Vinyl Carbenoid Research Articles

Asymmetric Catalytic Synthesis of Allylic Sulfenamides from Vinyl α-Diazo Compounds by a Rearrangement Route.

The asymmetric rearrangement of allylic sulfilimines is an effective route to synthetic attractive targets such as allylic sulfenamides and others. The current methods are limited to chirality transfer from chiral allylic sulfilimine precursors. Herein, we report a general and fundamentally new rearrangement route accessing optically enriched allylic sulfenamides and their derivatives. The process involves a S-alkylation and an unusual S-to-N rearrangement step. The chiral nickel complex enables the transformation of a broad scope of sulfenamides and vinyl α-diazo pyrazoleamides under mild conditions. Various allylic sulfenamides have been synthesized with excellent γ-regioselectivity and enantioselectivity, which can be efficiently converted to sulfinamide and 4-aminobutenoic acid derivatives. In addition, DFT calculations demonstrate the connection between the spin state and conformation of nickel vinyl carbenoid, as well as an unknown rearrangement process.

Asymmetric Catalytic Synthesis of Allylic Sulfenamides from Vinyl α‐Diazo Compounds by a Rearrangement Route

The asymmetric rearrangement of allylic sulfilimines is an effective route to synthetic attractive targets such as allylic sulfenamides and others. The current methods are limited to chirality transfer from chiral allylic sulfilimine precursors. Herein, we report a general and fundamentally new rearrangement route accessing optically enriched allylic sulfenamides and their derivatives. The process involves a S‐alkylation and an unusual S‐to‐N rearrangement step. The chiral nickel complex enables the transformation of a broad scope of sulfenamides and vinyl α‐diazo pyrazoleamides under mild conditions. Various allylic sulfenamides have been synthesized with excellent γ‐regioselectivity and enantioselectivity, which can be efficiently converted to sulfinamide and 4‐aminobutenoic acid derivatives. In addition, DFT calculations demonstrate the connection between the spin state and conformation of nickel vinyl carbenoid, as well as an unknown rearrangement process.

Stereoselective Synthesis of Vinylcyclopropa[b]indolines via a Rh-Migration Strategy.

A mild rhodium catalytic system has been developed to synthesize vinylcyclopropa[b]indolines through cyclopropanation of indoles with vinyl carbenoids generated from ring opening of cyclopropenes in situ. By employing a Rh-migration strategy, the products can be obtained with good to excellent E:Z ratios (≤99:1) and complete diastereoselectivity (≤99:1). This method is easy, has a low catalyst loading, and works for a broad range of functionalities.

Synthesis of a C(sp2)‐bridged Phosphine‐Borane by Ionic Coupling

The vinyl carbenoid H2C=CBr(Li) has been used as key precursor to prepare a geminal C(sp2)‐bridged phosphine‐borane. Starting from bromoethene, two sequences of lithiation/electrophilic trapping, with ClPiPr2 and FBMes2 respectively, affords iPr2P–C(=CH2)–BMes2 3 [Mes = 2,4,6‐(H3C)3C6H2]. This new phosphine‐borane 3 was characterized by multi‐nuclear NMR and mass spectroscopy. It adopts a monomeric open structure without P→B interaction. A few crystals of a secondary product 4 were analyzed by X‐ray diffraction, revealing an unusual dimeric structure.

Bromine-lithium exchange on gem-dibromoalkenes part 1: batch vs microflow conditions

The generation and reactivity of two model vinyl carbenoids from gem-dibromoalkenes 1 were studied in microflow systems. From substrate 1a (R = Ph, CF3), the lithium-bromine exchange could be simply performed within 30 ms at 20 °C with very good E-selectivity whereas the reaction was unselective under batch conditions, even at −78 °C. Moreover, the unstable carbenoid generated from 1b (R = Ph, H) could be trapped as the major product while only the Fritsch-Buttemberg-Wiechell rearrangement product was obtained in a flask under cryogenic conditions.

Synthesis of Functionalized α-Vinyl Aldehydes from Enaminones.

