Magnesium alkyl- bromide

Boiling-point elevation and freezing-point depression measurements have demonstrated that at all concentrations in THF and at low concentrations in ether (up to 0.1M) Grignard reagents prepared from alkyl bromides and iodides are monomeric, that is, there are few or no molecules with two magnesium atoms. Thus, part of the Schlenk equilibrium is operating... 

Schema name is reaction of magnesium with alkyl bromides. The starting values for ease, yield and confidence are 90, 96, 100. The reagent class for this schema is 0. This is a single application schema. The maximum no. of nonidentical subgoal molecules allowed for this schema is 9.

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

By treating the vinylic magnesium tellurolates with air while stirring, instead of with alkyl bromides, divinylic ditellurides are formed. ... 

N-Alkylation is favored by sodium and potassium cations, whereas C-alkylation is promoted by the more strongly coordinating lithium and magnesium cations (90JCS(P1)111). N-Alkylation is also favored by polar solvents and the use of tosylates rather than iodides. Under phase transfer conditions, the preference for A-alkylation in indoles ranges from 1.5 1 for benzyl bromide to 10 1 for benzyl chloride and -alkyl bromides, over 3-alkylation (90H(31)447). 

Reaction IX. (a) Action of Magnesium Alkyl or Aryl Halide on certain Alkyl or Aryl Halides in the presence of Absolute Ether (Grignard). (C. 1906, II., 748.)—The Grignard reaction has perhaps a wider application than even the Friedel-Crafts or the diazo reaction. When an alkyl or aryl bromide or iodide is treated with magnesium powder, usually in presence of absolute ether, a magnesium alkyl or aryl halide is formed. 

The Grignard reaction is often one of the first reactions encountered for the preparation of organometallic compounds. As such it provides a method for the conversion of an alkyl bromide to an alkane. From the example shown below it is seen that the overall oxidation level change from the organic reactants to the products is from 0 to —2, so a reduction has occurred. Magnesium is the reductant and is itself oxidized from 0 to +2 oxidation state. The actual reduction takes place in the first step of the process in which the C-Br bond is converted to a C-Mg-Br bond. The reaction with water is merely a hydrolysis that does not change the oxidation state of carbon. 

The discussion of acids and bases in the previous section helps us understand the chemistry of the Grignard reagents. Grignard reagents are made by reacting an alkyl bromide with magnesium metal in diethyl ether. 

Alkyl Aryl Telluriums2 (Reduction with Thiourea Dioxide) To a solution of 1.0 mmol diaryl ditellurium in 7.5 ml tetrahydrofuran are added 0.108 g (1.0 mmol) thiourea dioxide, 30 mg 2HT (phase transfer reagent), and 2.0 mmol of the alkyl bromide. To this mixture are added 7.5 ml of a 50% aqueous solution of sodium hydroxide. The mixture is stirred for several hours at 20°. The organic phase is separated. The aqueous phase is extracted three times with 20 ml ethyl acetate. The organic phase and the ethyl acetate extracts are combined and dried with anhydrous magnesium sulfate. The mixture is filtered and the solvents evaporated from the filtrate. The residue is chromatographed on silica gel with ethyl acetate as mobile phase. 

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