Brook Rearrangement as Trigger for Carbene Generation: Synthesis of Stereodefined and Fully Substituted Cyclobutenes

Visible-Light-Induced Insertion of Siloxycarbene into Amide N-H Bonds: Synthesis of Carbinolamides from Acylsilanes and Amides

The insertion of carbene into secondary amide N-H bonds remains underexplored in organic synthesis. In this work, we discovered the visible-light-induced insertion of siloxycarbene into amide N-H bonds. This metal-free, facile reaction proceeds with atom economy under mild conditions with a broad range of secondary N-H amides, including benzanilide, acetanilide, oxindole, isatin, quinolinone, and maleimide, affording stable N- and O-acetals in excellent isolated yields. In addition, the chemoselective insertion reveals the robustness of this chemical transformation.

Sc-Catalyzed Asymmetric [2 + 2] Annulation of 2-Alkynylnaphthols with Dienes to Access Cyclobutene Frameworks

Herein, we introduce a scandium-catalyzed synthetic strategy that provides access to a diverse and functionalized array of cyclobutene frameworks adorned with a quaternary carbon center. This approach broadens the synthetic repertoire of 2-alkynylnaphthols with alkenes, offering a versatile platform for the construction of complex molecular architectures. The asymmetric catalytic [2 + 2] cycloaddition reaction demonstrates a wide substrate scope and an impressive functional group tolerance, yielding products with high efficiency, up to 97% yield, and excellent enantiomeric excess of up to 97%. The simplicity of scaling up this process, coupled with the ease of converting these cyclobutene frameworks into a variety of substituted products, significantly enhances the synthetic utility of this method.

Recent synthetic strategies for the functionalization of fused bicyclic heteroaromatics using organo-Li, -Mg and -Zn reagents

This review highlights the use of functionalized organo-Li, -Mg and -Zn reagents for the construction and selective functionalization of 5- and 6-membered fused bicyclic heteroaromatics. Special attention is given to the discussion of advanced syntheses for the preparation of highly functionalized heteroaromatic scaffolds, including quinolines, naphthyridines, indoles, benzofurans, benzothiophenes, benzoxazoles, benzothiazoles, benzopyrimidines, anthranils, thienothiophenes, purine coumarins, chromones, quinolones and phthalazines and their fused heterocyclic derivatives. The organometallic reagents used for the desired functionalizations of these scaffolds are generally prepared in situ using the following methods: (i) through directed selective metalation reactions (DoM), (ii) by means of halogen/metal exchange reactions, (iii) through oxidative metal insertions (Li, Mg, Zn), and (iv) by transmetalation reactions (organo-Li and Mg transmetalations with ZnCl2 or ZnO(Piv)2). The resulting reactive organometallic reagents allow a wide range of C-C, C-N and C-X cross-coupling reactions with different electrophiles, employing in particular Kumada or Negishi protocols among other transition metal (Pd, Ni, Co, Cu, Cr, Fe, etc.)-catalyzed processes. In addition, key developments concerning selective metalation techniques will be presented, which rely on the use of RLi, LDA and TMP metal bases. These methods are now widely employed in organic synthetic chemistry and have proven to be particularly valuable for drug development programs in the pharmaceutical industry. New and improved protocols have resulted in many Li, Mg and Zn organyls now being compatible with functionalized aryl, heteroaryl, alkenyl, alkynyl and alkyl compounds even in the presence of labile functional groups, making these reagents well-suited for C(sp2)-C(sp2), C(sp2)-C(sp) and C(sp2)-C(sp3) cross-coupling reactions with fused heteroaryl halides. In addition, the use of some transition metal-catalyzed processes occasionally allows a reversed role of the reactants in cross-coupling reactions, providing alternative synthetic routes for the preparation of fused heteroaromatic-based bioactive drugs and natural products. In line with this, this article points to novel methods for the functionalization of bicyclic heteroaromatic scaffolds by organometallic reagents that have been published in the period 2010-2023.

Benzocyclobutenone synthesis exploiting acylsilanes as photofunctional directing groups

The visible-light irradiation of acylsilane tethered vinyl ketones promotes an intramolecular Stetter-type reaction via siloxycarbene intermediates. To exploit this unique mode of reactivity, we herein describe the innovative use of acylsilanes as photofunctional directing groups. First, an acylsilane directed ruthenium catalysed C-H olefination reaction was developed to generate benzoylsilanes bearing vinyl ketone functionality. Then, visible-light irradiation initiated the 1,4-conjugate addition of transient siloxycarbene intermediates with pendent vinyl ketones to afford unique benzocyclobutenone scaffolds primed for further synthetic elaboration.

