Efficient and stereoselective synthesis of beta-trifluoromethyl alpha,beta-unsaturated esters via iron(III) porphyrin-catalyzed olefination of ketones

beta-Trifluoromethyl alpha,beta-unsaturated esters were efficiently prepared by reactions of fluorine-containing ketones with diazo compounds via metalloporphyrin-catalyzed olefination in the presence of triphenylphosphine. The commercially available Fe(III)(TPP)Cl (TPP: tetraphenylporphyrin) is effective for catalyzing the olefination of a variety of trifluoromethyl ketones with different diazoacetate esters under mild conditions. The reactions proceeded with high yields (up to 95% isolated yield) and high stereoselectivity (up to 99% (E)-selectivity).     

Oxorhenium complexes as aldehyde-olefination catalysts

Several oxorhenium compounds in the formal oxidation states V and VII are examined as catalysts for the aldehyde-olefination starting from diazo compounds, phosphines, and aldehydes. Of these, [ReMeO2(eta2-alkyne)] complexes provide the simplest catalysts to study, although [ReOCl3(PPh3)2] still remains the most efficient rhenium catalyst for aldehyde-olefination described to date. Prior to the reaction with the Re catalysts the phosphine and the diazo compound react to form a phosphazine. No catalytic reaction occurs in cases where no phosphazine formation is observed. The first step of the catalytic cycle involves the formation of a carbene intermediate by the reaction of phosphazine and catalyst under extrusion of phosphine oxide and dinitrogen. In a second step the carbene reacts with aldehyde under olefin formation and catalyst regeneration. Excess of alkyne as well as the presence of ketones slows down the catalytic reaction. The olefination of 4-nitrobenzaldehyde with diazomalonate is possible with these Re catalysts. In contrast, this reaction does not take place either in the classical Wittig fashion from Ph3P=C(CO2Et)2 and aldehyde or by use of all other catalysts for aldehyde olefination reactions reported to date. Catalytic ylide formation from diazo compounds seems therefore not to be the only pathway through which catalytic aldehyde-olefination reactions can proceed.     

Synthesis of spiropyrrolidine oxindoles through Rh(II)-catalyzed olefination/cyclization of diazooxindoles and vinyl azides

A simple and efficient process involving the Rh(II)-catalyzed [1+1+3] annulation of diazooxindoles and vinyl azides has been developed for the synthesis of spiropyrrolidine oxindoles with potential biological activity and significant synthetic applications. This process involves a novel rhodium-catalyzed olefination of diazo compounds, followed by annulation with vinyl azides. This method is compatible with a broad range of substrates and affords moderate to good yields under mild reaction conditions.     

Acid-promoted olefination of ketones by an iron(III) porphyrin complex

[reaction: see text] The acid-promoted olefination of unactivated ketones with diazo reagents in the presence of triphenylphosphine can be catalyzed by the commercially available iron(III) porphyrin complex Fe(TPP)Cl. The reactions were effectively carried out under mild conditions in a one-pot fashion with the use of a stoichiometric diazo reagents and substoichiometric benzoic acid. Examples include aromatic, aliphatic, cyclic, and unsaturated ketones with ethyl diazoacetate or tert-butyl diazoacetate.     

Synthesis of vinylogous carbamates by rhodium(II)-catalyzed olefination of tertiary formamides with a silylated diazoester

Treatment of tertiary formamides with a silylated diazoester in the presence of a rhodium(II) catalyst leads to the formation of 3-amino-2-silyloxyacrylates in good yield. No olefination is observed if a nonsilylated diazo compound is employed. [reaction: see text]     

Generation of phosphoranes derived from phosphites. A new class of phosphorus ylides leading to high E selectivity with semi-stabilizing groups in Wittig olefinations

Tosylhydrazones derived from aryl aldehydes react with t-BuOK, ClFeTPP, (MeO)3P, and aldehydes to furnish olefins with high E selectivity. These reactions occur through a Wittig-type pathway via the corresponding diazo compounds, metal carbenes and phosphorous ylides, with the water-soluble trimethyl phosphate as the byproduct. Similar reactions can also be performed using ethyl diazoacetate; however, high E selectivity was only observed in the presence of LiBr. In this case, the reaction is believed to occur via the phosphonate anion, formed through an Arbuzov reaction. Thus this olefination reaction occurs through a Horner-Wadsworth-Emmons (HWE) reaction but the phosphonate anion is generated under completely base-free conditions.     

