sliver-Catalyzed 1,3-Aza-Benzyl Migration of Allyl Alcohol

Carbon migration of alkenyl alcohols has been recognized as an increasingly viable methodology in organic synthesis. Herein, we disclose a silver-catalyzed 1,3-aza-benzyl migration of allyl alcohols by utilizing chelation-assisted selective cleavage of an unstrained C(sp³)-C(sp³) bond. This approach provides an available, efficient, high atom-economic, and environmentally benign procedure, leading to alkylation products with broad substrate scopes and excellent yields. The migration proceeds via a one-pot, two-step process involving a free-state alkyl metal species.

Synthesis of Imidized Cyclobutene Derivatives by Strain Release of [1.1.1]Propellane

Herein, we report the metal-free synthesis of imidized methylene cyclobutane derivatives via a strain-release driven addition reaction of [1.1.1]propellane. Using this strategy, the methylene cyclobutyl cation intermediate generated by protonation of [1.1.1]propellane was found to be trapped by nitriles to form a nitrilium ion intermediate, which subsequently reacted with carboxylic acids to produce imidized methylene cyclobutene derivatives via a Mumm-type rearrangement.

C-C Bond Activation of Cyclopropanes Enabled by Phosphine-Catalyzed In Situ Formation of High-Strain Methylenecycopropane Intermediate

An effective strategy for the ring-opening/elaboration of cyclopropanes by phosphine catalyst is documented, providing the 2,4-pentadiene sulfonamides and isoindolines in moderate to good yields. The key to the success of this reaction is phosphine-catalyzed introduction of a trigonal center into cyclopropanes, which results in the formation of higher ring strain cyclopropylidenemethyl phosphonium salt. Moreover, this methodology is employed as the key step for the synthesis of bioactive molecules.

Iron-catalyzed deconstructive alkylation through chlorine radical induced C-C single bond cleavage under visible light

Selective C-C single bond cleavage of simple compounds is a highly challenging and desired process. Herein, a chlorine radical-induced deconstructive C-C bond alkylation with alcohols and alkenes catalyzed by iron salts was reported for the first time. Readily available alcohols and various electron-deficient alkenes were tolerated. Late-stage and large-scale reactions proceed smoothly. This catalyst system shows potential for diversified deconstructive functionalization of simple C-C bonds.

Sequentially Regulating the Structural Transformation of Copper Metal-Organic Frameworks (Cu-MOFs) for Controlling Site-Selective Reaction

Regulating atomically precise sites in catalysts to achieve site-selective reactions is remarkable but challenging. In this work, a convenient and facile solid-gas/liquid reaction strategy was used to construct controllable active sites in metal-organic frameworks (MOFs) to guide an orientation site-selective reaction. A flexible Cu^I-MOF-1 with dynamics originating from an anionic and tailorable framework could undergo a reversible structural transformation to engineer a topologically equivalent mixed-valent Cu^ICu^II-MOF-2 via a solid-gas/liquid oxidation/reduction process. More importantly, Cu^I-MOF-1 and Cu^ICu^II-MOF-2 could further execute the solid-gas/liquid reaction under ammonia vapor/solution to generate Cu^II-MOF-3. Furthermore, the transformation from Cu^I-MOF-1 to Cu^ICu^II-MOF-2 and Cu^II-MOF-3 served as controllable catalysts to facilitate site-selective reactions to realize direct C-N bond arylations. The results demonstrated that Cu^I-MOF-1 and Cu^II-MOF-3 possessed well-defined platforms with uniformly and accurately active sites to attain a "turn-on/off" process via different reaction routes, providing the desired site-selective ring-opening products.

