Organomagnesium reactions

FIGURE 4.16 Method D organomagnesium reactions leading to monoalkylated products. 

FIG. 2.8. Typical assembly for an organomagnesium reaction. For reactions in boiling solvent the thermometer is replaced by a condenser. 

FIG. 2.9. Typical assembly for a small-scale organomagnesium reaction. 

Organohthium and organomagnesium compounds find their chief use m the prepa ration of alcohols by reaction with aldehydes and ketones Before discussing these reac tions let us first examine the reactions of these organometallic compounds with proton donors... 

Grignard reagent (Section 14 4) An organomagnesium com pound of the type RMgX formed by the reaction of magne Slum with an alkyl or aryl halide... 

The term Grignard reaction refers to both the preparation of a class of organomagnesium haUde compounds and their subsequent reaction with a wide variety of organic and inorganic substrates. As such it has had a wide and profound influence on synthetic chemistry since its first elucidation by Victor Grignard at the beginning of the twentieth century. 

Magnesium ethylate is used as a drying agent for organic solvents, as an intermediate for the manufacture of organomagnesium and other organic products, as catalyst in the Tishchenko (85) and other reactions (86—91), in the condensation of esters (92), in alkylation reactions, and in polymeri2ations (84,91). 

Anhydrous diethyl ether is the customary solvent used when preparing organomagnesium compounds. Sometimes the reaction does not begin readily, but once started, it is exothermic and maintains the temperature of the reaction mixture at the boiling point of diethyl ether (35°C). 

Reactions of Enamine Salts with OrganometalUc Compounds Organolithium and organomagnesium compounds react with enamine salts to give amines substituted on the ix-carbon atoms. The treatment of. -dehydroquinolizidinium perchlorate (163) with alkylmagnesium halides gives 9-alkylated quinolizidines (164) (252,256). Formation of... 

Organomagnesium derivatives have not so far been isolated [80JA994 80JOM(193)C13]. Bis[bis(trimethylsilyl)phosphonamide] with diphenylbutadi-yne and calcium or strontium in THE yields the TiyP)-coordinated species 129. Reaction of (THE)4Ba[P(SiMc3)2]2 with diphenylbutadiyne gives an ri -coordinated species 130 (98JA6722). 1-Chloro-and l-cyclopentadienyl-3,4-dimethyl-2,5-bis-(trimethylsilyl)-l-phosphacyclopenta-2,4-dienes both react with calcium to give... 

The experimental conditions for the syntheses starting from acid chlorides of hydroxamic acids and from nitrile oxides are somewhat different. In the former case the other component of the reaction is organometallic, usually an organomagnesium derivative of an acetylene or, less frequently, a sodium enolate of a /8-diketone. Nitrile oxides condense directly with unsaturated compounds. 

The nucleophilic substitution reactions are still more limited in scope owing to the instability of the isoxazole ring toward nucleophilic reagents. Homolytic reactions appear to be unknown though some of the reactions being studied are possibly of this type. Besides those reactions which are characteristic of the reactivity of the isoxazole nucleus itself, we shall consider in this section some substitution reactions in the side chain organomagnesium synthesis in the isoxazole series, condensation reactions of the methyl groups of methyl-isoxazoles, and finally some miscellaneous reactions. 

A related cleavage reaction (165 166) occurs on the treatment of isoxazoles with organomagnesium compounds.This reaction considerably hinders the use of the Grignard reaction in the isoxazole series (see Section IV,D) in it the activity of the organo-... 

The cleavage of the isoxazole ring by organomagnesium compounds may proceed by either one or both of two alternative mechanisms. Magnesium subhalides produced during the associated reaction may act as reducing agents as proved in specific cases.Another possibility is that the reduction involves a six-membered cyclic complex (171). 

Under the conditions used for the reductive cleavage of the O—N bond in isoxazoles, a similar reaction readily occurs with isoxa-zolines, e.g. on treatment with organomagnesium compounds and on catalytic hydrogenation.- Hydrogenolysis of the O—N bond (180 181 182 183) was used to elucidate the structure of isoxa-zolones from -ketoesters. -... 

The actual mechanism by which a particular reaction proceeds strongly depends on the nature of the organomagnesium reagent. For instance benzophenone reacts with methylmagnesium bromide by a polar mechanism, while the reaction with t-butylmagnesium chloride proceeds for steric reasons by a SET-mechanism. 

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