Dimethoxyethane zinc complex

The reaction of EtjSnH with EtjZn results in decomposition with the formation of Zn metal, ethane, tetraethyltin, hexaethylditin and a polymerp The reaction of PhjSnH with EtjZn in pentane or benzene forms metallic Zn. However, when hydrostannolysis is carried out with preformed complexes of Et2Zn or with a complexing solvent, such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THE), or by using a preformed coordination complex of EtjZn with tetramethylethylenediamine (TMED) or bipy, the bis(triphenyltin)zinc complexes are obtained in high yields under mild conditions (—20° to 0°C)2 ... 

If either the olefin or the cyclopropane produced is sensitive to zinc iodide, a Lewis acid, the usual procedure can be modified by using as solvent diethyl ether containing 1 equivalent of glyme (1,2-dimethoxyethane). Zinc iodide is removed quantitatively from the solution as the crystalline, insoluble 1 1 complex with glyme. 

A homogeneous solution of the bis(iodomethyl)zinc DME complex in dichloromethane can be prepared by adding diethylzinc to 1 equivalent of 1,2-dimethoxyethane in dichloromethane followed by 2 equivalents of diiodomethane . The presence of DME makes the preparation of the reagent safer by ensuring that the mixture is constantly homogeneous. This reagent has been useful in enantioselective cyclopropanation reactions vide infra). 

For many years the elucidation of the mechanism of the McMurry reaction has been complicated by the fact that the most commonly used low-valent titanium was derived from hour-long pre-reducing DME-complexed TiCl3 with a zinc/copper couple (DME is 1,2-dimethoxyethane). Apparently, however, the zinc/copper couple reacts with TiCl3 only if the carbonyl compound is present, too. This fact has become part of the current mechanistic view of the McMurry reaction (Figure 17.58). At the same time, it became the starting point of the following variants of the McMurry reaction ... 

With regard to the preparation of the Zn/Cu couple, several methods have been developed Dibromomethane may be used instead of diiodomethane when a specifically prepared Zn/Cu couple is applied . The use of Zn/Ag couple often gives better results . Ultrasound irradiation of the reaction mixture has been shown to facilitate the reaction . The cyclopropanation of alkenes with diiodomethane and diethylzinc can be carried out in hydrocarbon solvents and is particularly suitable with easily polymerizable olefins such as vinyl ethers . It has been reported that molecular oxygen remarkably promotes the reaction of diiodomethane with diethylzinc and substantially increases the yield of the adducts . A convenient modification which avoids the handling of pyrophoric diethylzinc has been reported . In reaction of olefins which are sensitive to the unavoidably produced zinc iodide (the Lewis acid), the addition of one equivalent of dimethoxyethane (DME) to the solvent has been recommended . Zinc iodide is then precipitated as the 1 1 DME complex as it is formed. Zinc salts, which often complicate the workup of the reaction mixture, can also be removed as precipitates by the addition of pyridine prior to the workup . ... 

Since Cd(CF3)2DME is ordinarily vastly superior to Hg(CF3)2 as a tii-fluoromethylating agent, it seems only reasonable to assume that trifluoromethyl zinc compounds might well be even more effective. Diligand Zn(CF3)2 complexes have been prepared by a number of routes, including the reactions of Zn(CH3)2 with Hg(CF3)2 (87) orCF3I (88) in basic solvents like pyridine, dimethoxyethane, and acetonitrile. 

However, this asymmetric cyclopropanation of allylic alcohols can be used only up to 1 mmol equivalent. When run on a larger scale, e.g. 8 mmol, a violent explosion of the reaction mixture was observed. Accordingly, the authors have improved the procedure by adding the bis(iod-omethyl)zinc reagent as its 1,2-dimethoxyethane complex in dichloromethane. The cyclo-propanations were safely carried out on > 1 mmol scale with 93% ee and > 98% yield, e.g. cyclopropanation to give 95. The chiral alcohol and butylboronic acid could be recovered and used again. 

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