Deconstructive Synthesis of Bridged and Fused Rings via Transition-Metal-Catalyzed "Cut-and-Sew" Reactions of Benzocyclobutenones and Cyclobutanones

Bridged and fused rings are commonly found in biologically important molecules. Current tactics to construct these ring systems are primarily based on stepwise ring formation (i.e., making one ring first followed by making another) and cycloaddition reactions (e.g., Diels-Alder reaction). To seek a complementary and perhaps more unified ring-forming approach, a deconstructive strategy based on C-C bond activation of cyclic ketones has been conceived. The named "cut-and-sew" reaction uses cyclic ketones with a tethered unsaturated moiety as substrates, which involves oxidative addition of a transition metal into the ketone C-C bond followed by intramolecular insertion of the unsaturated unit. This strategy has proved successful to access diverse ring scaffolds that are nontrivial to construct otherwise.This Account offers a concise summary of our laboratory's systematic efforts in developing transition metal-catalyzed cut-and-sew reactions for the synthesis of bridged and fused rings over the past 10 years. In particular, we will focus on the reactions using readily available benzocyclobutenones and cyclobutanones. To date, the scope of the cut-and-sew reactions has been greatly expanded. First, diverse unsaturated moieties can serve as suitable coupling partners, such as alkenyl, alkynyl, allenyl, carbonyl, and iminyl groups. Second, a variety of reaction modes have been uncovered. In this account, (4 + 2), (4 + 2 - 1), and (4 + 1) cycloadditions that lead to a range of bridged or fused scaffolds will be summarized. Third, enantioselective transformations have been realized to efficiently construct chiral scaffolds, which are enabled by two strategies: enantio-determining migratory insertion and desymmetrization of cyclobutanones. Fourth, the synthetic applications have been demonstrated in streamlined total syntheses of a number of complex natural products. Compared to conventional synthetic logics, the cut-and-sew reaction allows the development of new bond-disconnecting strategies. Thus, the syntheses of (-)-cycloclavine, (-)-thebainone A, penicibilaenes, and the proposed cycloinumakiol are discussed in more detail.In addition to the narrative of the development of the cut-and-sew chemistry, this Account also aims to provide core guiding foundations and inspirations toward broader deconstructive synthetic applications through C-C bond cleavage. It is anticipated that more classes of cyclic compounds could serve as the substrates beyond benzocyclobutenones and cyclobutanones, and more diverse unsaturated moieties could be coupled. It can also be envisaged that more innovative utilization of this cut-and-sew strategy in complex organic syntheses will be revealed in the near future.

An endo-Directing-Group Strategy Unlocks Enantioselective (3+1+2) Carbonylative Cycloadditions of Aminocyclopropanes

An endo-directing group strategy enables enantioselective (3+1+2) cycloadditions that are triggered by carbonylative C-C bond activation of cyclopropanes. These processes are rare examples of cycloadditions where C-C bond oxidative addition is enantiodetermining, and the first where this is achieved within the context of a multicomponent (higher order) reaction design.

KOtBu-promoted Michael/aldol/ring-opening cascade reaction of cyclobutanones with chalcones

An efficient Michael/aldol/ring-opening cascade reaction of cyclobutanones with chalcones has been developed. This protocol employs inexpensive potassium tert-butoxide (KOtBu) as a promoter and enables an efficient synthesis of densely substituted cyclohex-3-ene-carboxylic acids in high yields with high diastereoselectivities, which are difficult to prepare through conventional approaches. The significant advantages of this methodology include transition-metal-free conditions, readily available starting materials, wide scope and high atom economy.

Fe-Catalyzed Selective Formal Insertion of Diazo Compounds into C(sp)-C(sp3) Bonds of Propargyl Alcohols: Access to Alkyne-Substituted All-Carbon Quaternary Centers

The construction of all-carbon quaternary centers, especially those containing an alkyne-substituted framework, represents an important challenge in organic synthesis. Here we present a novel Fe-catalyzed selective formal insertion of diazo compounds into C(sp)-C(sp³) bonds of propargyl alcohols under mild conditions that enables the streamlined construction of alkyne-substituted all-carbon quaternary centers. This unique strategy starts with in situ generation of an ester group in the presence of carboxylic acids, followed by insertion of metal-carbene into C(sp)-C(sp³) bonds, which may open up a new reaction mode for exploring metal-carbene insertion into acyclic C-C bonds.