Divergent Enantioselective Access to Diverse Chiral Compounds from Bicyclo[1.1.0]butanes and α,β-Unsaturated Ketones under Catalyst Control

Achieving structural and stereogenic diversity from the same starting materials remains a fundamental challenge in organic synthesis, requiring precise control over the selectivity. Here, we report divergent catalytic methods that selectively yield either cycloaddition or addition/elimination products from bicyclo[1.1.0]butanes and α,β-unsaturated ketones. By employing chiral Lewis acid or Brønsted acid catalysts, we achieved excellent regio-, diastereo-, and enantioselectivity across all three distinct transformations, affording a diverse array of synthetically valuable chiral bicyclo[2.1.1]hexanes and cyclobutenes. The divergent outcomes are controlled by the differential activation of the substrates by the specific chiral catalyst with the reaction conditions dictating the pathway selectivity. This strategy demonstrates the power of divergent catalysis in creating molecular complexity and diversity, offering a valuable tool for the synthesis of enantioenriched chiral building blocks.

Electroreductive Cross-Coupling between Aromatic Aldehydes and Chlorosilanes Enabling the Synthesis of α-Silyl Alcohols

α-Silyl alcohols are powerful structural motifs for pharmaceutical chemistry, materials chemistry, and organic synthesis. The limitations of current synthetic techniques encompass a requirement for difficult-to-obtain silyl precursors, noble-metal catalysts, and narrow substrate scopes. Here, we developed a general synthetic method for α-silyl alcohols through electroreductive cross-coupling of aldehydes and chlorosilane. This method features easily available reagents, mild conditions, and a wide substrate scope. The establishment of this protocol will provide an alternative for access to α-silyl alcohols.

N-H Insertion of Anilines on N-Tosylhydrazones Induced by Visible Light Irradiation

Diazo compounds and their precursors represent an interesting chemical category for organic synthesis. Particularly, N-tosylhydrazones have attracted attention for their easy accessibility and diverse reactivity, including carbene transfer reactions. We described a visible light-induced N-H insertion reaction of anilines on in situ-generated diazo compounds. Optimal conditions using DBU in toluene efficiently yielded the desired products. Mechanistic studies enabled us to trap a carbene intermediate that plays a key role in the transformation.

Chemoenzymatic synthesis of optically active α-cyclopropyl-pyruvates and cyclobutenoates via enzyme-catalyzed carbene transfer with diazopyruvate

Cyclopropanes are recurrent structural motifs in natural products and bioactive molecules. Recently, biocatalytic cyclopropanations have emerged as a powerful approach to access enantioenriched cyclopropanes, complementing chemocatalytic approaches developed over the last several decades. Here, we report the development of a first biocatalytic strategy for cyclopropanation using ethyl α-diazopyruvate as a novel enzyme-compatible carbene precursor. Using myoglobin variant Mb(H64V,V68G) as the biocatalyst, this method afforded the efficient synthesis of α-cyclopropylpyruvates in high diastereomeric ratios and enantiomeric excess (up to 99% ee). The ketoester moiety in the cyclopropane products can be used to synthesize diverse optically pure cyclopropane derivatives. Furthermore, the enzymatically obtained α-cyclopropylpyruvate products could be converted into enantiopure cyclobutenoates via a metal-free photochemical ring expansion without loss of optical activity.

Ring-opening-isomerization anionic polymerization via Brook rearrangement

Ring-opening isomerization polymerization was developed using a combination of a ring-opening reaction of epoxides and subsequent Brook rearrangement. An epoxy monomer with a benzyltrimethylsilyl group was designed for the polymerization. Characterization of the obtained polymer by NMR and MALDI-TOF-MS indicated that polymerization proceeded exclusively via a ring-opening-isomerization anionic polymerization mechanism.

Synthesis of silyl indenes by ruthenium-catalyzed aldehyde- and acylsilane-enabled C-H alkylation/cyclization

A ruthenium-catalyzed C-H alkylation/cyclization sequence is presented to prepare silyl indenes with atom and step-economy. This domino reaction is triggered by acyl silane-directed C-H activation, and an aldehyde controlled the following enol cyclization/condensation other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyl indenes allows access to a diverse range of interesting indene and indanone derivatives.

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.

