Dinitrogen extrusion from enoldiazo compounds under thermal conditions: synthesis of donor-acceptor cyclopropenes

Photo-cycloaddition reactions of vinyldiazo compounds

Heterocyclic rings are important structural scaffolds encountered in both natural and synthetic compounds, and their biological activity often depends on these motifs. They are predominantly accessible via cycloaddition reactions, realized by either thermal, photochemical, or catalytic means. Various starting materials are utilized for this purpose, and, among them, diazo compounds are often encountered, especially vinyldiazo compounds that give access to donor-acceptor cyclopropenes which engage in [2+n] cycloaddition reactions. Herein, we describe the development of photochemical processes that produce diverse heterocyclic scaffolds from multisubstituted oximidovinyldiazo compounds. High chemoselectivity, good functional group tolerance, and excellent scalability characterize this methodology, thus predisposing it for broader applications. Experimental and computational studies reveal that under light irradiation these diazo reagents selectively transform into cyclopropenes which engage in cycloaddition reactions with various dipoles, while under thermal conditions the formation of pyrazole from vinyldiazo compounds is favored.

Straightforward access to α-carbonyloxy esters and β-keto thioethers from aryldiazoacetates

A metal- and additive-free approach has been described for synthesizing α-carbonyloxy esters and β-keto thioethers from readily available aryldiazoacetates with carboxylic acids and thiol derivatives, respectively. α-Carbonyloxy esters and β-keto thioether derivatives were synthesized in good to high yields from aryldiazoacetates, carboxylic acids, and thiol derivatives decorated with various functional groups. Finally, the potential of the new approach is demonstrated through its application in gram-scale reactions and the synthesis of a few bioactive molecules.

Construction of heterocyclic rings from cyclopropenes

Heterocyclic rings are the fundamental building blocks of biological systems and have wide applications in synthetic chemistry and medicinal science. The development of novel synthetic methodology for heterocyclic skeletons from a variety of starting materials has made great progress in the past decades. Meanwhile, highly strained cyclopropenes as reactive reagents in organic transformations have drawn much attention from chemists. The rich chemical reactivity and reaction routes have been well investigated, and some review articles related to the reactivity of cyclopropenes and the construction of carbocycles and acyclic compounds have appeared in these years. Thus, this review mainly focuses on the progress in the construction of heterocyclic rings starting from various cyclopropenes including the reactions of commonly available stable cyclopropenes, in situ generated reactive cyclopropenes and cyclopropene precursors during this decade. Firstly, the transformations of common cyclopropenes into donor-type vinyl metal carbenes via transition metal induced ring opening, direct metalation of the CC bond of metal complexes, and cycloaddition reactions with 1,3-dipoles are described. Next, the annulation reactions of reactive cyclopropenes generated in situ with donor-acceptor reagents, intramolecular nucleophilic addition, and the cycloaddition reactions with 1,3-dipoles are introduced. Then, the transformation of cyclopropene precursors such as alkyl 1-chloro- or 1-alkoxy-2-aroylcyclopropanecarboxylates into five-membered heteroaromatic compounds is also mentioned. In addition, a brief outlook of the opportunity and challenges in the field of bio-orthogonal reactions related to cyclopropenes is given.

Copper(II) Acetate-Induced Oxidation of Hydrazones to Diazo Compounds under Flow Conditions Followed by Dirhodium-Catalyzed Enantioselective Cyclopropanation Reactions

A tandem system comprising in-line diazo compound synthesis and downstream consumption in a rhodium-catalyzed cyclopropanation reaction has been developed. Passing hydrazone through a silica column absorbed with Cu(OAc)₂-H₂O/N,N-dimethylaminopyridine oxidized the hydrazone to generate an aryldiazoacetate in flow. The crude aryldiazoacetate elutes from this column directly into a downstream cyclopropanation reaction, catalyzed by the chiral dirhodium tetracarboxylates, Rh₂(R-p-Ph-TPCP)₄ and Rh₂(R-PTAD)₄. This convenient flow to batch method was applied to the synthesis of a range of 1,2-diarylcyclopropane-1-carboxylates in high yields and with high levels of enantioselectivity.

Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor-Acceptor Cis-Cyclobutenecarboxamide

A substituted donor-acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature.

Nucleophilic Migratory Cyclopropenation of Activated Alkynes: A Nonmetal Approach to Unbiased Cyclopropenes

An unprecedented reductive [2 + 1] annulation of α-keto esters with alkynones mediated by P(NMe₂)₃ is described. Although this nonmetal cyclopropenation is a nucleophilic process, attributed to the ester migration via a formal [2 + 2] cycloaddition reaction of Kukhtin-Ramirez adducts and alkynones followed by a fragmentation, cyclopropenes with an unbiased alkene scaffold are formed in good to excellent yields, thus providing a promising complementarity to electrophilic metal-catalyzed cyclopropenation.

Formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloaddition reactions of donor-acceptor cyclobutenes, cyclopropenes and siloxyalkynes induced by Brønsted acid catalysis

Brønsted acid catalyzed formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloadditions of donor-acceptor cyclobutenes, cyclopropenes, and siloxyalkynes with benzopyrylium ions are reported. [4 + 2]-cyclization/deMayo-type ring-extension cascade processes produce highly functionalized benzocyclooctatrienes, benzocycloheptatrienes, and 2-naphthols in good to excellent yields and selectivities. Moreover, the optical purity of reactant donor-acceptor cyclobutenes is fully retained during the cascade. The 1,3-dicarbonyl product framework of the reaction products provides opportunities for salen-type ligand syntheses and the construction of fused pyrazoles and isoxazoles that reveal a novel rotamer-diastereoisomerism.

Strain-Induced Nucleophilic Ring Opening of Donor-Acceptor Cyclopropenes for Synthesis of Monosubstituted Succinic Acid Derivatives

1,2,3-Trisubstituted donor-acceptor cyclopropenes (DACPs) generated in situ from enoldiazo compounds react with nucleophiles to form α-substituted succinic acid derivatives in high yields. Initial dirhodium(II) carboxylate catalysis rapidly converts enoldiazo-acetates or -acetamides to DACPs that undergo catalyst-free Favorskii ring opening with amines, and also with anilines, alcohols, and thiols, when facilitated by catalytic amounts of 4-dimethylaminopyridine (DMAP). This methodology provides easy access to mixed esters and amides of monosubstituted succinic acids, including derivatives of naturally occurring compounds. It also affords dihydrazide, dihydroxamic acid, and diamide derivatives, as well as α-substituted tetrahydropyridazine-3,6-diones in high yields. Attempts to generate optically enriched DACPs were not successful because their populations having the R and S configurations formed with a chiral dirhodium catalyst are quite similar, and the loss of enantiocontrol likely originates from the DACP ring forming step which is reversible with its intermediate metal carbene.

Copper(i)-catalyzed highly enantioselective [3 + 3]-cycloaddition of γ-alkyl enoldiazoacetates with nitrones

Chiral copper(i) catalysts are preferred over chiral dirhodium(ii) catalysts for [3 + 3]-cycloaddition reactions of γ-alkyl-substituted enoldiazoacetates compounds with nitrones.

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.

Chiral donor-acceptor azetines as powerful reactants for synthesis of amino acid derivatives

Coupling reactions of amines and alcohols are of central importance for applications in chemistry and biology. These transformations typically involve the use of a reagent, activated as an electrophile, onto which nucleophile coupling results in the formation of a carbon-nitrogen or a carbon–oxygen bond. Several promising reagents and procedures have been developed to achieve these bond forming processes in high yields with excellent stereocontrol, but few offer direct coupling without the intervention of a catalyst. Herein, we report the synthesis of chiral donor–acceptor azetines by highly enantioselective [3 + 1]-cycloaddition of enoldiazoacetates with aza-ylides and their selective coupling with nitrogen and oxygen nucleophiles via 3-azetidinones to form amino acid derivatives, including those of peptides and natural products. The overall process is general for a broad spectrum of nucleophiles, has a high degree of electronic and steric selectivity, and retains the enantiopurity of the original azetine.

Chiral 3-azetidinones are structural analogues of medicinally relevant β-lactams, however their synthesis and reactivity are underexplored. Here, the authors show a highly enantioselective copper-catalyzed [3 + 1]-cycloaddition generating 2-azetidines, which react with nucleophiles yielding amino acids via 3-azetidinones.

Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor-acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity.

Synthesis of spiro-4H-pyrazole-oxindoles and fused 1H-pyrazoles via divergent, thermally induced tandem cyclization/migration of alkyne-tethered diazo compounds

A thermally induced, substrate-dependent reaction of alkynyl diazo compounds has been developed. This transformation produces spiro-4H-pyrazole-oxindoles and fused 1H-pyrazoles in good to high yields from the corresponding alpha-cyano and alpha-sulfonyl diazo compounds. The salient features of this reaction include excellent chemoselectivity and atom-economy, mild reaction conditions, simple purification and potential for large scale production.

Intramolecular cycloaddition/rearrangement cascade from gold(iii)-catalysed reactions of propargyl aryldiazoesters with cinnamyl imines

Surprising rearrangement of dihydroazepinyl aryldiazoacetates from a gold-catalysed [4+3]-cycloaddition to conjugated cycloheptene-1,4-dione-enamines.

Rhodium(ii)-catalysed generation of cycloprop-1-en-1-yl ketones and their rearrangement to 5-aryl-2-siloxyfurans

A new mechanistic pathway for the formation of 2,5-disubstituted furans from cycloprop-1-en-1-yl ketones generated from silyl-protected enoldiazoketones.

