Ethers reactions with magnesium

Benzyl chloride readily forms a Grignard compound by reaction with magnesium in ether with the concomitant formation of substantial coupling product, 1,2-diphenylethane [103-29-7]. Benzyl chloride is oxidized first to benzaldehyde [100-52-7] and then to benzoic acid. Nitric acid oxidizes directly to benzoic acid [65-85-0]. Reaction with ethylene oxide produces the benzyl chlorohydrin ether, CgH CH20CH2CH2Cl (18). Benzylphosphonic acid [10542-07-1] is formed from the reaction of benzyl chloride and triethyl phosphite followed by hydrolysis (19). 

Magnesium enolates play an important role in C-acylation reactions. The magnesium enolate of diethyl malonate, for example, can be prepared by reaction with magnesium metal in ethanol. It is soluble in ether and undergoes C-acylation by acid anhydrides and acyl chlorides (entries 1 and 3 in Scheme 2.14). Monoalkyl esters of malonic acid react with Grignard reagents to give a chelated enolate of the malonate monoanion. 

Caution. Owing to the poisonous nature of 3-bromo-l-propene, the following procedure must be carried out in a well-ventilated fume hood. No further 3-bromo-l-propene should be added until it is certain that the reaction with magnesium turnings, as evidenced by a slightly milky appearance of the diethyl ether solution, has been successfully initiated. 

BIBBWBBif Write the structure of the Grignard reagent formed from each of the following compounds on reaction with magnesium in diethyl ether ... 

Method A. Huang-Minlon modification of the Wolff-Kishner reduction. Place 36.0 g (0.3 mol) of redistilled acetophenone, b.p. 201 °C, 300 ml of diethylene glycol, 30ml of 90 per cent hydrazine hydrate (CAUTION) and 40g of potassium hydroxide pellets in a 500-ml two-necked round-bottomed flask fitted with a reflux condenser insert a thermometer supported in a screw-capped adapter in the side-neck so that the bulb dips into the reaction mixture. Warm the mixture on a boiling water bath until most of the potassium hydroxide has dissolved and then heat under reflux for 1 hour either by means of a free flame or by using a heating mantle. Remove the reflux condenser and fit a still-head and condenser for downward distillation. Distil until the temperature of the liquid rises to 175 °C (1). Separate the upper hydrocarbon layer from the distillate and extract the aqueous layer twice with 20 ml portions of ether. Dry the combined upper layer and ethereal extracts with magnesium sulphate, remove the ether on a water bath and distil the residue. Collect ethylbenzene at 135-136 °C the yield is 20 g (62.5%). 

An alternative to the above eliminations, which in principle offers a great many advantages, is the metal induced elimination of the elements of ROBr from a 2-halocyclopropyl ether. Thus the sequence of addition of dibromocarbene to an enol ether, reduction to a monobromide and reaction with magnesium in tetrahydro-furan provides a route to cyclopropene itself45). 

By the action of ethyl magnesium bromide, the bromine atom bound to the tin is replaced by the ethyl group, with the formation of tin triethyl-5-bromoamyL In absolute ether solution the reaction with magnesium goes quite easily, a 75 per cent, yield being obtained, the (5-triethylstannyl-n-amyl) magnesium bromide,... 

Inspection of Table 11 shows that p for the reaction of aryl halides with tri-u-butyltin hydride was not solvent-dependent, whereas p for the reaction with magnesium was. The small values of p for the reaction of aryl bromide with magnesium in a polar solvent were again interpreted [81c] in terms of a mass transport-limited process. Thus reactions of aryl bromides would be transport-limited in THF and more polar solvents, but not in diethyl ether and less polar solvent. On the other hand, the rate of reaction of aryl iodides with magnesium seemed to be transport-limited, even in diethyl ether, whereas the rate of reaction of chlorobenzene with magnesium, which was lO slower than that of bromobenzene, was not. 

Brigas, A. F. and Johnstone, R. A. W. 2000. Hetero aromatic ethers of phenols in nickel-catalysed dipso-replacement reactions with magnesium, zinc and tin organometallic compounds./. Ghent. SPl. 11 1735-1739. 

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