Ligand-free nickel-catalyzed Kumada couplings of aryl bromides with tert-butyl Grignard reagents

A ligand-free nickel-catalyzed Kumada cross-coupling of aryl bromides and tert-butyl Grignard reagents led to the formation of a series of tert-butyl aryls in moderate to good yields, excellent tBu/iBu ratios, and good functional group compatibility. A radical coupling process is indicated and a mechanism with a Ni(I)-Ni(III) catalytic cycle is proposed.     

Stereoselective synthesis of isoindolinones and tert-butyl sulfoxides

A reaction of Grignard reagents with an optically pure N-sulfinylimine derived from methyl 2-formylbenzoate yields enantioenriched isoindolinones and tert-butyl sulfoxides. The products are formed by the addition of the nucleophile to N-sulfinylimine followed by cyclization to form N-tert-butylsulfinylisoindolinone, which readily undergoes substitution with a second equivalent of Grignard reagent. The reaction can be carried out in dichloromethane at room temperature or at elevated temperatures without any loss of stereoselectivity. The use of nucleophiles other than Grignard reagents has also been investigated.     

Synthesis of new α-aminophosphonates containing sterically hindered phenol fragments based on the reaction of 3,5-di(tert-butyl)-4-oxo-2,5-cyclohexadienylidenemethylphosphonates with aliphatic amines

A reaction of aliphatic amines (n-butylamine, sec-butylamine, tert-butylamine, dodecylamine, 1,12-diaminododecane) with α-phosphorylated methylenequinones (dimethyl {[3,5-di-(tert-butyl)-4-oxo-2,5-cyclohexadienylidene]methyl}phosphonate and diphenyl {[3,5-di(tertbutyl)-4-oxo-2,5-cyclohexadienylidene]methyl}phosphonate)) to form 1,6-nucleophilic addition products was studied. This approach was used to obtain a-aminophosphonates containing sterically hindered phenol fragments in high yields.     

Asymmetric addition reactions of Grignard reagents to chiral 2-trifluoromethyl tert-butyl (Ellman) sulfinimine-ethanol adducts

The addition of Grignard reagents to chiral trifluoromethyl tert-butyl sulfinimine-ethanol adducts affords protected trifluoromethylamines in high yields with good to excellent diastereoselectivities. The stereochemical outcome of the addition is opposite to that expected via a chelation controlled transition state.     

Reversal of diastereoselection in the addition of Grignard reagents to chiral 2-pyridyl tert-butyl (Ellman) sulfinimines

Addition of Grignard reagents to chiral tert-butyl sulfinimines derived from pyridine 2-carboxaldehyde affords protected 2-pyridyl amines in high yields and diastereoselectivities. The sense of chiral induction is opposite to that predicted via a chelation-controlled transition state.     

Synthesis of modified Weinreb amides: N-tert-butoxy-N-methylamides as effective acylating agents

An efficient preparation of N-methyl-O-tert-butylhydroxylamine hydrochloride has been settled, which allowed the synthesis of modified Weinreb amides. Nucleophilic addition of organolithium and Grignard reagents on these N-tert-butoxy-N-methylamides afforded efficiently the corresponding ketones and reduction with DIBAL furnished the corresponding aldehydes in good yields up to 97%.     

Synthesis of N,N′-di(tert-butyl)bispidin-9-ones

Condensation of 1,3,5-tri(tert-butyl)-1,3,5-triazacyclohexane with acetone gave 3,7-di(tert-butyl)-1,5-bis[(tert-butylamino)methyl]bispidin-9-one. Reactions with ethyl methyl ketone and other ketones of the formula RCH₂COCH₃ yielded 5-R-3,7-di(tert-butyl)-1-[(tert-butylamino)methyl]bispidin-9-ones, while reactions with diethyl ketone and other symmetrical ketones of the formula RCH₂COCH₂R afforded 1,5-R-3,7-di(tert-butyl)bispidin-9-ones.     

Design of ionic liquids as a medium for the Grignard reaction

Design of novel phosphonium ionic liquids that are compatible with Grignard reagents have been investigated; several types of phosphonium salts that have an alkyl ether moiety have been synthesized and their capability evaluated as solvents for Grignard reagents. It has been established that even basic aliphatic Grignard reagent-mediated reactions are possible when methoxyethyl(tri-n-butyl)phosphonium bis(trifluoromethanesulfonyl)imide is used as the solvent.     