Catalytic Divergent [3+3]- and [3+2]-Cycloaddition by Discrimination Between Diazo Compounds

Highly selective divergent cycloaddition reactions of enoldiazo compounds and α-diazocarboximides catalyzed by copper(I) or dirhodium(II) have been developed. With tetrakis(acetonitrile)copper(I) tetrafluoroborate as the catalyst epoxypyrrolo[1,2-a]azepine derivatives were prepared in good yields and excellent diastereoselectivities through the first reported [3+3]-cycloaddition of a carbonyl ylide. Use of Rh2 (pfb)4 or Rh2 (esp)2 directs the reactants to regioselective [3+2]-cycloaddition generating cyclopenta[2,3]pyrrolo[2,1-b]oxazoles with good yields and excellent diastereoselectivities.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.

Rac-2′,3a,6,6,6′,6′-Hexamethyl-3a,3b,6,7-tetra-hydrospiro-[benzo[2,3]cyclopropa[1,2-c]pyrazole-1,1′-cyclo-hepta[2,4]diene]

This note describes a novel reaction cascade in which a tosylhydrazone derivative of eucarvone undergoes a non-classical dimerization process under basic conditions. The key step in this sequence is a dipolar cycloaddition between a diazo species and a transient cyclopropene. A proposed mechanism for this sequence is presented that is supported by single crystal X-ray analysis of the resulting dimer. We believe this unique transformation is of note as it highlights a neat and efficient entry to complex polycyclic architectures containing an embedded pyrazoline moiety.

Catalytic Asymmetric [3+1]-Cycloaddition Reaction of Ylides with Electrophilic Metallo-enolcarbene Intermediates

The first asymmetric [3+1]-cycloaddition was successfully achieved by copper(I) triflate/double-sidearmed bisoxazoline complex catalyzed reactions of β-triisopropylsilyl-substituted enoldiazo compounds with sulfur ylides. This methodology delivered a series of chiral cyclobutenes in good yields with high enantio- and diastereoselectivities (up to 99 % ee, and >20:1 d.r.). Additionally, the [3+1]-cycloaddition of catalytically generated metallo-enolcarbenes was successfully extended to reaction with a stable benzylidene dichlororuthenium complex.

Catalyst-Free Rearrangement of Allenyl Aryldiazoacetates into 1,5-Dihydro-4H-pyrazol-4-ones

Phenylpropargyl diazoacetates exist in equilibrium with 1-phenyl-1,2-dien-1-yl diazoacetate - allenes that are rapidly formed at room temperature through 1,3-acyloxy migration catalyzed by gold(I) or gold(III) compounds, and these catalysts react solely with the π-donor rather than with the diazo group. The product allene of the aryldiazoacetates undergoes rearrangement that is not catalyzed by gold in which the terminal nitrogen of the diazo functional group adds to the central carbon of the allene, initiating a sequence of bond-forming reactions, resulting in the production of 1,5-dihydro-4H-pyrazol-4-ones in good yields. These 1,5-dihydro-4H-pyrazol-4-ones undergo intramolecular 1,3-acyl migration to form an equilibrium mixture and can quantitatively transfer the acyl group to an external nucleophile with formation of 4-hydroxypyrazoles. Reactions of phenylpropargyl phenyldiazoacetates catalyzed by cationic gold complexes are initiated at the diazo functional group to form a gold carbene whose subsequent cascade process (intramolecular addition, then aromatic substitution) results in the formation of a product that is uniquely characteristic of this pathway.

NBS-mediated dinitrogen extrusion of diazoacetamides under catalyst-free conditions: practical access to 3-bromooxindole derivatives

A synthetically useful reaction via NBS-mediated dinitrogen extrusion of N-aryl diazoacetamides under catalyst-free conditions is disclosed, which gives the 3-bromooxindoles in high to excellent yields with high selectivity via a non carbene process.

Cyclopentadiene Construction via Rh-Catalyzed Carbene/Alkyne Metathesis Terminated with Intramolecular Formal [3 + 2] Cycloaddition

A new type of intramolecular carbene cascade reaction of alkynyl-tethered styryl diazoesters is presented, which is terminated with a formal [3 + 2] cycloaddition to give the bicyclic cyclopentadiene derivatives in high yields and selectivity. Additionally, it was found that the β-H shift is the dominating process in the case of alkyl alkyne-tethered substrates.

Hg(OTf)2 Catalyzed Intramolecular 1,4-Addition of Donor-Acceptor Cyclopropenes to Arenes

A Hg(OTf)2 catalyzed intramolecular arene 1,4-addition reaction of N-benzyl donor-acceptor cyclopropenecarboxamides was developed to synthesize a series of [3.2.2]nonatriene derivatives. This novel reaction is also observed with silver(I) catalysts known to form metal carbene intermediates in competition with the Buchner reaction.