Dimethoxyethane zinc

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. 

The addition of (Z)-3-(alkylamino)-2-butenoic acid esters to nitroalkenes derived from open-chain sugars gave a 50 50 mixture of diastereomeric products 32. The reaction of chiral 2-sub-stituted l-(2-nitroethenyl)pyrrolidines with zinc enolates of 3-substituted tetrahydro-2//-pyran-2-oncs in 1,2-dimethoxyethane at — 78 °C afforded the corresponding 3,3-disubstituted products in 82-96% ee via an addition-elimination process33. The stereochemical course of the reaction was determined by chemical correlation of (S)-( )-3-ethyltetrahydro-3-(2-ni-troethenyl)-2//-pyran-2-one with ( + )-quebrachamine. 

Nalliah et al. (202,203) developed a novel entry to protopine alkaloids from a protoberberine. 13-Oxotetrahydroberberine (ophiocarpinone) metho salt (291) was reduced with zinc in 30% acetic acid to afford a new type of the protopine analog 401 (Scheme 74) (202). On the other hand, 291 gave 13-oxoallocryptopine (402) via the hydroperoxide on exposure to air in the presence of sodium hydride and potassium iodide in dimethoxyethane (203). 

Vanadium compounds in a low oxidation state are known to be effective for inducing one-electron reduction. The highly stereoselective monodebromination of gem-dibromocyclopropanes proceeds with the help of a low-valent vanadium species generated from vanadium(III) chloride and zinc in dimethoxyethane in cooperation with diethyl phosphonate or triethyl phosphite... 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

Replacement of sulfonyloxy groups by hydrogen is also achieved by refluxing the methanesulfonyl and p-toluenesulfonyl esters of alcohols with sodium iodide and zinc dust in wet dimethoxyethane. Evidently iodine displaces sulfonyloxy group and is replaced by hydrogen by means of zinc. Yields range from 26% to 65% [700] (Procedure 33, p. 213). 

An interesting deoxygenation of ketones takes place on treatment with low valence state titanium. Reagents prepared by treatment of titanium trichloride in tetrahydrofuran with lithium aluminum hydride [205], with potassium [206], with magnesium [207], or in dimethoxyethane with lithium [206] or zinc-copper couple [206,209] convert ketones to alkenes formed by coupling of the ketone carbon skeleton at the carbonyl carbon. Diisopropyl ketone thus gave tetraisopropylethylene (yield 37%) [206], and cyclic and aromatic ketones afforded much better yields of symmetrical or mixed coupled products [206,207,209]. The formation of the alkene may be preceded by pinacol coupling. In some cases a pinacol was actually isolated and reduced by low valence state titanium to the alkene [206] (p. 118). 

An interesting reaction takes place when diketones with the keto groups in positions 1,4 or more remote are refluxed in dimethoxyethane with titanium dichloride prepared by reduction of titanium trichloride with a zinc-copper couple. By deoxygenation and intramolecular coupling, cycloalkenes with up to 22 members in the ring are obtained in yields of 50-95%. For example, 1-methyl-2-phenylcyclopentene was prepared in 70% yield from 1-phenyl-1,5-hexanedione, and 1,2-dimethylcyclohexadecene in 90% yield from 2,17-octa-decanedione [206, 210]. 

A mixture of a solution of 0.300 g of an alkyl methanesulfonate or p-toluenesulfonate in 3-6 ml of 1,2-dimethoxyethane, 0.300 g of sodium iodide, 0.300 g of zinc dust, and 0.3 ml of water is stirred and refluxed for 4-5 hours. After dilution with ether the mixture is filtered the solution is washed with water, with 5% aqueous hydrochloric acid, with 5% aqueous solution of potassium hydrogen carbonate, with 5% aqueous solution of sodium thiosulfate and with water. After drying with anhydrous sodium sulfate the solution is evaporated and the residue worked up, giving 26-84% yield of alkane. 

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). 

Cyclopropanation.1 Ultrasound irradiation facilitates Simmons-Smith cyclopro-panation. Ordinary zinc can be used. The reaction is Caster and yields are considerably improved. 1,2-Dimethoxyethane is recommended as solvent in place of the usual solvent (ether). 

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 ... 

The reaction of phenanthrene with zinc dust and methylene iodide in 1,2-dimethoxyethane gives 9,10-dihydro-9,10-methanophenanthrene (XIX) in 25% yield 408). The reaction of naphthalene with diethylzinc... 

Metals commonly utilized include the alkali metals, mainly lithium, sodium and potassium, and also calcium, zinc, magnesium, tin and iron. Alkali metals and calcium have been used in liquid ammonia," in low molecular weight aliphatic amines," in hexamethylphosphoramide, in ether or in THF containing crown ethers, or in very dilute solutions in polyethers such as 1,2-dimethoxyethane (DME)." Reactions with metal solutions in liquid ammonia often use a cosolvent, such as ether, THF or DME, to increase solubility of the organic substrate in the reaction mixture. These same metals, as well as zinc and magnesium, have been used as suspensions in various solvents including ether, toluene, xylene, etc. 

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 . ... 

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 ... 

An especially active Ti(0) metal is obtained by converting TiCl3 into its crystalline TiCl3-dimethoxyethane solvate [TiCljfDME)], which is then reduced with a zinc-copper couple to Ti(0) metal. The Ti-mediated coupling may also be carried out in the presence of carbon-carbon double bonds, as exemplified below. 

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. 

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