Jean-Claude Négrel

New perspectives in the formation of the Grignard reagent

Abstract Inhibitors, in very low concentration, inhibit the reaction between 1-bromo-3-methylbutane and magnesium obtained by vaporization of metal. For such a magnesium derivative, the inhibitors cannot play the role of “killers” for active sites. A chain reaction for the formation of the Grignard reagent is proposed.

Further Evidence for the Radical Chain Character of Grignard's Reagent Formation. Use of Free Radical Clock in Conjunction with Changes in Concentration of Active Mg.

Abstract The reaction of endo-5-(2′-haloethyl)-2-norbortiene (X=I, Br, Cl) with active Mg obtained by metal vaporization yields both cyclized and uncyclized organomagnesium compounds. The ratio cyclized / uncyclized increases in the order Cl

Drastic effects of dioxygen on the selectivity of reduction by LiAlH4. Milder conditions made possible by strictly anaerobic conditions

Abstract Reduction of the gem-disubstituted cyclopropane 1 with LiAlH 4 yields different results under strictly anaerobic conditions and loosely anaerobic ones. Under strictly anaerobic conditions, (+)-1-bromo-2,2-diphenylcyclopropanecarbinol 2 is quantitatively reduced to 3 with complete racemization. These observations are explained by a radical chain mechanism. They open the way to reductions by LiAlH 4 under mild conditions.

Occurence of a radical intermediate during the reductive decyanation of an α-sulfonitrile with LiAlH4

Abstract LiAlH 4 reduction of 2-(isopropylsulfonyl)-bicyclo[2.2.1]hept-5-ene-2-carbonitrile 1 gave the reduced product 2 resulting from the first reported LiAlH 4 induced α-sulfonitrile decyanation. Experiments using LiAlD 4 suggest the involvement of a radical intermediate. Using a radical-probe, the spiranic compound 6 characteristic of a radical intermediate has been detected. Electrochemical measurements do fit with a SET from LiAlH 4 to the nitrile.

Extension of the e.t. mechanism of aromatic nitration to the formation of Grignard reagents of RX (R = alkyl, aryl, cyclopropyl, vinyl). The ion pair collapse versus radical pair collapse hypothesis

Abstract The ion pair collapse versus radical pair collapse hypothesis is proposed for the Grignard reagent formation as an extension of the e.t. mechanism of aromatic nitration. Such a scheme, never examined before for the Grignard reaction, could explain the evolution of the ‘free radical character’ depending on experimental conditions and the nature of the substrate.

Metal Vapor Reactivity of Copper Toward Substituted Bromobenzenes. Comparison with the Classical Ullmann Coupling Reaction

The reaction of Copper vapors with a film of bromobenzenes in a solution of methylcyclohexane - tetrahydrofuran maintained at - 120°C mainly yields reduction and coupling products. The low temperature contrasts with the severe reaction conditions used when metallic copper is used as a promoter. In terms of overall conversion (percent of copper effectively used to induce chemical transformation) o-chloro; o-amino and p-RO substituents play a beneficial role. In terms of coupling /reduction ratio, unsubstituted bromobenzene yields the best results. As in the classical Ullmann reaction nitro group induces far more coupling when it is situated in ortho of bromine than when it occupies the meta or (even worse) para position. For some substrates, THF used as cosolvent reacts with the intermediates organometallic to yield the condensation of benzene with an open form of THF. Ortho substituents of apropriate structure seem able to stabilize metallic atoms of Cu.