Electrophotochemical Ce-Catalyzed Ring-Opening Functionalization of Cycloalkanols under Redox-Neutral Conditions: Scope and Mechanism

Selective cleavage and functionalization of C-C bonds in alcohols is gaining increasing interest in organic synthesis and biomass conversion. In particular, the development of redox-neutral catalytic methods with cheap catalysts and clean energy is of utmost interest. In this work, we report a versatile redox-neutral method for the ring-opening functionalization of cycloalkanols by electrophotochemical (EPC) cerium (Ce) catalysis. The EPC-Ce-enabled catalysis allows for cycloalkanols with different ring sizes to be cleaved while tolerating a broad range of functional groups. Notably, in the presence of chloride as a counteranion and electrolyte, this protocol selectively leads to the formation of C-CN, C-C, C-S, or C-oxime bonds instead of a C-halide bond after β-scission. A preliminary mechanistic investigation indicates that the redox-active Ce catalyst can be tuned by electro-oxidation and photo-reduction, thus avoiding the use of an external oxidant. Spectroscopic characterizations (cyclic voltammetry, UV-vis, electron paramagnetic resonance, and X-ray absorption fine structure) suggest a Ce(III)/Ce(IV) catalytic pathway for this transformation, in which a Ce(IV)-alkoxide is involved.

Carbonylative N-Heterocyclization via Nitrogen-Directed C-C Bond Activation of Nonactivated Cyclopropanes

Under Rh-catalyzed conditions, secondary amines and anilines function as directing groups to facilitate regioselective C–C bond activation of nonactivated cyclopropanes. The resulting amino-stabilized rhodacycles undergo carbonylative C–N bond formation en route to challenging seven- and eight-membered lactams. The processes represent rare examples where C–C bond oxidative addition of nonactivated cyclopropanes is exploited in reaction design.

Metal-free hypervalent iodine-promoted tandem carbonyl migration and unactivated C(Ph)-C(Alkyl) bond cleavage for quinolone scaffold synthesis

An unexpected iodine(III)-mediated C(sp³)-C(sp²) bond cleavage of 3-(methylamino)-2-(2-substitutedbenzoyl)acrylates for efficient synthesis of privileged scaffold 4-quinolones was described. Notably, a wide range of alkyl groups (e.g. methyl, tert-butyl or alkyl chain) can be conveniently cleaved in this system. The detailed mechanism studies revealed that the transformation proceeded through cascade ipso-cyclization and 1,2-carbonyl migration, the smaller bond energy determined ortho C-C bond cleavage rather than C-H bond cleavage, via an enamine radical intermediate.

Heterocycles from cyclopropenones

Great attention has been paid to cyclopropenones as they are present in many natural sources. Various synthesized cyclopropenone derivatives also show a wide range of biological activities. The cyclopropenone derivatives undergo a variety of reactions such as ring-opening reactions, isomerization reactions, C-C coupling reactions, C-H activation, cycloaddition reactions, thermal and photo-irradiation reactions, and acid-base-catalyzed reactions under the influence of various chemical reagents (electrophiles, nucleophiles, radicals, and organometallics) and external forces (heat and light). Many previous reviews have dealt with the chemistry and reactions of cyclopropenones. However and to the best of our knowledge, the utility of cyclopropenones in the synthesis of heterocycles has not been reported before. Therefore, it would be interesting to shed light on this new topic. The present review article provides, for the first time, a comprehensive compilation of synthetic methods for the synthesis of various heterocyclic ring systems, as a significant family in the field of organic chemistry.

Palladium-Catalyzed Stagewise Strain-Release-Driven C-C Activation of Bicyclo[1.1.1]pentanyl Alcohols

A palladium-catalyzed chemoselective coupling of readily available bicyclo[1.1.1]pentanyl alcohols (BCP-OH) with various halides is reported, which offers expedient approaches to a wide range of cyclobutanone and β,γ-enone skeletons via single or double C-C activation. The chemistry shows a broad substrate scope in terms of both the range of BCP-OH and halides including a series of natural product derivatives. Moreover, dependency of reaction chemodivergence on base additive has been investigated through experimental and density functional theory (DFT) studies, which is expected to significantly enrich the reaction modes and increase the synthetic potential of BCP-OH in preparing more complex molecules.

