Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Synthesis of Tetrasubstituted Allenes via Visible-Light-Promoted Radical 1,3-Difunctionalization of Alkynyl Diazo Compounds

Herein, we described an unprecedented process for generating allenyl radicals through radical addition to alkynyl diazo compounds followed by a 1,2-radical shift with the loss of nitrogen. Using this protocol, radical 1,3-difunctionalization of alkynyl diazo compounds for the synthesis of tetrasubstituted allenes with RSO₂X (X = SeR', SR', and I) as the radical sources was developed. The reactions were promoted by visible light without photocatalyst and any additives.

Silver-catalyzed cyclopropanation of alkenes with alkynyl N-nosylhydrazones leading to alkynyl cyclopropanes

Here, a novel method for the stereoselective synthesis of alkynyl cyclopropanes, by the silver-catalyzed alkynylcyclopropanation of alkenes using alkynyl N-nosylhydrazones as alkynyl carbene precursors, is reported. This method provides a straightforward and powerful approach for preparing various alkynyl cyclopropanes in high yield with excellent stereoselectivities. In addition, the practicality of this method was demonstrated by gram-scale synthesis and late-stage modification of bioactive molecules.

Ag-Catalyzed Insertion of Alkynyl Carbenes into C-C Bonds of β-Ketocarbonyls: A Formal C(sp2) Insertion

Here we report a silver-catalyzed alkynyl carbene insertion into β-ketocarbonyls using alkynyl N-nosylhydrazones as alkynyl carbene precursors, which provides access to trisubstituted allenyl ketones. This reaction represents the first example of an alkynyl carbene insertion into a C-C σ bond, affording products homologated with an sp² carbon center. The products are useful substrates for further transformations. Experimental investigations and theoretical calculations suggest the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.

Transition metal-free reductive coupling of allylic sulfonylhydrazones with aryl boronic acids for C(sp3)–C(sp2) bond formation

The reductive coupling between allylic sulfonylhydrazones and aryl boronic acids gives 1,3-diarylpropene systems with good to excellent yields under very simple reaction conditions without metal catalysts and an inert atmosphere.

Zinc Iodide Catalyzed Synthesis of Trisubstituted Allenes from Terminal Alkynes and Ketones

A straightforward, user-friendly, efficient protocol for the one pot, ZnI₂-catalyzed allenylation of terminal alkynes with pyrrolidine and ketones, toward trisubstituted allenes, is described. Trisubstituted allenes can be obtained under either conventional heating or microwave irradiation conditions, which significantly reduces the reaction time. A sustainable, widely available, and low-cost metal salt catalyst is employed, and the reactions are carried out under solvent-free conditions. Among others, synthetically valuable allenes bearing functionalities such as amide, hydroxyl, or phthalimide can be efficiently prepared. Mechanistic experiments, including kinetic isotope effect measurements and density functional theory (DFT) calculations, suggest a rate-determining [1,5]-hydride transfer during the transformation of the intermediate propargylamine to the final allene.

Synthesis of Trifluoromethylated Tetrasubstituted Allenes via Palladium-Catalyzed Carbene Transfer Reaction

Herein, we report on the palladium-catalyzed synthesis of trifluoromethylated, tetrasubstituted allenes from vinyl bromides and trifluoromethylated diazoalkanes in good to excellent yield. This reaction proceeds via oxidative addition of a Pd(0) complex with vinyl bromide. Subsequent base-promoted reductive elimination generates the allene. This methodology provides an efficient strategy even on gram scale to valuable trifluoromethylated, tetrasubstituted allenes under mild reaction conditions. The allene products can be used in acid catalyzed cyclization reactions to give trifluoromethylated indene products.