Acylsilanes in Transition-Metal-Catalyzed and Photochemical Reactions: Clarifying Product Formation

Acylsilanes are able to react as nucleophilic carbene precursors, electrophiles, and directing groups in C-H functionalization. To date, some of the products reportedly formed during transition-metal-catalyzed and photochemical reactions involving acylsilanes have been incorrectly assigned. To provide clarity, we herein address these structural misassignments and detail the revised structures. New insights into the reactivity of acylsilanes were also afforded via the discovery that light-induced siloxy carbenes participate in intramolecular 1,2-carbonyl addition to proximal esters.

Enantioselective Synthesis of Chiral Cyclobutenes Enabled by Brønsted Acid-Catalyzed Isomerization of BCBs

Chiral cyclobutene units are commonly found in natural products and biologically active molecules. Transition-metal-catalysis has been extensively used in asymmetric synthesis of such structures, while organocatalytic approaches remain elusive. In this study, bicyclo[1.1.0]butanes are involved in enantioselective transformation for the first time to offer a highly efficient route toward cyclobutenes with good regio- and enantiocontrol. The utilization of N-triflyl phosphoramide as a chiral Brønsted acid promoter enables this isomerization process to proceed under mild conditions with low catalyst loading as well as good functional group compatibility. The resulting chiral cyclobutenes could serve as platform molecules for downstream manipulations with excellent reservation of stereochemical integrity, demonstrating the synthetic practicality of the developed method. Control experiments have also been performed to verify the formation of a key carbocation intermediate at the benzylic position.

Friedel-Crafts Reaction of Acylsilanes: Highly Chemoselective Synthesis of 1-Hydroxy-bis(indolyl)methanes and 1-Silyl-bis(indolyl)methanes Derivatives

A novel double Friedel-Crafts reaction of acylsilanes in water is described. This strategy enables synthesis of bis(indolyl)methane derivatives with 1-hydroxy or 1-silyl substituents in moderate to high yield. Compared to the 1-silyl-bis(indolyl)methane derivatives from indole substrate, 1-hydroxy-bis(indolyl)methane derivatives were synthesized from the 5-hydroxyindole, and the hydrogen bonds in the 5-hydroxyindole play a crucial role in regulating the reaction selectivity.

Four-Component Strain-Release-Driven Synthesis of Functionalized Azetidines

Despite the favorable properties that azetidine rings can engender on drug-compounds, methods for the diversity-oriented synthesis of azetidine-based structures are significantly underdeveloped. Herein, we report the successful realization of a multicomponent [1,2]-Brook rearrangement/strain-release-driven anion relay sequence and its application to the modular synthesis of substituted azetidines. The rapidity of the reaction, as confirmed by in situ infra-red spectroscopy, leverages the strain-release ring-opening of azabicyclo[1.1.0]butane to drive the equilibrium of the Brook rearrangement. The three electrophilic coupling partners, added sequentially to azabicyclo[1.1.0]butyl-lithium, could be individually varied to access a diverse compound library. The utility of this methodology was demonstrated in a 4-step synthesis of the EP2 receptor antagonist PF-04418948.

Regio- and Diastereoselective Carbometalation Reaction of Cyclopropenes

The various facets of the chemistry of cyclopropane derivatives, the smallest carbocycle, are amazingly diverse and continue to fascinate theoreticians, synthetic or structural chemists having interest in fundamental physical, medicinal chemistry, and natural product synthesis. The challenges generated by this intriguing cyclic arrangement of only three tetravalent carbons represent a wide area of the chemical spectrum. From fundamental aspects of bonding through the synthesis of highly strained molecules, the understanding of the mode of action in biological systems to the selective cleavage into acyclic substrates makes the chemistry of these small rings fascinating. Therefore, efficient routes to prepare differently polysubstituted cyclopropanes have always been of a primordial importance. In the past decade, we and others have expanded the scope of the carbometalation reaction of cyclopropenes as a broad and general method to the formation of stereodefined cyclopropane derivatives. Although cyclopropenes, with their even higher strain energy, easily undergo addition reactions of organometallic reagents, their carbometalation reactions generate new regio-, diastereo-, and enantioselectivity issues that needed to be addressed. These various stereochemical aspects accompanied our research from its origins to today, and we are proposing in this Account, a didactic overview of the different ways by which cyclopropenes can lead to the formation of polysubstituted cyclopropanes or open-products possessing several stereogenic centers as a single regio- and diastereomer.We initially launched our research campaign on the chemistry of these strained three-membered rings by the regio- and diastereoselective copper-catalyzed carbomagnesiation of enantiomerically enriched cyclopropenyl carbinols. The directing alcohol governed both the regio- and diastereoselectivity of the addition and also served as a good leaving group as it undergoes a selective 1,2-elimination reaction to provide enantioenriched alkylidenecyclopropanes in excellent yields and enantiomeric excesses. Then, we turned our attention to the regio- and stereoselective synthesis of stereodefined tri- and tetrasubstituted cyclopropanes through the diastereoselective addition to sp2- monosubstituted cyclopropenyl ester derivatives. With the aim to further expand this concept to the formation of penta- and hexa-substituted cyclopropanes as single isomer, we had first to design the preparation of the required 1,2-disubstituted cyclopropenes that would control the regioselective addition of the organometallic derivatives. The synthesis of penta- and hexa-substituted cyclopropanes was then reported for the first time as a single regio- and diastereomer. It should be noted that the in situ formed cyclopropyl-metal intermediate is configurationally stable and can be subsequently functionalized with pure retention of the configuration by addition of electrophiles. Then, the enantioselective-catalyzed carbometalation reaction of achiral cyclopropenes allowed the synthesis of several new classes of cyclopropane derivatives in high enantiomeric ratios. Finally, by combining the regio- and diastereoselective carbometalation reaction of a cyclopropene with a subsequent reaction of the resulting cyclopropylmetal species, a selective carbon-carbon bond cleavage was observed to lead to the preparation of acyclic substrates possessing several stereocenters including a quaternary carbon stereogenic center. Our original vision of using strain within an embedded double bond in a three-membered ring has provided new routes to the stereoselective synthesis of polysubstituted cyclopropanes and has been extremely successful, as it represents a current new tool for the synthesis of persubstituted cyclopropanes as a single diastereomer.

