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This study provides evidence of the enhancement of the oxygen reduction reaction (ORR) activity of Pt/C and Pd-core Pt-shell catalysts upon ionic liquid modification. Under high potential at 1.15 V (vs. reversible hydrogen electrode), the lower coverage of the oxide species on the Pt surface modified with [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)][bis(trifluoromethylsulfonyl)imide (NTf2)] ionic liquid (IL) was confirmed by electrochemical analysis. Operando X-ray absorption spectra (XAS) of the ionic liquid-modified Pd-core Pt-shell catalyst displayed smaller 5d orbital vacancies than the naked catalyst. Shrinkage of Pt-Pt bond length was also displayed, indicating the suppression of oxidation. Thus, we conclude that the IL-modified systems are effective in avoiding the adsorption of oxide species on the Pt surface and improving the oxygen reduction activity.

About 40 years have passed since methylene dizinc reagent was discovered as a substitute for Wittig reagent. Density functional theory (DFT) calculations have been performed to understand the reaction pathways of methylenation of carbonyl compounds with bis(iodozincio)methane. The present computational/theoretical study concluded that the methylenation reaction with gem-dizinc reagent proceeds as a two-step reaction, that is, methylene addition (RDS) and olefination. In the first step, the nucleophilic attack of the CH2 group enhanced by two Zn proceeds under the assistance of the electrophilic activation of the carbonyl group with the Zn atom. In the second step, the olefination is facilitated by both Zn atoms of the gem-dizinc reagent without an electron transfer process.

Transmetalation between [CpFe(CO)2I] and arylzinc iodide-lithium chloride complexes proceeds in the presence of catalytic amounts of palladium acetate and N,N,N’,N’-tetramethyl-1,2-cyclohexanediamine to yield the corresponding aryliron complexes [CpFe(CO)2Ar]. Phenylation of [CpFe(CO)2I] also takes place when triphenylindium is used under similar conditions. Arylboronic acids undergo arylation in the presence of cesium carbonate and a palladium-N-heterocyclic carbene complex, PEPPSI. The present methods are useful for the facile synthesis of various functionalized [CpFe(CO)2Ar]. The products [CpFe(CO)2Ar] represent an interesting class of aryl metals that undergo several transformation.

Radical additions of S-thiophosphinyl dithiocarbonate to terminal aromatic alkynes afford (E)-1-aryl-1-thio-2-thiophosphinylethene derivatives regio- and stereoselectively in high yields. The transformations of the products are also described.

Metal-mediated or -catalyzed retro-allylation of homoallyl alcohol that proceeds via a six-membered transition state has appeared as a new useful method to prepare allylmetals. A crotylzirconium re...

The reactions of aryl halides with 2,3,3-trimethyl-4-penten-2-ol in the presence of a palladium catalyst result in prenyl transfer from the alcohol to aryl halides via retro-allylation, yielding prenylarenes. Other multisubstituted allyl groups such as the 2,3-dimethyl-2-butenyl group are introduced to aromatic rings.

Regioselective and stereoselective radical additions of arenethiols to various ynamides have been developed. Mixing yn­amides and arenethiols in the presence of a catalytic amount of triethylborane affords the corresponding adducts, (Z)-1-amino-2-thio-1-alkenes, in excellent yields with high selectivities. The products can be reduced by means of trifluoroacetic acid and triethylsilane to yield 1-amino-2-thioalkanes.