Difluoroacetaldehyde N-Triftosylhydrazone (DFHZ-Tfs) as a Bench-Stable Crystalline Diazo Surrogate for Diazoacetaldehyde and Difluorodiazoethane

Despite the growing importance of volatile functionalized diazoalkanes in organic synthesis, their safe generation and utilization remain a formidable challenge because of their difficult handling along with storage and security issues. In this study, we developed a bench-stable difluoroacetaldehyde N-triftosylhydrazone (DFHZ-Tfs) as an operationally safe diazo surrogate that can release in situ two low-molecular-weight diazoalkanes, diazoacetaldehyde (CHOCHN₂ ) or difluorodiazoethane (CF₂ HCHN₂ ), in a controlled fashion under specific conditions. DFHZ-Tfs has been successfully employed in the Fe-catalyzed cyclopropanation and Doyle-Kirmse reactions, thus highlighting the synthetic utility of DFHZ-Tfs in the efficient construction of molecule frameworks containing CHO or CF₂ H groups. Moreover, the reaction mechanism for the generation of CHOCHN₂ from CF₂ HCHN₂ was elucidated by density functional theory (DFT) calculations.

Syntheses of 12H-benzo[a]xanthen-12-ones and benzo[a]acridin-12(7H)-ones through Au(i)-catalyzed Michael addition/6-endo-trig cyclization/aromatization cascade annulation

A multifaceted gold(i)-catalyzed aromaticity-driven double 6-endo cascade cyclization strategy to synthesize both 12H-benzo[a]xanthen-12-ones and benzo[a]acridin-12(7H)-ones, whose core motifs xanthone and acridone both exist as important scaffolds in an immense number of bioactive compounds, was developed. The scopes of this strategy were examined by using a batch of synthetic 1,3-diphenylprop-2-yn-1-one substrates. To probe the mechanism of this cyclization a control experiment for synthesizing intermediates was performed. Thus, a putative mechanism was determined according to this experiment and previous studies.

Influence of sulfonyl substituents on the decomposition of N-sulfonylhydrazones at room temperature

A systematic investigation on the influence of sulfonyl substituents on the decomposition of N-sulfonylhydrazones at room temperature is described.

Thioether-functionalized trifluoromethyl-alkynes, 1,3-dienes and allenes: divergent synthesis from reaction of 2-trifluoromethyl-1,3-conjugated enynes with sulfur nucleophiles

A divergent synthesis of thioether-functionalized trifluoromethyl-alkynes, 1,3-dienes and allenes via regioselective nucleophilic addition of sulfur nucleophiles to 2-trifluoromethyl-1,3-conjugated enynes was developed. The addition patterns depend on the type of enyne, sulfur nucleophile and reaction conditions used. 1,4-Addition leading to thioether-functionalized trifluoromethyl-allenes was realized when enynes possessing electron-withdrawing aryl groups on the alkyne moiety were used as reaction partners and alkanethiols were used as nucleophiles, whereas solvent-controlled construction of thioether-functionalized 1,3-dienes and alkynes was realized, respectively, via a 3,4-addition pattern or 1,2-addition pattern if thiophenols were applied as nucleophiles. The three types of compounds containing both sulfur and fluorine elements are valuable building blocks for synthesis of multifunctional fluorinated vinyl sulfides and thiophene derivatives.

A divergent synthesis of thioether-functionalized trifluoromethyl-alkynes, 1,3-dienes and allenes viaregioselective nucleophilic addition of sulfur nucleophiles to 2-trifluoromethyl-1,3-conjugated enynes was developed.

Silver-Catalyzed Cyclopropanation of Alkenes Using N-Nosylhydrazones as Diazo Surrogates

An efficient silver-catalyzed [2 + 1] cyclopropanation of sterically hindered internal alkenes with diazo compounds in which room-temperature-decomposable N-nosylhydrazones are used as diazo surrogates is reported. The unexpected unique catalytic activity of silver was ascribed to its dual role as a Lewis acid activating alkene substrates and as a transition metal forming silver carbenoids. A wide range of internal alkenes, including challenging diarylethenes, were suitable for this protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions.