Triethylborane-induced bromine atom-transfer radical addition in aqueous media: study of the solvent effect on radical addition reactions.

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.

TiCl4-n-Bu4NX (X = I, Br, and Cl) combination-induced coupling of alpha,beta-unsaturated ketones with aldehydes.

A three-component coupling reaction between vinyl ketones, aldehydes, and halides has been developed with TiCl(4)-n-Bu(4)NX combined reagents. Treatment of vinyl ketones with a TiCl(4)-n-Bu(4)NI combination followed by an addition of a variety of aldehydes provides syn-alpha-iodomethyl-beta-hydroxy ketones with high stereoselectivity. Methyltriphenylphosphonium iodide as well as n-Bu(4)NI acts efficiently as a halide source. The combination of TiCl(4)-n-Bu(4)NBr provides the corresponding bromo compounds in good yields. syn-alpha-Chloromethyl-beta-hydroxy ketones are obtained with the TiCl(4)-n-Bu(4)NCl combination. A competitive experiment reveals that the order of relative reactivity of the combinations is TiCl(4)-n-Bu(4)NI > TiCl(4)-n-Bu(4)NBr > TiCl(4)-n-Bu(4)NCl.

Remarkable rate acceleration of Pd(0)-catalyzed hydrogermylation of alkynes and dienes in water.

[reaction: see text] Without the use of hydrophilic cosolvents or phase transfer catalysts, Pd(0)-catalyzed hydrogermylation of alkynes in water provides dienylgermanes efficiently with high stereo- and regioselectivity. The reaction in water proceeds much faster than the reaction under neat conditions. By using an aqueous system, hydrogermylation can be conducted at a low catalyst loading.

Radical [3+2] annulation of N-allyl-N-chlorotosylamide with alkenes via atom-transfer process.

[reaction: see text]. A radical [3+2] annulation reaction with an N-centered radical has been developed. The reaction of alkenes with N-allyl-N-chlorotosylamide yields the corresponding pyrrolidine derivatives in good yields in the presence of Et3B as a radical initiator.

Triethylborane-induced radical reactions with gallium hydride reagent HGaCl2.

[see reaction]. A gallium hydride reagent, HGaCl2, was found to act as a radical mediator, like tributyltin hydride. 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.

Selective halogen-magnesium exchange reaction via organomagnesium ate complex.

Halogen-magnesium exchange of various aryl halides is achieved with a magnesium ate complex at low temperatures. Tributylmagnesate (nBu3MgLi) induces facile iodine-magnesium exchange at -78 degrees C. Dibutylisopropylmagnesate (iPr(n)Bu2MgLi) is more reactive than nBu3MgLi, and this reagent accomplishes selective bromine-magnesium exchange at -78 degrees C. This procedure is utilized for the preparation of various polyfunctionalized arylmagnesium species. The exchange of alkenyl halides using this method proceeds with retention of configuration of the double bond.

Palladium-catalyzed cross-coupling reaction of organoindiums with aryl halides in aqueous media.

[reaction: see text] Diaryl-, divinyl-, and dialkylindium proved to be stable in aqueous media and to undergo a palladium-catalyzed cross-coupling reaction with aryl halides in aqueous THF. Treatment of 3-iodophenol with diphenylindium compound, generated from indium trichloride and two equimolar amounts of a phenyl Grignard reagent, in aqueous media under palladium catalysis provided the corresponding coupling product in excellent yield. Divinyl- and diethylindium can be used for the coupling reaction in the presence of water. A wide range of functional groups, including a hydroxy group and a formyl group, are compatible with this reaction.

Intramolecular Aryl Migration from Tin to Carbon via a Radical Atom-Transfer Process.

Triethylborane-induced intramolecular 1,4-aryl migration from tin to a carbon-centered radical is described. Treatment of 3-iodoalkyldimethylphenylstannanes with Et(3)B in benzene at reflux followed by an addition of MeMgI provides 3-phenylalkyltrimethylstannanes in good yield via a radical atom-transfer process.

Et(3)B-Induced Hydrogermylation of Alkenes and Silyl Enol Ethers.

Tri-2-furanylgermane in the presence of a catalytic amount of Et(3)B adds to internal alkenes as well as terminal alkenes effectively to give the corresponding adducts in good to excellent yields. The addition of tri-2-furanylgermane to silyl enol ethers followed by elimination of germyl and siloxy moieties provides a new route for the conversion of ketones into alkenes.

Formation of organomagnesium compounds via EtMgBr-mediated radical cyclization of allyl beta-iodoacetals

[reaction: see text] Treatment of allyl beta-iodoacetals with ethylmagnesium bromide in THF provided tetrahydrofuran derivatives in good yields. On the other hand, the reaction in DME provided tetrahydrofuranylmethylmagnesium compounds in good yields.