During these thirty years, we have pursued new methodologies in organic synthesis and developed many synthetically useful reactions. Among them, four topics are described in this article. (1) Regio- and stereoselective silylmetalation of acetylenes has been examined. Whereas platinum- or copper-catalyzed silylmagnesation of terminal acetylenes afforded (E)-1-silyl-1-alkene exclusively, palladium-catalyzed silylalumination with PhMe 2 Si-AlEt 2 provided 2-silyl-1-alkenes with high regioselectivity. (2) Triethylborane induced radical addition of Ph 3 SnH to acetylenes gave 1-triphenyl-stannyl-1-alkenes in the presence of small amount of oxygen. The reaction has two distinguishing characteristics. One feature is that Et 3 B can initiate the radical reaction at low temperature, such as -78 °C. (3) The other distinctive feature of Et 3 B-induced radical reaction is that many solvents could be used for the reaction. Thus, water was chosen as a solvent, and Et 3 B-induced atom-transfer radical cyclization of iodo acetals and iodoacetates in water was examined. (4) Three types of organometallic ate complexes, R 3 MnMgBr, R 3 MgLi, and R 3 Co(L 2 )MgBr, were prepared and used for organic synthesis. Treatment of gem-dibromocyclopropane with trialkylmanganate provided alkylated cyclopropane after aqueous workup. Aryl and alkenyl halides could be converted into the corresponding magnesium reagents by the action of trialkylmagnesate via halogen-magnesium exchange. Finally, synthetic reactions catalyzed by cobalt complexes are described. Without suffering from β-elimination, cobalt complexes allow cross-coupling reactions of alkyl halides with Grignard reagents.

Treatment of 1-alkynylphosphane sulfides with 1,6- or 1,7-diynes in the presence of a cationic rhodium catalyst results in a formal [2+2+2] cycloaddition reaction to afford the corresponding aromatic phosphane sulfides. The aromatic rings formed in the cycloaddition naturally bear one or two substituents at the ortho positions to the phosphorus atom, which creates a sterically hindered environment around the phosphorus atom. The following desulfidation of the products is facile under radical conditions or with the aid of tris(dimethylamino)phosphane, providing the corresponding bulky phosphanes. Dicyclohexyl(2,6-diphenylaryl)phosphane, which is available through this sequence, proves to serve as an efficient ligand in palladium-catalyzed cross-coupling amination reactions.

The treatment of an aldehyde with a tertiary homoallylic alcohol at 100-250 degrees C in the presence of cesium carbonate and a rhodium catalyst leads to allyl transfer from the homoallylic alcohol to the aldehyde. The process includes Rh-mediated retroallylation to form an allyl rhodium species as the key intermediate. The homoallylic alcohol formed initially through allyl transfer is converted under the reaction conditions into the corresponding saturated ketone when bulky ligands are used. Microwave heating at 250 degrees C accelerates the reaction significantly.

The reaction of aryl-X or alkenyl-X (X=I, Br, OTf, ONf) with terminal acetylenes in the presence of a catalytic amount of Pd(OAc)2 provided the alkynylated products in good yields by using Cp∗Li (Cp∗=1,2,3,4,5-pentamethylcyclopentadienyl) as a base.

Treatment of 1-alkynylphosphines with a rhodium catalyst in 1,4-dioxane/H2O at reflux provides (E)-1-alkenylphosphine oxides in good yields with perfect stereoselectivity. The reaction proceeds as ...

Cobalt/N-heterocyclic carbene system or cobalt/diamine combination effectively catalyzes sequential cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives with trialkylsilylmethyl, 1-alkynyl, and aryl Grignard reagents. The sequential cyclization/cross-coupling reactions are applied to the synthesis of 1,3-diols starting from siloxy-tethered 6-halo-1-hexene derivatives.

Water enables the alkylation of aldehydes with trialkylboranes under nickel catalysis without the addition of a base. Trialkylboranes prepared from borane-dimethyl sulfide and terminal olefins via hydroboration as well as commercially available trialkylboranes could be employed for the reaction. The reaction would proceed via η 2 -coordinated nickel complexes with aldehydes as the key intermediates.