An efficient RhII -catalyzed synthesis of functionalized α-vinyl aldehydes with high E/Z stereoselectivity was developed. The reaction mediates the cyclopropanation of enaminones with vinyl carbenoids that are generated from cyclopropenes in situ to give the aminocyclopropane intermediates. Selective C-C bond cleavage of the cyclopropane intermediates leads to formation of α-vinyl aldehyde derivatives with high E/Z selectivity. This method proceeds at room temperature under very mild reaction conditions and works with a broad substrate scope.

Synthesis of Functionalized α‐Vinyl Aldehydes from Enaminones

AbstractAn efficient RhII‐catalyzed synthesis of functionalized α‐vinyl aldehydes with high E/Z stereoselectivity was developed. The reaction mediates the cyclopropanation of enaminones with vinyl carbenoids that are generated from cyclopropenes in situ to give the aminocyclopropane intermediates. Selective C−C bond cleavage of the cyclopropane intermediates leads to formation of α‐vinyl aldehyde derivatives with high E/Z selectivity. This method proceeds at room temperature under very mild reaction conditions and works with a broad substrate scope.

Vinyl Sulfoxonium Ylide: A New Vinyl Carbenoid Transfer Reagent for the Synthesis of Heterocycles

Sulfoxonium ylides have recently gained prominence as safe carbenoid precursors in metal-catalyzed reactions. The stability and reactivity of sulfoxonium ylides depend on the substitution of the ylide carbon. The reactivity of vinyl-substituted sulfoxonium ylides is different and offers several advantages over known stabilized sulfoxonium ylides in the case of carbenoid transfer reactions. Herein, we provide an overview of early efforts in this area, with particular emphasis on our own recent development of sulfoxonium ylide-derived vinyl carbenoid transformations for N-Heterocycles.1 Introduction2 Classification of Sulfoxonium Ylides3 Synthesis of Vinyl Sulfoxonium Ylides4 [3+2] Annulation of Vinyl Sulfoxonium Ylides5 [4+1] Annulation of Vinyl Sulfoxonium Ylides6 Conclusion

Iron-Catalyzed Carbenoid-Transfer Reactions of Vinyl Sulfoxonium Ylides: An Experimental and Computational Study.

A method for the generation of unprecedented vinyl carbenoids from sulfoxonium ylides has been developed and applied in the synthesis of a diverse array of heterocycles such as indolizines, pyrroles, 3-pyrrolin-2-ones, and furans. The reactions proceed by FeBr2 catalysis under mild reaction conditions with a broad substrate scope. A reaction pathway involving iron carbenoids is proposed based on a series of control experiments and DFT calculations.

Zinc-Catalyzed Synthesis of Allylsilanes by Si-H Bond Insertion of Vinyl Carbenoids Generated from Cyclopropenes.

Allylsilanes have long been recognized as valuable building blocks for organic synthesis. A zinc-catalyzed reaction of cyclopropenes and hydrosilanes provides a convenient route to these versatile unsaturated organosilanes. In this transformation, ZnBr2 serves as an efficient catalyst, allowing the generation of a zinc vinyl carbenoid intermediate, which is subsequently involved in a Si-H bond insertion. The process shows broad scope, and is amenable to substituted and functionalized cyclopropenes or the functionalization of polysiloxanes. Moreover, zinc-catalyzed carbene insertion into a Ge-H bond is reported for the first time.

Zinc‐Catalyzed Synthesis of Allylsilanes by Si−H Bond Insertion of Vinyl Carbenoids Generated from Cyclopropenes

AbstractAllylsilanes have long been recognized as valuable building blocks for organic synthesis. A zinc‐catalyzed reaction of cyclopropenes and hydrosilanes provides a convenient route to these versatile unsaturated organosilanes. In this transformation, ZnBr2 serves as an efficient catalyst, allowing the generation of a zinc vinyl carbenoid intermediate, which is subsequently involved in a Si−H bond insertion. The process shows broad scope, and is amenable to substituted and functionalized cyclopropenes or the functionalization of polysiloxanes. Moreover, zinc‐catalyzed carbene insertion into a Ge−H bond is reported for the first time.