BINOL derived C2-symmetric bis-sulfinates as efficient sulfinyl transfer agents in the synthesis of tert-butyl sulfoxides

The reaction of novel C₂-symmetric bis-sulfinate esters derived from (R)-BINOL with Grignard reagents affords tert-butyl sulfoxides in ee up to 97%. The desired enantiomer can be generated at will by the proper selection of BINOL.     

Copper(II) tetrafluoroborate as a novel and highly efficient catalyst for N-tert-butoxycarbonylation of amines under solvent-free conditions at room temperature

Commercially available copper(II) tetrafluoroborate hydrate was found to be a highly efficient catalyst for chemoselective N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions and at room temperature. Various aromatic amines were protected as their N-tert-butyl carbamates in high yields and in short times. No competitive side reactions such as isocyanate, urea, and N,N-di-t-Boc formation was observed. Chemoselective N-tert-butoxycarbonylation was achieved with substrates bearing OH and SH groups. Chiral α-amino acid esters afforded the corresponding N-t-Boc derivatives in excellent yields.     

The synthesis of chlorosilanes from alkoxysilanes,silanols, and hydrosilanes with bulky substituents

We have found that commercially important trialkylchlorosilanes can readily be synthesized by the reaction of alkoxysilanes, silanols, and hydrosilanes with aqueous concentrated hydorochloric acid. Treatment of trialkylalkoxysilanes bearing the bulky alkyl substituents, such as the i-Pr, sec-Bu, tert-Bu, and cyclo-Hex group, with 35% aqueous hydrochloric acid afforded the corresponding trialkylchlorosilanes in excellent yields. Similar treatment of trialkylsilanols with 35% aqueous hydrochloric acid also gave trialkylchlorosilanes in almost quantitative yields. The reaction of methyltrichlorosilane and dimethyldichlorosilane with alkyl-Grignard reagents bearing a bulky alkyl group, followed by treatment of the resulting mixtures with aqueous concentrated hydrochloric acid, produced the respective dialkylmethyl- and alkyldimethylchlorosilanes in high yields. Treatment of trialkylhydrosilanes with concentrated hydrochloric acid in the presence of a palladium catalyst afforded trialkylchlorosilanes in high yields.     

Direct synthesis of 4,4-disubstituted N-silyl-1,4-dihydropyridines

An unprecedented method for the preparation of 4,4-disubstituted 1,4-dihydropyridines is presented. It is based on the trapping reaction of 4-substitued N-silylpyridinium ions. When performed with dialkylmagnesium reagents, such as iPr₂Mg, silyl protected 4,4-disubstituted 1,4-dihydropyridines were obtained in up to quantitative yields. High 1,4-selectivity was found for sterically demanding nucleophiles, whereas small nucleophiles (Me₂Mg) tend to yield 1,2-addition-products. Grignard, dialkylzinc and organocopper reagents were found to give either no addition products or less favorable results. Reduction of the obtained 1,4-dihydropyridine with NaCNBH₃ in the presence of HCl, followed by treatment with tert-butyl dicarbonate provided the corresponding N-Boc protected piperidines with high yields.     

Solid state conformations of α,β-unsaturated hydroxamates derived from the ‘chiral Weinreb amide’ auxiliary (S)-N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine

α,β-Unsaturated hydroxamates derived from the ‘chiral Weinreb amide’ auxiliary (S)-N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine consistently adopt a defined conformation and undergo highly diastereoselective conjugate addition reactions with lithium amide reagents. The configuration of the N-1-(1′-naphthyl)ethyl group dictates the position of the O-tert-butyl group and also the configuration adopted by the pyramidal nitrogen atom via a ‘chiral relay’ effect. Conjugate addition of lithium amide reagents to these substrates proceeds on the face opposite to both the O-tert-butyl group and nitrogen lone-pair with high levels of diastereoselectivity.     

Preparation of azulenyllithium and magnesium reagents utilizing halogen-metal exchange reaction of several iodoazulenes with organolithium or magnesium ate complex

Preparation of several azulenyllithium and magnesium reagents was achieved by the halogen-metal exchange reaction of iodoazulenes with n-butyllithium or lithium tri(n-butyl)magnesate at low temperature and the synthetic application of the reagents was explored.     