Reactions of 2-alkenyl methyl ether with phenyl, trimethylsilylmethyl, and allyl Grignard reagents in the presence of cobalt(II) complexes are discussed. The success of the reactions heavily depends on the combination of the substrate, ligand, and Grignard reagent. In the reaction of cinnamyl methyl ether, the formation of the linear coupling products predominates over that of the relevant branched products. In the cobalt-catalyzed allylation of allylic ethers, addition of a diphosphine ligand can change the regioselectivity, mainly providing the corresponding branched products. Rhodium complexes catalyze the reactions of allylic ethers and halides with allylmagnesium chloride and allylzinc bromide, respectively, in which the branched coupling product is the major product.

Cobalt salts such as cobalt(II) chloride catalyze allylzincation reactions of 1-aryl-1-alkynes. The allylzincations proceed in a syn fashion to form 2-allyl-1-aryl-1-alkenylzinc species. Other inte...

[reaction: see text] This paper describes cobalt-mediated cross-coupling reactions of alkyl halides with 1-(trimethylsilyl)ethenylmagnesium bromide and 2-(trimethylsilyl)ethynylmagnesium bromide, respectively. The cobalt system allows for employing secondary as well as primary alkyl halides as the substrates. The reactions offer facile formations of alkyl-alkenyl and alkyl-alkynyl bonds. The reaction mechanism would include single-electron transfer from a cobalt complex to alkyl halide to generate the corresponding alkyl radical. The cobalt system thus enables sequential radical cyclization/alkenylation and cyclization/alkynylation reactions of 6-halo-1-hexene derivatives.

Tri(2-furyl)germane is a readily prepared, easy-to-handle, and storable triorganogermane. The three 2-furyl groups confer unique reactivity on it in a variety of reactions. Tri(2-furyl)germane is a promising alternative to tributyltin hydride in radical reaction, especially superior in radical addition to alkenes. The hydrogen of tri(2-furyl)germane is so acidic that generation of the corresponding germyl anion is achieved with the aid of weak bases, such as potassium tert-butoxide and cesium carbonate. The anion participates in nucleophilic addition to aldehydes, 1,4-addition to α,β-unsaturated carbonyl compounds, and coupling reaction with aryl and vinyl halides under palladium catalysis. Palladium-catalyzed hydrogermylation with tri(2-furyl)germane create some novel phenomena. Some chemistry of triphenylgermane will be described.

Bis(iodozincio)methane was prepared from diiodomethane and zinc in the presence of a catalytic amount of lead. The compound showed a high potential as an organic synthetic reagent. The reaction with carbonyl compounds proceeded to give methylenated products. The reagent also reacted with a couple of electrophiles sequentially. The function as Lewis acid is also unique. It coordinated with 1, 2-dicarbonyl compounds in face-to-face manner and assisted the stepwise nucleophilic attack on the carbon center of the reagent; the products are cis-1, 2-cyclopropanediols. A structural study was also examined.(Communicated by Hitosi NOZAKI, M. J. A., March 12, 2003)

A gallium hydride reagent, HGaCl2, was found to act as a radical mediator. Treatment of alkyl halides with the gallium hydride reagent, generated from gallium trichloride and sodium bis(2-methoxyethoxy)aluminum hydride, provided the corresponding reduced products in excellent yields. Radical cyclization of halo acetals was also successful with not only the stoichiometric gallium reagent but also a catalytic amount of gallium trichloride combined with stoichiometric aluminum hydride as a hydride source. An indium hydride reagent, HInCl2, prepared from indium trichloride and diisobutylaluminum hydride also worked as a radical mediator. HInCl2 could reduce aryl iodides and bromides in the presence of Et3B as a radical initiator. Download : Download full-size image

A highly regioselective radical addition of N,N-dichlorobenzenesulfonamide (dichloramine-B) to 1-alkenes is achieved at -78 degrees C by the use of triethylborane as a radical initiator. The reaction of 1,3-dienes with N,N-dichlorosulfonamide in the presence of Et(3)B regioselectively provides N-chloro-N-allylamide derivatives. N-chloro-N-allylamides thus obtained react with a variety of alkenes to furnish pyrrolidine derivatives in good yields. A radical annulation reaction among N,N-dichlorosulfonamide, 1,3-dienes, and alkenes has been developed.

A reaction of 1,2-diketone with bis(iodozincio)methane gave a cyclopropanediol derivative via [2+1] cycloaddition. The reaction proceeded via a sequential nucleophilic attack of the dizinc reagent to a couple of carbonyl group in the substrate. The reaction proceeded with high diastereoselectivity to give cis-isomer. Detailed mechanistic studies have been examined by ab initio calculation.

This account focuses on the discovery and the rationalization of solvent effects, especially those of water, on synthetically useful radical reactions. Noteworthy advantages were observed during the course of our study onradical reactions in aqueous media. The established advantages of water as a solvent for radical reactions are (1) facile room-temperature atom transfer radical cyclization of allyl iodoacetate, (2) highly efficient construction of medium and large rings via radical cyclization of ω-alkenyl iodoacetate, (3) enhancement of intermolecular radical addition, and (4) the highest deuterium incorporation in the deuterative reduction of organic halides. The favorable solvent effects of water are mainly due to its extremely high polarity, strong hydrogen bonding ability, and lack of easily transferable hydrogen via a radical process.

The palladium-catalyzed Kharasch reaction of alkenes and alkynes is enhanced by the use of a heterogeneous aqueous system, without the use of hydrophilic co-solvents or phase transfer catalysts. The Pd(0)-catalyzed reaction of terminal alkenes with bromotrichloromethane in water goes to completion within 2 h at room temperature, whereas no reaction occurs in benzene otherwise under the same conditions. The Pd(0) catalyst in aqueous media also effects the radical addition of iodoperfluoroalkanes toward terminal alkenes and alkynes at room temperature.

Reduction of Li 2 MnCl 4 with magnesium metal provided activated manganese as a black suspension in THF. Treatment of organic halides such as allyl bromides, α-halo esters or aryl halides with activated manganese furnished various organomanganese reagents which reacted further with electrophiles to afford the corresponding adducts. The reaction of a ketone bearing an iodoaryl moiety with this active manganese induced cyclization to provide dihydroindene derivative.

Tri-2-furylgermane in the presence of a catalytic amount of triethylborane adds to internal alkenes as well as to terminal alkenes effectively, giving the corresponding adducts in good to excellent yields. The addition of tri-2-furylgermane to silyl enol ethers followed by elimination of germyl and siloxy moieties provides a new route for the conversion of ketones into alkenes.

Bis(iodozincio)methane was prepared from diiodomethane and zinc in the presence of a catalytic amount of lead. The compound showed a high potential as an organic synthetic reagent. The reaction with carbonyl compounds proceeded to give methylenated products. The reagent also reacted with a couple of electrophiles. A structural study was also examined.

A mixture of ethyl bromoacetate and 1-octene was treated with triethylborane in water at ambient temperature to provide ethyl 4-bromodecanoate in good yield. The bromine atom-transfer radical addition in benzene was not satisfactory. The addition proceeded smoothly in polar solvents such as DMF and DMSO, protic solvents such as 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol, and aqueous media. Ab initio calculations were conducted to reveal the origin of the solvent effect of water in the addition reaction. The polar effect of solvents, which is judged by the dielectric constant, on the transition states in the bromine atom-transfer and radical addition steps is moderately important. Calculations show that a polar solvent tends to lower the relative energies of the transition states. The coordination of a carbonyl group to a proton in a protic solvent, like a Lewis acid, would also increase the efficiency of the propagation.

Treatment of cyclopropyl ketones with TiCl4-n-Bu4NI mixed reagent provides (Z)-titanium enolates which afford syn-α-iodethyl-β-hydroxyketones stereoselectively upon subsequent reaction with various aldehydes. The aldol adducts are cyclized into trans-acyltetrahydrofurans in good yield by active alumina. In contrast, the use of Et2AlI in place of TiCl4-n-Bu4NI provides the corresponding anti aldol adducts with high stereoselectivity. These methods can complementarily provide both syn and anti isomers of α-iodoethyl-β-hydroxyketones from cyclopropyl ketones and aldehydes.

