Search for: All records

Abstract A Brønsted acid catalyzed C–H functionalization of vinyldiazoacetates with 3-hydroxyisoindolinone is developed. This methodology provides a general access to E-substituted isoindolinone vinyldiazo compounds in good yields and excellent diastereoselectivities with broad substrate generality under mild conditions, and with 4-substituted 2-diazo-3-butenoates produces fused bicyclic pyrrolidines. The reaction generally involves addition of the N-acyl ketiminium electrophile, formed from the 3-hydroxyisoindolinone, to the vinylogous position of the vinyldiazo compound resulting in vinyldiazonium ion intermediates that undergo deprotonation to new vinyldiazo compounds or ring closure to fused bicyclic pyrrolidines. 

Abstract 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. 

Abstract [3+n]‐Cycloaddition reactions that employ donor‐acceptor cyclopropanes using either chiral catalysts and racemic cyclopropanes or achiral catalysts and chiral, non‐racemic, cyclopropanes have become useful transformations for the construction of carbocyclic and heterocyclic compounds, with both processes offering mechanistic and structural advantages in ring formation. Although the vast majority of asymmetric cycloaddition reactions of donor‐acceptor cyclopropanes have been performed with racemic cyclopropane compounds catalyzed by Lewis acids with chiral ligands, optically active cyclopropane compounds can serve the same role using Lewis acids without chiral ligands. This review covers the use of chiral catalysts with racemic donor‐acceptor cyclopropanes and the use of chiral non‐racemic donor‐acceptor cyclopropanes with achiral Lewis acid catalysts. 

Intramolecular addition reactions of electrophilic metallovinylcarbenes with nucleophiles that do not have access to the carbene center undergo addition to the vinylogous position, forming products that rely on subsequent transformations of vinylmetal intermediates. Catalytic addition to a carbon-carbon double bond elicits the formation of an intermediate carbocation whose proton loss causes protodemetalation of the vinylmetal intermediate. Addition to the azido group results in the formation of aliphatic 1,2,3-triazines by [3 + 3]-cycloaddition. Catalytic intramolecular reactions with a carbamate nucleophile yield a carbonyl ylide whose loss of isobutylene produces oximidovinyl-oxazolidinone esters with high enantioselectivity. Comparisons are made between rhodium, copper, gold, and silver catalysts 

Triflimide catalysis of the [3 + 2]-cycloaddition of 3-indolymethanols with vinyldiazoacetates provides general access to β-tetrahydrocyclopenta[b]indol-substituted α-diazoesters. Initiated by addition of the in situ generated vinylogous iminium electrophile from 3-indolymethanol to the vinylogous position of the vinyldiazo compound and completed by intramolecular cyclization from the vinyldiazonium ion intermediate, this transformation occurs in good yields and excellent diastereoselectivity with a broad substrate scope under mild conditions. The resulting α-diazoesters undergo Rh2(OAc)4-catalyzed substrate-dependent 1,2-migration to form multisubstituted carbazoles in high yields. 

Highly selective formal [3 + 2]-cycloaddition of vinyldiazoacetates with quinone ketals and quinoneimine ketals has been accomplished at room temperature with catalytic amounts of the Brønsted acid triflimide, leading to highly functionalized diazoacetates in good yields. The vinyldiazonium ion generated by electrophilic addition to the vinylogous position of the reactant vinyldiazo compound is the key intermediate in this selective transformation. Both oximidovinyldiazoacetates and those with other vinyl substituents undergo cycloaddition reactions with quinone ketals whose products, after extended reaction times, undergo substrate-dependent 1,2-migration; catalysis by Rh2(OAc)4, HNTf2, and Sc(OTf)3 effects these 1,2-migrations to the same products. However, the products from HNTf2-catalyzed reactions between quinoneimine and oximidovinyldiazoacetates undergo Rh2(OAc)4-catalyzed 1,3-C−H insertion. 1,3-Difunctionalization products are obtained for electrophilic reactions of Eschenmoser’s salt with selected vinyldiazoacetates, but with α-dibenzylaminomethyl ether, 1,6-hydride transfer reactions are observed with oximidovinyldiazoacetates. 

Highly selective formal [4 + 2]-cycloaddition of vinyldiazoacetates with azoalkenes from a-halohydrazones, as well as with cyclopentadiene and furan, occurs with light irradiation at room temperature, producing highly functionalized heterocyclic and bicyclic compounds in good yields and excellent diastereoseletivity. Under blue light these vinyldiazoacetate reagents selectively form unstable cyclopropenes that undergo intermolecular cycloaddition reactions at a faster rate than their competitive ene dimerization. [4 + 2]-cycloaddition of vinyldiazoacetates with in situ formed azoalkenes produces bicyclo[4.1.0]tetrahydropyridazine derivatives and, together with their cycloaddition using cyclopentadiene and furan that form tricyclic compounds, they occur with high chemoselectivity and diastereocontrol, good functional group tolerance, and excellent scalability. Subsequent transformations portray the synthetic versatility of these structures. 

The catalytic uses of metal carbenes for addition, insertion, and cycloaddition reactions are dependent on their carbene precursor. The limited methods available for the preparation of diazo esters, which are the most common carbene precursors, restricts their ability to impart structural diversity in metal carbene reactions. Here we report a new methodology for the preparation of diverse vinyldiazoacetate esters and their effective uses in highly enantiocontrolled cyclopropanation, N-H bond insertion, O-H bond insertion, and [3+2] cycloaddition reactions. 1,2,3-Triazine 1-oxides with a sp3-C-H bond at the 5-position undergo base catalyzed Dimroth-type rearrangement to form multiply substituted oximidovinyldiazoacetates in high yields at or below room temperature, and these diverse vinyldiazo compounds undergo catalytic metal carbene transformations to produce oximidovinylcyclopropanes, α-oximidovinyl-α-amino acids and α-hydroxy acids, as well as tricyclic indole derivatives in high yields and enantioselectivities. The new access to vinyldiazo compounds has applicability to a vast array of metal carbene transformations.