Trifluoroacetaldehyde: A useful industrial bulk material for the synthesis of trifluoromethylated amino compounds

The synthesis of various trifluoromethylated amino compounds was studied using trifluoroacetaldehyde, an industrial bulk material, as a starting compound. One general application of trifluoroacetaldehyde is the preparation of trifluoroethylamino derivatives via reductive amination reaction. This synthesis includes the formation of the corresponding N,O-acetal intermediates followed by their reduction using NaBH₄ or 2-picoline borane complex, affording the target trifluoroethylamino compounds in moderate to good yields (47-87%).Another general application of trifluoroacetaldehyde is the synthesis of chiral α-substituted trifluoroethylamino compounds. In this synthesis, trifluoroacetaldehyde was first converted into the chiral trifluoromethyl tert-butyl sulfinimine, which was subjected to 1,2-nucleophilic addition reactions across its CN double bond. The addition of phenyllithium afforded α-(phenyl)trifluoroethylamino derivative in 83% yield and with 96% de. Allylation and Reformatsky reactions produced the corresponding α-substituted trifluoroethylamino derivatives in 82 and 58% yields and with 90 and 91% de, respectively.     

Diastereocontrolled addition of organometallic reagents to S-chiral N-(tert-butanesulfinyl)-α-fluoroenimines

Grignard and organolithium reagents efficiently react with (S)-N-(tert-butanesulfinyl)-α-fluoroenimines to provide chiral allylamines in excellent yields and with diastereomeric ratios of up to 96:4. Acidic removal of the sulfinyl group and simple functional group transformations allow to get enantiopure fluoroolefin dipeptide mimics.     

Some new aspects of conjugate addition of grignard and alkyllithium reagents to vinylsilanes

A structure and reactivity relationship has been determined on the reaction of isopropyl and n-butyl Grignard reagents and n-butyllithium with vinyl-(alkoxy)silanes and (amino)silanes. The addition to the vinyl group has been analyzed in terms of the nucleophile HOMO-vinyl group LUMO interaction.     

The retro Grignard addition reaction revisited: the reversible addition of benzyl reagents to ketones

The Grignard addition reaction is known to be a reversible process with allylic reagents, but so far the reversibility has not been demonstrated with other alkylmagnesium halides. By using crossover experiments it has been established that the benzyl addition reaction is also a reversible transformation. The retro benzyl reaction was shown by the addition of benzylmagnesium chloride to di-tert-butyl ketone followed by exchange of both the benzyl and the ketone moiety with another substrate. Similar experiments were performed with phenylmagnesium bromide and tert-butylmagnesium chloride, but in these two cases the Grignard addition reaction did not show any sign of a reverse transformation.     

Silver-catalyzed cross-coupling reactions of alkyl bromides with alkyl or aryl Grignard reagents

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.     

A new synthesis of β,γ-unsaturated esters from three components, aldehydes, chloromethyl p-tolyl sulfoxide, and tert-butyl acetate, via magnesium carbenoid 1,2-CH and 1,2-CC insertion as the key reaction

Addition reaction of 1-chlorovinyl p-tolyl sulfoxides, which were derived from various aldehydes, with lithium enolate of tert-butyl acetate at −78 °C in THF gave adducts in high yields. Magnesium carbenoids were generated by treatment of these adducts with Grignard reagents via the sulfoxide-magnesium exchange reaction. When the adducts were derived from alkyl aldehydes or electron-deficient aromatic aldehydes, carbenoid 1,2-CH insertion reaction took place from the magnesium carbenoids to afford β,γ-unsaturated butyric esters having a substituent at the β-position. On the contrary, when the adducts were derived from electron-rich aromatic aldehydes, carbenoid 1,2-CC insertion reaction took place from the magnesium carbenoids to give β,γ-unsaturated butyric esters having the aromatic group at the γ-position. Highly stereospecific 1,2-CC insertion reactions were observed in the latter reactions. This procedure provides a good way for a synthesis of β,γ-unsaturated esters from aldehydes with two carbon-carbon bond-formations.