Treatment of various organic halides with phosphinic acid (hypophosphorous acid) in aqueous ethanol in the presence of a radical initiator and a base gave the corresponding reduced products in high yields. Addition of a base is indispensable for the reduction of halides by phosphinic acid. Allylic ether of o-iodophenol or 2-haloalkanal allylic acetal underwent radical cyclization under the same conditions to afford the corresponding cyclic product in excellent yield. Deuterated phosphinic acid was found to be an efficient chain carrier for the radical deuteration of organic halides. For example, a deuterium oxide solution of deuterated phosphinic acid, potassium carbonate, 2,2′-azobis(isobutyramidine) dihydrochloride as an initiator, and p-iodobenzoic acid was heated at reflux to give p-deuteriobenzoic acid in 94% yield. A mixed dioxane/D2O solvent system combined with DBU and potassium peroxodisulfate was crucial to deuterate hydrophobic substrates in high yields and with high deuterium incorporation. Co...

Treatment of trialkyl(dibromomethyl)silanes with trialkylmanganates, derived from manganese(II) chloride and three molar amounts of Grignard reagents or alkyllithiums, provided (E)-1-trialkylsilyl-1-alkenes with high stereoselectivity in good yields. The reaction of trialkyl(dibromomethyl)silanes with alkylmagnesium halides proceeded in the presence of a catalytic amount of manganese(II) chloride. Treatment of 1,2-bis(dibromomethyl)benzene with triphenylmanganate gave 1,2-diphenyl-1,2-dihydrobenzocyclobutene.

Triethylborane-induced atom-transfer radical cyclization of iodo acetals and iodoacetates in water is described. Radical cyclization of iodo acetal proceeded smoothly both in aqueous methanol and in water. Atom-transfer radical cyclization of allyl iodoacetate (3a) is much more efficient in water than in benzene or hexane. For instance, treatment of 3a with triethylborane in benzene or hexane at room temperature did not yield the desired lactone. In contrast, 3a cyclized much more smoothly in water and yielded the corresponding γ-lactone in high yield. The remarkable solvent effect of water was observed in this reaction, although the medium effect is believed to be small in radical reactions. Powerful solvent effects also operate in the preparation of medium- and large-ring lactones. Water as a reaction solvent strikingly promoted the cyclization reaction of large-membered rings. Stirring a solution of 3,6-dioxa-8-nonenyl iodoacetate in water in the presence of triethylborane at 25 °C for 10 h provided a ...

A combined reagent derived from Ti(O- n -Bu) 4 and KO- t -Bu (1:1) has proved to be very effective for the self-aldol addition and also for the cross-aldol condensation between aldehyde and α,β-unsaturated aldehyde such as 2-alkenal or 2-alkynal.

Oxidative self-coupling reactions of various arylmagnesium bromides with TiCl 4 affords the corresponding symmetric biaryls in moderate to good yields at 0°C or lower. Tributylmagnesate-induced halogen–magnesium exchange of aryl halides followed by the coupling reaction provides biaryls in good yields under mild conditions. This method can achieve a one-pot synthesis of biaryls containing functional groups such as esters, amides, or nitriles.

Treatment of gem -dibromocyclopropanes with trialkylmanganate, derived from manganese(II) chloride and three equivalents of Grignard reagent or alkyllithium, followed by an addition of electrophiles provided dialkylated cyclopropanes in good yields. It was found that the reaction of gem -dibromocyclopropanes with alkylmagnesium halide proceeded in the presence of a catalytic amount of manganese(II) chloride. The use of iodobenzofuran in place of gem -dibromocyclopropane gave a ring opening product, 2-alkenylphenol.