Asymmetric Rhodium-Catalyzed C–C Activation of Cyclobutanones

Significance: C–C activation is an attractive method to functionalize strained four-membered ring systems. Zhou and Dong demonstrate the utility of allenes as a formal vinyl carbenoid in a rhodium-catalyzed asymmetric intramolecular ring expansion of cyclobutanones. Comment: An impressive substrate scope for this C–C activation protocol is demonstrated. Challenging cyclobutanone substrates such as those that are α-substituted also worked for this methodology, albeit with slightly diminished yields. General reaction: O R1

Zinc-catalyzed alkene cyclopropanation through zinc vinyl carbenoids generated from cyclopropenes.

The zinc-catalyzed reaction of cyclopropenes with alkenes leading to vinylcyclopropane derivatives is reported. A broad range of alkenes (including highly substituted or functionalized alkenes) is compatible with this protocol. On the basis of trapping experiments and computational studies, this cyclopropanation reaction is proposed to proceed through initial formation of an electrophilic zinc vinyl carbenoid intermediate, which may be involved in a concerted cyclopropanation reaction. The reported protocol represents an unprecedented and simple strategy for the catalytic generation of zinc vinyl carbenoids, which are promising intermediates in organic synthesis.

Zinc‐Catalyzed Alkene Cyclopropanation through Zinc Vinyl Carbenoids Generated from Cyclopropenes

AbstractThe zinc‐catalyzed reaction of cyclopropenes with alkenes leading to vinylcyclopropane derivatives is reported. A broad range of alkenes (including highly substituted or functionalized alkenes) is compatible with this protocol. On the basis of trapping experiments and computational studies, this cyclopropanation reaction is proposed to proceed through initial formation of an electrophilic zinc vinyl carbenoid intermediate, which may be involved in a concerted cyclopropanation reaction. The reported protocol represents an unprecedented and simple strategy for the catalytic generation of zinc vinyl carbenoids, which are promising intermediates in organic synthesis.

Gold-Catalyzed C–H Bond Functionalization of Metallocenes: Synthesis of Densely Functionalized Ferrocene Derivatives

A convenient process for the direct functionalization of ferrocene that exploits the high electrophilic character of gold–vinyl carbenoids catalytically generated from vinyldiazo compounds and gold complexes is reported. This process takes place with complete regioselectivity (vinilogous reactivity). The synthesis of the corresponding ruthenocene analogues has also been achieved. Preliminary studies on the reactivity of these new organometallic compounds seem to suggest that the presence of the adjacent ferrocenyl group confers a specific reactivity on the functionalized carbon chain manifested in the easy dry state aerobic allylic oxidation.

Platinum(II)-Catalyzed Cyclization Sequence of Aryl Alkynes via C(sp3)–H Activation: A DFT Study

The mechanism and intermediates of hydroalkylation of aryl alkynes via C(sp(3))-H activation through a platinum(II)-centered catalyst are investigated with density functional theory at the B3LYP/[6-31G(d) for H, O, C; 6-31+G(d,p) for F, Cl; SDD for Pt] level of theory. Solvent effects on reactions were explored using calculations that included a polarizable continuum model for the solvent (THF). Free energy diagrams for three suggested mechanisms were computed: (a) one that leads to formation of a Pt(II) vinyl carbenoid (Mechanism A), (b) another where the transition state implies a directed 1,4-hydrogen shift (Mechanism B), and (c) one with a Pt-aided 1,4-hydrogen migration (Mechanism C). Results suggest that the insertion reaction pathway of Mechanism A is reasonable. Through 4,5-hydrogen transfer, the Pt(II) vinyl carbenoid is formed. Thus, the stepwise insertion mechanism is favored while the electrocyclization mechanism is implausible. Electron-withdrawing/electron-donating groups substituted at the phenyl and benzyl sp(3) C atoms slightly change the thermodynamic properties of the first half of Mechanism A, but electronic effects cause a substantial shift in relative energies for the second half of Mechanism A. The rate-limiting step can be varied between the 4,5-hydrogen shift process and the 1,5-hydrogen shift step by altering electron-withdrawing/electron-donating groups on the benzyl C atom. Additionally, NBO and AIM analyses are applied to further investigate electronic structure changes during the mechanism.

A DFT study on the mechanism of gold(iii)-catalyzed synthesis of highly substituted furans via [3, 3]-sigmatropic rearrangements and/or [1, 2]-acyloxy migration based on propargyl ketones

The mechanisms of gold(III)-catalyzed synthesis of highly substituted furans via [3,3]-sigmatropic rearrangements and/or [1,2]-acyloxy migration based on propargyl ketones have been investigated using density functional theory calculations at BHandHLYP/6-31G(d,p) (SDD for Au) level of theory. Solvent effects on these reactions were explored using calculations that included a polarizable continuum model (PCM) for the solvent (toluene). Two plausible pathways that lead to the formation of Au(III) vinyl carbenoid and an allenyl structure through [3,3]-sigmatropic rearrangements, [1,2]-acyloxy migration via oxirenium and dioxolenylium were performed. Our calculated results suggested: (1) the major pathway of the cycle causes an initial Rautenstrauch-type [1,2]-migration via oxirenium to form an Au(III) vinyl carbenoid. Subsequent cycloisomerization of this intermediate then provides the corresponding furan whether for the methyl-substituted propargylic acetates or the phenyl-substituted propargylic acetates; (2) for the methyl-substituted propargylic acetates, the formation of Au(III) vinyl carbenoid structures was the rate-determining step. However, intramolecular nucleophilic attack and subsequent cycloisomerization to give the final product was rate-determining for the phenyl-substituted propargylic acetates. The computational results are consistent with the experimental observations of Gevorgyan, et al. for gold(III)-catalyzed synthesis of highly substituted furans based on propargyl ketones.

ChemInform Abstract: Rhodium(II)-Catalyzed Cyclization Reactions of Alkynyl-Substituted . alpha.-Diazo Ketones.

Treatment of several o-alkynyl-substituted α-diazoacetophenone derivatives with rhodium(II) carboxylates results in the formation of substituted indenones. The simplest mechanism accounting for the results involves addition of a rhodium-stabilized carbenoid onto the acetylenic π-bond to generate a vinyl carbenoid directly or possibly a highly strained cyclopropene derivative. The vinyl carbenoid was found to undergo addition to a neighboring alkene tethered on the side chain to give an indenyl-substituted bicyclo[3.1.0]hexane derivative. A number of related systems were examined so as to probe the scope and generality of the process

ChemInform Abstract: Insertion of 1-Chloro-1-lithioalkenes into Organozirconocenes. A Versatile Synthesis of Stereodefined Unsaturated Systems.

Hydrozirconation of alkenes and alkynes, followed by insertion of 1-halo-1-lithio-1-alkenes, generated in situ by lithium tetramethylpiperidide deprotonation of vinyl halides, affords vinylzirconocene species which may be further elaborated. The method provides easy access to many structures including terminal (3E)- and (3Z)-1,3-dienes and (3E,5E)- and (3Z,5E)-1,3,5-trienes, and internal (E,Z)-dienes, (E,Z,E)-trienes, and (1E,3Z)-1,3-dien-5-ynes. Insertion of 2-monosubstituted 1-halo-1-lithio-1-alkenes occurs with clean inversion of configuration of the sp2-carbenoid carbon. Carbenoids derived by deprotonation of 2,2-disubstituted 1-halo-1-alkenes gave poor stereocontrol probably due to isomerization before insertion into the organozirconium species. Insertion of vinyl carbenoids into alkynylzirconocenes affords terminal (3E)- or (3Z)-1,3-dien-5-ynes, internal (1E,3Z)-1,3-dien-5-ynes, and (Z)-3-en-1,5-diynes.

Au-catalyzed isomerization of cyclopropenes: a novel approach to indene derivatives

AuPPh 3Cl/AgOTf-catalyzed reaction of cyclopropenes is found to be highly efficient, giving indene derivatives in high yields. The reaction is suggested to proceed through gold vinyl carbenoid intermediate.