Intermolecular silacarbonyl ylide cycloadditions: a direct pathway to oxasilacyclopentenes

Silacarbonyl ylides, generated by metal-catalyzed silylene transfer to carbonyls, participate in formal intermolecular 1,3-dipolar cycloaddition reactions with carbonyl compounds and alkynes to form dioxasilacyclopentane acetals and oxasilacyclopentenes in an efficient, one-step process.     

Formation and utility of oxasilacyclopentenes derived from functionalized alkynes

Oxasilacyclopentenes were shown to be synthetically useful masked allylic alcohols constructed in high yields and regioselectivities from terminal and internal alkynes. Several functional groups were shown to be tolerated utilizing silver-catalyzed silacyclopropenation of alkynes. In situ insertion of various carbonyl compounds into silacyclopropenes afforded regioselective formation of oxasilacyclopentenes. Elaboration of the oxasilacyclopentenes displayed the synthetic utility of these substrates. Both diastereoselective hydrogenation and Diels-Alder reactions utilizing the vinylsilane functionality demonstrated the reactivity and synthetic utility of oxasilacyclopentenes.     

Ruthenium-catalysed double trans-hydrosilylation of 1,4-diarylbuta-1,3-diynes leading to 2,5-diarylsiloles

Dihydrosilanes undergo double trans-hydrosilylation with 1,4-diarylbuta-1,3-diynes in the presence of a cationic ruthenium catalyst to afford 2,5-diarylsiloles: in particular, 9-silafluorene is a good hydrosilylating agent to produce spiro-type siloles in good yield.     

Dialkylzinc-assisted alkynylation of nitrones

[reaction: see text]. Reaction of nitrones with terminal alkynes takes place readily in the presence of a substoichiometric amount of diethylzinc in toluene, affording N-propargyl-hydroxylamines in excellent yields and purity.     

Bisoxazolidine-catalyzed enantioselective alkynylation of aldehydes

A C2-symmetric bisoxazolidine derived from aminoindanol has been successfully applied in the asymmetric alkynylation of aldehydes. The ligand is readily available, has a wide substrate scope, and catalyzes the formation of chiral propargylic alcohols with excellent yields and enantioselectivties.     

Catalytic alkynylation of 6-bromosteroids

An efficient method for synthesis of 6-alkynyl-substituted androstane derivatives was developed via the Pd-catalyzed Sonogashira—Hagihara coupling reaction. The use of AgCl as the cocatalyst (instead of traditionally used CuI) was shown to increase the activity of the catalytic system in several cases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 486–490, March, 2007.     

Transition-metal-free alkynylation of aryl chlorides

Two sets of conditions have been developed for a base-mediated, transition-metal-free alkynylation of aryl chlorides that proceeds via benzyne intermediates. The first set of conditions involves the use of TMPLi base in a pentane/THF mixture at 25 °C. The second set involves use of a metal alkoxide base in dioxane at elevated temperature. Reasonable functional group tolerance has been observed. Fluoro, trifluoromethyl, silyl, cyano, and alcohol functionalities are compatible with the reaction conditions.     

Unusual condensation ofN-phenylsemiquinonediimine ligands coordinated to the NiII atom with an acetone molecule to form a heterocycle

The reaction of a mononuclear Ni^II semiquinonediimine complex,viz., Ni[1,2-(NPh)(NH)C₆H₄]₂, with silver triflate was investigated. The starting monomer was not oxidized in benzene or CH₂Cl₂ solutions; instead, a solvate of the complex with C₆H₆ was formed. According to the X-ray diffraction data, the solvate is characterized by an unusual molecular packing with short C−H(C₆H₆)... Ni and C−H(C₆H₆)... N−H contacts. An analogous effect is observed in the crystals of the solvate Pt[1,2-(NPh)(NH)C₆H₄]₂·C₆H₆. Grinding of the nickel complex with silver triflate (the reagent ratio was 1:5) in the presence of acetone led to the oxidation of the complex, yielding tetrahydroimidazophenazinium triflate, whose structure was established by X-ray diffraction analysis. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1794–1798, October, 2000.     