Palladium-Catalyzed Access to Benzocyclobutenone-Derived Ketonitrones via C(sp2)-H Functionalization

The palladium-catalyzed C(sp2)-H functionalization of bromoaryl aldonitrones leading to benzocyclobutenone-derived ketonitrones is described. This method allows for the preparation of a wide range of strained, four-membered ketonitrones with broad functional group tolerance. Downstream transformations of the formed products were readily demonstrated, illustrating the synthetic utility of the obtained benzocyclobutenone-derived nitrones for the construction of polycyclic nitrogen-containing scaffolds.

Rapid syntheses of N-fused heterocycles via acyl-transfer in heteroaryl ketones

The wide-ranging potencies of bioactive N-fused heterocycles inspire the development of synthetic transformations that simplify preparation of their complex, diverse structural motifs. Heteroaryl ketones are ubiquitous, readily available, and inexpensive molecular scaffolds, and are thus synthetically appealing as precursors in preparing N-fused heterocycles via intramolecular acyl-transfer. To best of our knowledge, acyl-transfer of unstrained heteroaryl ketones remains to be demonstrated. Here, we show an acyl transfer-annulation to convert heteroaryl ketones to N-fused heterocycles. Driven via aromatisation, the acyl of a heteroaryl ketone can be transferred from the carbon to the nitrogen of the corresponding heterocycle. The reaction commences with the spiroannulation of a heteroaryl ketone and an alkyl bromide, with the resulting spirocyclic intermediate undergoing aromatisation-driven intramolecular acyl transfer. The reaction conditions are optimised, with the reaction exhibiting a broad substrate scope in terms of the ketone and alkyl bromide. The utility of this protocol is further demonstrated via application to complex natural products and drug derivatives to yield heavily functionalised N-fused heterocycles.

Heteroaryl ketones are ubiquitous molecular scaffolds but seldom used as synthetic precusors. Here, the authors develop an acyl transfer-annulation to convert heteroaryl ketones to N-fused heterocycles, which are prevalent in bioactive molecules.

Catalytic Enantioselective Synthesis of γ-Lactams with β-Quaternary Centers via Merging of C-C Activation and Sulfonyl Radical Migration

Transition-metal-catalyzed C-C activation has become synthetically valuable; however, it rarely involves single-electron downstream processes. To expand the repertoire of C-C activation, here we describe the discovery of a Rh-catalyzed enantioselective C-C activation involving migration of a sulfonyl radical. This reaction directly transforms cyclobutanones containing a sulfonamide-tethered 1,3-diene moiety into γ-lactams containing a β-quaternary center with excellent enantioselectivity. This unusual process involves cleavage of C-C and N-S bonds and subsequent formation of C-N and C-S bonds. The reaction also exhibits broad functional group tolerance and a good substrate scope. A combined experimental and computational mechanistic study suggested that the reaction goes through a Rh(I)-mediated oxidative addition into the cyclobutanone C-C bond followed by a Rh(III)-triggered N-S bond homolysis and sulfonyl radical migration.

Merging strain-release and copper catalysis: the selective ring-opening cross-coupling of 1,2-oxazetidines with boronic acids

An unprecedented ring-opening cross-coupling of 1,2-oxazetidines with readily available arylboronic acids is achieved for the first time by copper catalysis. Unlike the known electrophilic oxygen reactivity in coupling with organometallic reagents, 1,2-oxazetidines were utilized as formaldimine precursors in this protocol. Remarkable features of this reaction include simple operation, inexpensive catalyst, broad scope and high regioselectivity, delivering a wide array of aminomethylation products. The practicality of this reaction was validated in the one-step downstream transformation of the obtained products into synthetically important molecules and late-stage modification of bioactive acids.

Enantioselective Iridium-Catalyzed Reductive Coupling of Dienes with Oxetanones and N-Acyl-Azetidinones Mediated by 2-Propanol

Cyclometallated iridium-PhanePhos complexes generated in situ from [Ir(cod)Cl]2 and (R)-PhanePhos catalyze 2-propanol-mediated reductive couplings of 2-substituted dienes with oxetanone and N-acyl-azetidinones to form branched homoallylic oxetanols and azetidinols with excellent control of regio- and enantioselectivity without C-C cleavage of the strained ring via enantiotopic π-facial selection of transient allyliridium nucleophiles. This work represents the first systematic study of enantioselective additions to symmetric ketones.

Intramolecular One-Carbon Homologation of Unstrained Ketones via C-C Activation-Enabled 1,1-Insertion of Alkenes

Here, we describe the development of a Rh-catalyzed intramolecular one-carbon homologation of unstrained aryl ketones through a formal 1,1-insertion process of olefins, enabled by temporary directing group (TDG)-aided C-C activation. The reaction provides a distinct approach to access various substituted 1-indanones. Computational mechanistic studies reveal that the formal 1,1-insertion is realized by a selective C(sp²)-C(sp³) activation and turnover limiting 2,1-insertion into the alkene, followed by a facile β-H elimination and reinsertion process.

Computational, Mechanistic, and Experimental Insights into Regioselective Catalytic C-C Bond Activation in Linear 1-Aza-[3]triphenylene

C-C bond activation by transition metal complexes in ring-strained compounds followed by annulation with unsaturated compounds is an efficient approach to generate structurally more complex compounds. However, the site of catalytic C-C bond activation is difficult to predict in unsymmetrically substituted polycyclic systems. Here, we report a study on the (regio)selective catalytic cleavage of selected C-C bonds in 1-aza-[3]triphenylene, followed by annulation with alkynes, forming products with extended π-conjugated frameworks. Based on density functional theory (DFT) calculations, we established the stability of possible transition metal intermediates formed by oxidative addition to the C-C bond and thus identified the likely site of C-C bond activation. The computationally predicted selectivity was confirmed by the following experimental tests for the corresponding Ir-catalyzed C-C cleavage reaction followed by an alkyne insertion that yielded mixtures of two mono-insertion products isolated with yields of 34-36%, due to the close reactivity of two bonds during the first C-C bond activation. Similar results were obtained for twofold Ir- or Rh-catalyzed insertion reactions, with higher yields of 72-77%. In a broader context, by combining DFT calculations, which provided insights into the relative reactivity of individual C-C bonds, with experimental results, our approach allows us to synthesize previously unknown pentacyclic azaaromatic compounds.

Synthesis of Benzofused O- and N-Heterocycles through Cascade Carbopalladation/Cross-Alkylation of Alkynes Involving the C–C Cleavage of Cyclobutanols

We report a Pd-catalyzed route to heterocycles bearing a tetrasubstituted alkene fragment. Our approach merges the intramolecular carbopalladation of tethered alkynes with an alkylation step produced by the C–C cleavage of cyclobutanol derivatives. An alkenyl-Pd(II) intermediate has been isolated and characterized by X-ray diffraction studies. Interestingly, the nature of the tethering alkynyl chain influences the E/Z stereochemistry of the alkenyl fragment in the functionalized heterocycles.

Copper-Catalyzed Ring-Opening Defluoroborylation of gem-Difluorinated Cyclobutenes: A General Route to Bifunctional 1,3-Dienes and Their Applications

The exploration of the reactivity of gem-difluorinated small-size rings has continuously drawn attention in recent years but is limited to three-membered carbocycles. Herein we report a copper-catalyzed reaction of gem-fluorinated cyclobutenes with bis(pinacolato)diboron (B₂pin₂). A sequence of defluoroborylation and a ring-opening process produces B,F-bifunctional 1,3-dienes in a stereoselective manner. The transformation together with the efficient downstream coupling of the boronate and the fluoride moieties collectively constitutes a modular route to highly functionalized and stereocontrolled 1,3-dienes.

C-C Bond Cleavage of α-Pinene Derivatives Prepared from Carvone as a General Strategy for Complex Molecule Synthesis

The preparation of complex molecules (e.g., biologically active secondary metabolites) remains an important pursuit in chemical synthesis. By virtue of their sophisticated architectures, complex natural products inspire total synthesis campaigns that can lead to completely new ways of building molecules. In the twentieth century, one such paradigm which emerged was the use of naturally occurring "chiral pool terpenes" as starting materials for total synthesis. These inexpensive and naturally abundant molecules provide an easily accessed source of enantioenriched material for the enantiospecific preparation of natural products. The most common applications of chiral pool terpenes are in syntheses where their structure can, entirely or largely, be superimposed directly onto a portion of the target structure. Less straightforward uses, where the structure of the starting chiral pool terpene is not immediately evident in the structure of the target, can be more challenging to implement. Nevertheless, these "nonintuitive" approaches illustrate the ultimate promise of chiral pool-based strategies: that any single chiral pool terpene could be applied to syntheses of an indefinite number of structurally diverse complex synthetic targets.By definition, such strategies require carefully orchestrated sequences of C-C bond forming and C-C cleaving reactions which result in remodeling of the terpene architecture. The combination of traditional rearrangement chemistry and transition-metal-catalyzed C-C cleavage methods, the latter of which were primarily developed in the early twenty-first century, provide a rich and powerful toolbox for implementing this remodeling approach. In this Account, we detail our efforts to use a variety of C-C cleavage tactics in the skeletal remodeling of carvone, a chiral pool terpene. This skeletal remodeling strategy enabled the reorganization of the carvone scaffold into synthetic intermediates with a variety of carboskeletons, which we, then, leveraged for the total syntheses of structurally disparate terpene natural products.We begin by describing our initial investigations into various, mechanistically distinct C-C cleavage processes involving cyclobutanols synthesized from carvone. These initial studies showcased how electrophile-mediated semipinacol rearrangements of these cyclobutanols can lead to [2.2.1]bicyclic intermediates, and how Rh- and Pd-catalyzed C-C cleavage can lead to a variety of densely functionalized cyclohexenes pertinent to natural product synthesis. We, then, present several total syntheses using these synthetic intermediates, beginning with the bridged, polycyclic sesquiterpenoid longiborneol, which was synthesized from a carvone-derived [2.2.1]bicycle following a key semipinacol rearrangement. Next, we discuss how several members of the macrocyclic phomactin family were synthesized from a cyclohexene derivative prepared through a Rh-catalyzed C-C cleavage reaction. Finally, we describe our synthesis of the marine diterpene xishacorene B, which was prepared using a key Pd-catalyzed C-C cleavage/cross-coupling that facilitated the assembly of the core [3.3.1]bicycle that is resident in the natural product structure.

Photoredox Organocatalysis for the Enantioselective Synthesis of 1,7-Dicarbonyl Compounds

We describe an asymmetric organocatalytic method to synthesize 1,7-dicarbonyl compounds containing a β-stereocenter. The chemistry relies on the formation of γ-keto radicals, generated upon oxidative ring opening of cyclobutanols mastered by an organic photoredox catalyst. These nonstabilized primary radicals are stereoselectively intercepted by an iminium ion intermediate, formed upon activation of aliphatic and aromatic enals by a chiral secondary amine catalyst. This organocatalytic photoredox method served to prepare scaffolds found in natural products and drug molecules.

Kinetic stabilization allows structural analysis of a benzoborirene

Formal reduction of (2-bromophenyl)chloro(2,2'',4,4'',6,6''-hexaisopropyl-[1,1':3',1''-terphenyl]-2'-yl)borane with tert-butyl lithium at low temperatures yields a highly strained benzoborirene that is kinetically stabilized by the bulky terphenyl substituent. The target compound withstands heating to 80 °C, and represents the first benzoborirene fully characterized by single-crystal X-ray crystallography. The bond length pattern of the six-membered ring of the parent benzoborirene follows an anti-Mills-Nixon distortion.

Catalytic Activation of Unstrained C(Aryl)-C(Alkyl) Bonds in 2,2'-Methylenediphenols

Catalytic activation of unstrained and nonpolar C-C bonds remains a largely unmet challenge. Here, we describe our detailed efforts in developing a rhodium-catalyzed hydrogenolysis of unstrained C(aryl)-C(alkyl) bonds in 2,2'-methylenediphenols aided by removable directing groups. Good yields of the monophenol products are obtained with tolerating a wide range of functional groups. In addition, the reaction is scalable, and the catalyst loading can be reduced to as low as 0.5 mol %. Moreover, this method proves to be effective to cleave C(aryl)-C(alkyl) linkages in both models of phenolic resins and commercial novolacs resins. Finally, detailed experimental and computational mechanistic studies show that with C-H activation being a competitive but reversible off-cycle reaction, this transformation goes through a directed C(aryl)-C(alkyl) oxidative addition pathway.