Mechanistic differences between aryl iodide electrophiles and pronucleophiles in Pd-catalyzed coupling with cyclopropenes: a DFT study

Computational studies were carried out to investigate the mechanisms of Pd-catalyzed ring-opening reactions of cyclopropenes with pronucleophiles (H-Nu) and aryl iodide electrophiles, respectively.

Asymmetric [3 + 1]-Cycloaddition Reaction via Diazo Discrimination

The enantioselective [3 + 1]-cycloaddition of two structurally different diazo compounds has been achieved using chiral bisoxazoline copper(I) complexes as a catalyst, providing a novel route for the synthesis of cyclobutenes containing a quaternary stereocenter. Typically, this reaction represents the first example of asymmetric cross-electrophile coupling of two diazo substrates via carbene discrimination.

Catalytic asymmetric addition of thiols to silyl glyoxylates for synthesis of multi-hetero-atom substituted carbon stereocenters

A chiral Lewis acid-catalyzed enantioselective addition of thiols to silyl glyoxylates was developed. The reaction proceeds well with a broad range of thiols and acylsilanes, affording the target tertiary chiral α-silyl–α-sulfydryl alcohols with multi-hetero-atom carbon stereocenters in excellent yields (up to 99%) and enantioselectivities (up to 98% ee). A series of control experiments were conducted to elucidate the reaction mechanism.

Enantioselective addition of thiols to silyl glyoxylates for construction of a multi-hetero-atom substituted carbon stereocenter was described.

Regio- and Diastereoselective Copper-Catalyzed Carbomagnesiation for the Synthesis of Penta- and Hexa-Substituted Cyclopropanes

Despite the highly strained nature of cyclopropanes possessing three vicinal quaternary carbon stereocenters, the regio- and diastereoselective copper-catalyzed carbomagnesiation reaction of cyclopropenes provides an easy and efficient access to these novel persubstituted cyclopropyl cores with a complete regio- and diastereoselectivity.

Ruthenium-Catalyzed Brook Rearrangement Involved Domino Sequence Enabled by Acylsilane-Aldehyde Corporation

A ruthenium-catalyzed [1,2]-Brook rearrangement involved domino sequence is presented to prepare highly functionalized silyloxy indenes with atomic- and step-economy. This domino reaction is triggered by acylsilane-directed C-H activation, and the aldehyde controlled the subsequent enol cyclization/Brook Rearrangement other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyloxy indenes allows access to a diverse range of interesting indene and indanone derivatives.

Cobalt-Catalyzed Diastereoselective and Enantioselective Hydrosilylation of Achiral Cyclopropenes

A mild diastereoselective and enantioselective cobalt-catalyzed hydrosilylation reaction of achiral cyclopropenes has been developed. In this protocol, various substituted cyclopropenes and arylsilanes were transformed, in the presence of readily available chiral cobalt complex, into silylcyclopropanes with high selectivities.