Copper-catalyzed enantioselective additions to oxocarbenium ions: alkynylation of isochroman acetals

We have developed an enantioselective, copper(I)-catalyzed addition of terminal alkynes to racemic isochroman acetals. This method is one of the first transition-metal-catalyzed approaches to enantioselective additions to prochiral oxocarbenium ions. In this reaction, TMSOTf is used to form the oxocarbenium ion in situ under conditions compatible with simultaneous formation of the chiral copper acetylide. By using a bis(oxazoline) ligand, good yields and enantioselectivities are observed for a variety of enantioenriched 1-alkynyl isochromans.     

Highly satisfactory alkynylation of alkenyl halides via Pd-catalyzed cross-coupling with alkynylzincs and its critical comparison with the sonogashira alkynylation

[reaction: see text] The Pd-catalyzed alkynylation of various alkenyl halides and triflates with alkynylzincs proceeds well even with alkynyl derivatives containing electron-withdrawing groups. The reaction appears to be highly general. Noteworthy is that the corresponding Sonogashira reactions under various reported conditions are significantly less satisfactory in all cases performed in this study.     

Dinuclear Zn-catalyzed asymmetric alkynylation of unsaturated aldehydes

Our recent development of a proline-derived bimetallic catalyst has led to a number of efficient, catalytic, enantioselective transformations. Herein, we report a practical and general alkynylation of aromatic and alpha,beta-unsaturated aldehydes using our zinc catalyst system.     

Indium-mediated alkynylation of Baylis-Hillman acetates: a novel route to 1,4-enynes

Baylis-Hillman acetates undergo smooth alkynylation with aryl-susbstituted iodoalkynes in the presence of indium metal in refluxing dichloromethane to furnish 1,4-enynes in high yields with (E)-stereoselectivity. In the absence of Lewis acid, the reaction follows both S_N2 and S_N2′ pathways affording 1:1 mixtures of 1,4-enynes. Upon addition of 10 mol % of InBr₃, the reaction proceeds preferably in the S_N2′ manner. In the case of adducts derived from acrylonitrile, the corresponding products are obtained in fairly good yields and with (Z)-stereoselectivity.     

Tandem radical-electrophilic annulations to pyrrole

Annulations to pyrrole arising from atom-transfer radical substitution, followed by electrophilic cyclization have been developed. These annulations provide for novel entries into the azabicyclo-[3.3.0] and azabicyclo-[3.4.0] ring systems.     

Unusual enzymatic hydrolysis of NAD by solubilized form of NAD(+) glycohydrolase

Using solubilized form (sNADase) of membrane-bound porcine brain NAD(+) glycohydrolase (pNADase), the NADase-catalyzed hydrolysis and transglycosidation reactions of NAD (1) were examined. Unexpectedly, products in the reactions were found to be nicotinamide (5'-O-diphosphono)-beta-D-ribofuranoside (4) and adenosine (5). Adenosine 5'-diphosphate (ADP)-ribose (2) and nicotinamide (3) as well as a transglycosylated product, which are formed in a usual NAD/pNADase reaction system, were scarcely produced in the NAD/sNADase system. Setting aside the mechanical aspects of this unusual cleaving, it is quite interesting that the sNADase-catalyzed hydrolytic reaction of NAD resulted in the selective cleavage of the P-O bond of the adenosine side without the appreciable hydrolysis of the labile quaternary nicotinamide-ribose pyridinium linkage.     

Stereoconservative Negishi arylation and alkynylation as an efficient approach to enantiopure 2,2'-diarylated 1,1'-binaphthyls

Negishi arylation and alkynylation of easily synthesized chiral 2,2'-diodo-1,1'-binaphthyl rapidly proceeds in refluxing THF utilizing controlled microwave irradiation, affording enantiopure 2,2'-diarylated 1,1'-binaphthyls in good to excellent yields.     

Carbohydrate-derived amino-alcohol ligands for asymmetric alkynylation of aldehydes

[reaction: see text] Conformationally restricted amino alcohols based on carbohydrate scaffolds provide flexible and fine-tuneable libraries that greatly expand the range of ligands available in the Zn(OTf)(2)-mediated addition of alkynes to aldehydes, in some cases with very high stereoselectivities.