New Syntheses of Diazo Compounds

Diazo compounds (R¹R²C?N₂) are known as versatile and useful substrates for an array of chemical transformations and, therefore, diazo chemistry is still far from losing anything of its long‐standing fascination. In addition to many studies on the subsequent chemistry of the diazo group, the inventory of methods for the preparation of diazo compounds is continuously supplemented by new methods and novel variations of established procedures. Several of these synthetic approaches take into account the lability and remarkable chemical reactivity of certain classes of diazo compounds, and environmentally more benign procedures also continue to be developed.     

A Photoenzymatic Strategy for Radical-Mediated Stereoselective Hydroalkylation with Diazo Compounds

Carbene insertion reactions initiated with diazo compounds have been widely used to develop unnatural enzymatic reactions. However, alternative functionalization of diazo compounds in enzymatic processes has been unexploited. Herein, we describe a photoenzymatic strategy for radical-mediated stereoselective hydroalkylation with diazo compounds. This method generates carbon-centered radicals through an ene reductase catalyzed photoinduced electron transfer process from diazo compounds, enabling the synthesis of γ-stereogenic carbonyl compounds in good yields and stereoselectivities. This study further expands the possible reaction patterns in photo-biocatalysis and offers a new approach to solving the selectivity challenges of radical-mediated reactions.     

Allenes through Horner-Wadsworth-Emmons Olefination of Alkenylphosphonates

Mono- and di-substituted allenes 5 were synthesized by successive Horner-Wadsworth-Emmons olefination starting from methylene-bisphosphonate 1 and two carbonyl compounds. The key to success is KH or KH-18-crown-6 as a base for the second HWE olefination of hydroxyalkenylphosphonates 4 .     

Synthesis of Fluoroolefins via Julia-Kocienski Olefination

The Julia-Kocienski olefination provides a versatile platform for the synthesis of fluorovinyl compounds. This review describes our efforts as well as those of others in the synthesis of various fluorinated aryl and heteroaryl sulfones and their utility as olefination reagents for the modular assembly of fluoroalkenes. Where data is available, the influence of the fluorine atom on the reactivity of the olefination reagents and the stereochemical outcome of the olefination are described.     

Alkyl Halide‐Free Heteroatom Alkylation and Epoxidation Facilitated by a Recyclable Polymer‐Supported Oxidant for the In‐Flow Preparation of Diazo Compounds

Highly reactive metal carbenes, generated from simple ketones via diazo compounds, including diazo‐amides and ‐phosphonates, using a recyclable reagent in‐flow, are transient but versatile electrophiles for heteroatom alkylation reactions and for epoxide formation. The method produces no organic waste, with the only by‐products being water, KI and nitrogen, without the attendant hazards of isolation of intermediate diazo compounds.     

Safe and Facile Access to Nonstabilized Diazoalkanes Using Continuous Flow Technology

Despite the high synthetic potential of nonstabilized diazo compounds, their utilization has always been hampered by stability, toxicity, and safety issues. The present method opens up access to the most reactive nonstabilized diazoalkanes. Among diazo compounds, nonstabilized alkyl diazo compounds are the least represented because of their propensity to degrade during preparation. The continuous flow oxidation process of hydrazones on a silver oxide column afforded an output stream of base‐ and metal‐free pure diazo solution in dichloromethane. Starting from innocuous ketones and aldehydes, this methodology allows the production of a broad range of unprecedented diazoalkanes compounds in excellent yields, while highlighting their synthetic potential and the possibility of safe large‐scale diazo production.     

Unusual Reaction of Diazo Compounds with Enamines

Transition metal catalyzed decomposition of diazo compounds and consequent transformations constitute various synthetic useful reactions in the past decades. The catalytic addition of diazo compounds to olefins generally provides cyclopropanes in high yield. However the reaction of electron-rich enamines with diazo compounds gave variable results. Wenkert et al. reported the reaction of enamines with ethyl diazoacetae (EDA) in the presence of cuprous chloride or silver oxide unexpectedly gave…     

(Eta2-alkyne)methyl(dioxo)rhenium complexes as aldehyde-olefination catalysts

Complexes CH3ReO2L (L = 2-butyne, 3-hexyne, diphenylacetylene) are catalysts for the olefination of aldehydes, using 4-nitrobenzaldehyde (4-nba) as the standard aldehyde and ethyldiazoacetate (eda) as the diazo compound. Spectroscopic studies including in situ 31P, 17O, 13C, and 1H NMR spectroscopy are used to elucidate the mechanism and the nature of the active species. One of the key steps of the mechanism is the rapid formation of phosphazine at the beginning of the cycle and its subsequent reaction with the metal dioxide complex to form the catalytically active carbene species.     