Copper-catalyzed non-directed hydrosilylation of cyclopropenes: highly diastereoselective synthesis of fully substituted cyclopropylsilanes

The non-directed hydrosilylation of cyclopropenes with earth-abundant and environmentally benign base metal catalysis was described.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Diastereoselective ring opening of fully-substituted cyclopropanes via intramolecular Friedel-Crafts alkylation

We herein disclose a diastereoselective ring opening of non-donor-acceptor cyclopropanes via an intramolecular Friedel-Crafts alkylation en route to functionalized dihydronaphthalene scaffolds possessing quaternary carbon stereocentres. The transformation proceeds through a selective bond breaking at the most alkylated carbon centre with a pure retention of configuration. Mechanistic investigations and computational studies revealed that alkoxy functionality is the key for selective bond breaking leading to a complete retention of configuration.

Versatility in the Brook Rearrangement for the Selective Ring-Opening of Three-Membered Rings

From a single α-silylated carbinol intermediate, easily accessible by carbometallation of cyclopropenes, various scaffolds featuring a quaternary carbon stereocenter could be obtained selectively. The selectivity towards these different products was achieved by either changing the experimental conditions or the nature of the organometallic species involved.

Synthesis of silacyclopent-2-en-4-ols via intramolecular [2 + 2] photocycloaddition of benzoyl(allyl)silanes

Organosilicon compounds are versatile units with a wide range of uses from medicinal chemistry to the field of organic electronics.

Synthesis of 2-acetyl trisubstituted furans via copper-mediated deacylation cleavage of unstrained C(sp3)–C(sp2) bonds

A copper-mediated tandem addition/cyclization/carbon–carbon cleavage reaction for the convenient synthesis of 2-acyl trisubstituted furans has been developed.

Synthesis of cis-/All- cis-Substituted Cyclopropanes through Stereocontrolled Metalation and Pd-Catalyzed Negishi Coupling

We have developed a direct method for the synthesis of cis-substituted cyclopropanes from a cyclopropanecarboxamide through stereocontrolled metalation and Negishi coupling. Under the optimized reaction conditions, various substituents, including di/trisubstituted alkenes and aryl groups, were introduced in a stereoselective manner using a simple amide directing group that could subsequently be converted into an ester. Furthermore, this method was applicable to the synthesis of all- cis-substituted cyclopropanes bearing three different substituents, which are highly congested and strained molecules.

Diastereo- and enantioselective copper catalyzed hydroallylation of disubstituted cyclopropenes

A highly diastereo- and enantioselective protocol for the hydroallylation of 1,1- and 1,2-disubstituted cyclopropenes has been developed utilizing an in situ formed copper hydride. A variety of allyl electrophiles could be utilized yielding a diverse range of trisubstituted cyclopropanes. Finally a preliminary enantioselective variant could be established employing a recently described P-stereogenic xantphos derivative as ligand.

Three-component reaction to synthesize E-vinyl silyl anti-1,2-diols via sequential [1,4]-O-to-O/[1,4]-C-to-O silyl migrations

A three-component reaction of geminal bis(silyl) allyl silyl ether, aldehyde and electrophile has been developed to synthesize trisubstituted E-vinyl silyl anti-1,2-diols. The approach features a sequential [1,4]-O-to-O/[1,4]-C-to-O silyl migrations process.

3-Hydroxy-2-(trialkylsilyl)phenyl Triflate: A Benzyne Precursor Triggered via 1,3-C-sp2-to-O Silyl Migration

3-Hydroxy-2-(trialkylsilyl)phenyl triflates are presented as new versatile hydroxyaryne precursors. These are base-activated aryne precursors induced via a C-sp²-to-O 1,3-Brook rearrangement. The reaction of various arynophiles and 3-trialkylsiloxybenzyne generated from 3-hydroxy-2-(trialkylsilyl)phenyl triflate efficiently afforded highly regioselective phenol derivatives. Furthermore, through crossover experiments, the intramolecular mechanism of silyl migration was demonstrated.

Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes: The Complete Mechanistic Picture

The intermolecular gold(I)-catalyzed reaction between arylalkynes and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which takes place stepwise, first by formation of cyclopropyl gold(I) carbenes, followed by a ring expansion. However, 1,3-butadienes are also formed in the case of ortho-substituted arylalkynes by a metathesis-type process. The corresponding reaction of alkenes with aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed reaction of alkynes with alkenes is proposed on the basis of density functional theory calculations, which shows that the two pathways leading to cyclobutenes or dienes are very close in energy. The key intermediates are cyclopropyl gold(I) carbenes, which have been independently generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa[a]naphthalenes.