Electrochemical Aerobic Oxidative Cleavage of (sp3)C-C(sp3)/H Bonds in Alkylarenes

An electrochemistry-promoted oxidative cleavage of (sp³)C-C(sp³)/H bonds in alkylarenes was developed. Various aryl alkanes can be smoothly converted into ketones/aldehydes under aerobic conditions using a user-friendly undivided cell setup. The features of air as oxidant, scalability, and mild conditions make them attractive in synthetic organic chemistry.

Phosphine-Catalyzed Dearomative [3 + 2] Cycloaddition of Benzoxazoles with a Cyclopropenone

The triphenylphosphine-catalyzed dearomative [3 + 2] cycloaddition of benzoxazoles with 1,2-diphenylcyclopropenone is herein described. The reaction scope, mechanism, and possible future applications of this rare organocatalyzed cycloaddition are herein discussed.

Cyclobutane based "overbred intermediates" and their exploration in organic synthesis

"Overbred intermediates" have been used in organic synthesis for a long time, but these intermediates are not categorized as such. This name was given recently in Hoffman's book Elements of Synthesis Planning. These intermediates are very useful to synthesize complex carbocyclic molecules. A number of powerful and efficient strategies have been developed by using overbred intermediates through innovative fragmentative transformations. This review is based on four-member overbred intermediates (cyclobutane based) that have been used in the total synthesis of natural products from 1968 to 2020. In the initial part, we have discussed synthetic methods (photochemical, metal-mediated, and other miscellaneous transformations) for the construction of cyclobutane overbred intermediates. In the later section, we have discussed how the overbred skeleton is cleaved through numerous fragmentation methods to access the desired target structure.

Electrochemical Ring-Opening Dicarboxylation of Strained Carbon-Carbon Single Bonds with CO2: Facile Synthesis of Diacids and Derivatization into Polyesters

Diacids are important monomers in the polymer industry to construct valuable materials. Dicarboxylation of unsaturated bonds, such as alkenes and alkynes, with CO₂ has been demonstrated as a promising synthetic method. However, dicarboxylation of C─C single bonds with CO₂ has rarely been investigated. Herein we report a novel electrochemical ring-opening dicarboxylation of C─C single bonds in strained rings with CO₂. Structurally diverse glutaric acid and adipic acid derivatives were synthesized from substituted cyclopropanes and cyclobutanes in moderate to high yields. In contrast to oxidative ring openings, this is also the first realization of an electroreductive ring-opening reaction of strained rings, including commercialized ones. Control experiments suggested that radical anions and carbanions might be the key intermediates in this reaction. Moreover, this process features high step and atom economy, mild reaction conditions (1 atm, room temperature), good chemoselectivity and functional group tolerance, low electrolyte concentration, and easy derivatization of the products. Furthermore, we conducted polymerization of the corresponding diesters with diols to obtain a potential UV-shielding material with a self-healing function and a fluorine-containing polyester, whose performance tests showed promising applications.

Nickel-Catalyzed Addition of C-C Bonds of Amides to Strained Alkenes: The 1,2-Carboaminocarbonylation Reaction

C(aryl)-C(═O) bonds of aryl amides can be activated and added across alkenes with the aid of a nickel catalyst. This 1,2-carboaminocarbonylation reaction enables the dicarbofunctionalization of alkenes with an atom economy of 100%.

Regioselective and stereospecific synthesis of functionalized 3,4-dihydro-2H-1,4-thiazines by catalyst-free [3 + 3] annulation of pyridinium 1,4-zwitterionic thiolates and aziridines

An efficient route to 3,4-dihydro-2H-1,4-thiazines was developed though [3 + 3] annulation of pyridinium 1,4-zwitterionic thiolates and aziridines under catalyst-free conditions.