Reactions between Diazo Compounds and Hypervalent Iodine(III) Reagents

Site‐selective “cut and sew” transformations employing diazo compounds and hypervalent iodine(III) compounds involve the departure of leaving groups, a “cut” process, followed by a reorganization of the fragments by bond formation, a “sew” process. Bearing controllable cleavage sites, diazo compounds and hypervalent iodine(III) compounds play a critical role as versatile reagents in a wide range of organic transformations because their excellent nucleofugality allows for a large number of unusual reactions to occur. In recent years, the combination of diazo compounds and hypervalent iodine(III) reagents has emerged as a promising tool for developing new and valuable approaches, and has met considerable success. In this Minireview, this combination is systematically illustrated with recent advances in the field, with the aim of elaborating the synthetic utility and potential of this concept as a powerful strategy in organic synthesis.     

A novel one-pot method for the preparation of pyrazoles by 1,3-dipolar cycloadditions of diazo compounds generated in situ

A convenient one-pot procedure for the preparation of pyrazoles by 1,3-dipolar cycloaddition of diazo compounds generated in situ has been developed. Diazo compounds derived from aldehydes were reacted with terminal alkynes to furnish regioselectively 3,5-disubstituted pyrazoles. Furthermore, the reaction of N-vinylimidazole and diazo compounds derived from aldehydes gave exclusively 3-substituted pyrazoles in a one-pot process.     

Photoredox-Enabled Self-(3+2) Cyclization of Vinyldiazo Reagents: Synthesis of Cyclopentenyl α-Diazo Compounds

A photocatalytic self-(3+2) cycloaddition of vinyldiazo compounds is described, which provides cyclopentene derivatives with conservation of one diazo functional group. Experimental insights and density functional theory indicate that the reaction is triggered by an unusual single electron oxidation of vinyldiazo compounds, while the photolysis for the generation of free carbene species is not involved. The synthetic applications of the resulting cyclopentenyl α-diazo compounds were demonstrated based on the rich chemistry of the diazo functional group.     

酸促进2-乙烯基苯胺与重氮的[5+1]环化合成2-芳基喹啉

利用廉价易得的2-乙烯基苯胺与重氮化合物在三氟乙酸催化下发生[5+1]环化反应, 以中等至良好的产率合成了24种2-芳基喹啉衍生物. 该方法无需金属催化剂, 官能团兼容性好, 反应成本低, 不仅拓展了重氮化合物参与的新型有机反应, 同时也为2-芳基取代喹啉的合成提供了高效、 经济且简洁的新方法.     

1,2-Thio group migration in Rh(II) carbene reactions

[reaction: see text] A series of beta-thio group substituted alpha-diazo carbonyl compounds have been prepared by nucleophilic substitution reactions of thiophenol, thionaphthol, or benzyl mercaptan with beta-acetoxy-alpha-diazo carbonyl compounds. The diazo decomposition of these diazo carbonyl compounds with various transition metal catalysts, including Rh(II) carboxylates and Cu(I) and Cu(II) complexes, has been investigated. It was found that the diazo decomposition of these compounds gave 1,2-thio group migration products. No 1,2-hydride or 1,2-aryl migration products were observed in all cases.     

From C1 to C3: Copper‐Catalyzed gem‐Bis(trifluoromethyl)olefination of α‐Diazo Esters with TMSCF3

A Cu‐catalyzed gem‐bis(trifluoromethyl)olefination of α‐diazo esters, using TMSCF₃ as the only fluorocarbon source, has been developed and provides an exquisite method to access gem‐bis(trifluoromethyl)alkenes. This unprecedented olefination process involves a carbene migratory insertion into “CuCF₃” to generate the α‐CF₃‐substituted organocopper species, which then undergoes β‐fluoride elimination and two consecutive addition‐elimination processes to give the desired products. The key to this efficient one‐pot C₁‐to‐C₃ synthetic protocol lies in the controllable double (over single and triple) trifluoromethylations of the gem‐difluoroalkene intermediates.