Treatment of [CpFe(CO)2I] with terminal alkynes in the presence of catalytic amounts of dichlorobis(triphenylphosphine)palladium and copper iodide in aliphatic amine/THF results in Sonogashira-type carbon–iron bond formation to yield [CpFe(CO)2(Ctriple bond; length of mdashCR)] in good yields.

Treatment of secondary or tertiary alkyl bromides with alkyl Grignard reagents in the presence of catalytic amounts of silver bromide and potassium fluoride in CH2Cl2 afforded the corresponding cross-coupling products in reasonable yields. Moreover, silver showed catalytic activity for the cross-coupling reactions of alkyl bromides with aryl Grignard reagents.

Treatment of aryl bromides with homoallyl alcohols bearing a trimethylene group at the allylic position in the presence of cesium carbonate under palladium catalysis affords (2-aryl­ethylidene)cyclobutanes selectively. The selective formation of the alkylidenecyclobutane skeleton results from regiospecific retro-­allylation of the homoallyl alcohols, which accompanies the transposition of the double bonds.

Cobalt(II) bromide catalyzes arylzincation of alkynes with arylzinc iodide x lithium chloride complexes in acetonitrile. The scope of the arylzincation is wide enough to use unfunctionalized alkynes, such as 6-dodecyne, as well as arylacetylenes. The inherent functional group compatibility of arylzinc reagents allows preparation of various functionalized styrene derivatives. The reaction is applicable to the efficient and stereoselective synthesis of a synthetic estrogen and its derivative.

Aryl cyclopropyl ketones undergo nickel-catalyzed borylative ring opening with bis(pinacolato)diboron to yield 4-oxoalkylboronates.

Retro-aldol reactions of β-hydroxy ketones take place under rhodium catalysis, leading to regioselective formation of the corresponding rhodium enolates. The enolates react with aldehydes in situ to afford the corresponding aldol adducts in high yields.

Treatment of silacyclobutanes with enones under palladium catalysis resulted in formal cycloaddition to yield the corresponding eight-membered ring. The reaction provides a facile and convergent access to synthetically useful eight-membered cyclic silyl enolates.

Treatment of alkyl halides, including tertiary alkyl bromides, with benzylic or allylic Grignard reagent in the presence of a catalytic amount of silver nitrate in ether yielded the corresponding cross-coupling products in high yields. The coupling reactions of tertiary alkyl halides provide efficient access to quaternary carbon centers.

Treatment of 2-methylene-1,3-dithiane 1-oxide with arylboronic acid under rhodium catalysis in aqueous dioxane at 25 °C provided the corresponding adduct, which is a useful 2-aryl-alkanal equivalent.

[structure: see text]. Treatment of 1-alkynylphosphine with thiol in the presence of a catalytic amount of palladium acetate results in regio- and stereoselective anti-hydrothiolation, yielding the corresponding (Z)-1-phosphino-2-thio-1-alkene.

[reaction: see text]. The nickel-catalyzed ring opening reaction of silacyclobutanes with aldehydes affords the corresponding alkoxyallylsilanes. In contrast, the ring expansion reaction of benzosilacyclobutene with aldehydes occurs under nickel catalysis to give oxasilacyclohexenes.

Retro-allylation of homoallyl alcohol by rhodium catalysis occurs to generate allylrhodium species. This allylrhodium reacts with aldehydes to give the corresponding secondary alcohols in situ. Isomerization of these alcohols proceeds in the same pots to furnish the corresponding saturated ketones in good yields. [reaction: see text]

A new ligand, Cp*CH2PPh2 (Cp* = 1,2,3,4,5-pentamethyl-2,4-cyclopentadienyl), was prepared, and was used as a ligand for nickel-catalysed cross-coupling reaction of alkyl halides with aryl Grignard reagents, which nickel–phosphine complexes had never made possible.