Zinc-Copper/silver couple

The Pd-catalyzed homocoupling of tenninal alkynes with (Z)-l,2-dichloroethylene furnishes the corresponding (Z)-conjugated enediynes, whose structural feature is encountered in a class of antitumor antibiotics and is the object of current intense synthetic and mechanistic studies. " Unsymmetrical (E)- and (Z)-enediynes are prepared in a one-pot procedure by two sequential Pd-catalyzed cross-coupling reactions of (E)- and (Z)-l,2-dichloroethylene with the appropriate l-alkynes. Reduction of enediynes A with a zinc copper-silver couple in aqueous methanol provides access to (Zj-trienes. " ... 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

The direct reaction of zinc metal with organic iodides dates back to the work of Frankland(67). Several modifications have been suggested since that time to increase the reactivity of the metal. The majority of these modifications have employed zinc-copper couples(68-72), sodium-zinc alloys(73), or zinc-silver couples(77). Some recent work has indicated that certain zinc-copper couples will react with alkyl bromides to give modest yields of dialkylzinc compounds(74,73). However, all attempts to react zinc with aryl iodides or bromides have met with failure. The primary use of zinc couples has been in the Simmons-Smith reaction. This reaction has been primarily used with diiodomethane as 1,1-dibromides or longer chain diiodides have proven to be too unneactive even with the most reactive zinc couples. 

Mykytiuk et al. [184] have described a stable isotope dilution sparksource mass spectrometric method for the determination of cadmium, zinc, copper, nickel, lead, uranium, and iron in seawater, and have compared results with those obtained by graphite furnace atomic absorption spectrometry and inductively coupled plasma emission spectrometry. These workers found that to achieve the required sensitivity it was necessary to preconcentrate elements in the seawater using Chelex 100 [121] followed by evaporation of the desorbed metal concentrate onto a graphite or silver electrode for isotope dilution mass spectrometry. 

A branched-chain iodo sugar derivative, l,5-anhydro-4,6-0-benzyl-idene-2,3-dideoxy-3-C-(iodomethyl)-D-rifoo-hex-l-enitol [4,6-O-ben-zylidene-3-deoxy-3-C-(iodomethyl)-D-allal] (200), is one of the products formed on treatment of methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-en/thro-hex-2-enopyranoside (77) with the Simmons-Smith reagent (diiodomethane and zinc-copper couple).123,212 Compound 200 displays high solvolytic reactivity, an observation that has been rationalized by supposing the formation of the highly stabilized carbonium ion213 (201). Thus, under conditions wherein methyl 2,3,4-tri-0-acetyl-6-deoxy-6-iodo-a-D-glucopyranoside required more than 24 hours to react appreciably with an excess of silver nitrate in 50% aqueous p-dioxane buffered with silver carbonate, the iodide 200 was hydrolyzed completely in less than 1 minute the product of hydrolysis of 200 is the cyclopropyl aldehyde 202. Methanolysis of... 

Ishihara and coworkers have reported that the reaction of 2-[(trimethylsilyl)methyl]-3-chloro-3,3-difluoropropene couples regioselectively with a variety of carbonyl compounds in the presence of zinc-copper chloride or silver acetate to give 2,2-difluoro-3-(trimethylsilyl)methyl-3-buten-l-ol derivatives (equation 87)81. Note again that the difluo-roallyl zinc species generated in situ reacts exclusively on the difluoromethylene terminus. 

Fluorine is simultaneously eliminated with other halogens if it is the only halogen vicinal to iodine, bromine or chlorine. The elimination tendency decreases in the series IF > BrF > C1F. The group CF2CHFX is relatively stable towards elimination and requires stronger reaction conditions. The dehalogenation of saturated fluorohalo compounds with zinc dust or zinc/ copper and zinc/silver couples is one of the most useful methods for the preparation of fluoroalkenes (see Table 8). Zinc/zinc(II) chloride has also been used instead of zinc alone. 

Conia and his co-workers144 modified the Simmons-Smith reaction145,146 by using enol ether plus predominantly diiodomethane/zinc-silver couple instead of the common known diiodomethane/zinc-copper couple147. This method is an excellent access to cyclopropanols and related compounds148 149 (Scheme 29). Furthermore,... 

The Simmons-Smith reaction often starts very slowly and proceeds quite slowly at room temperature. The method of the preparation of active zinc-copper couple has been improved 93, 196, 298, 376, 443, 466). The use of zinc dust/cuprous chloride instead of zinc-copper couple gives higher yields in the Simmons-Smith reaction 407). The reaction with homoallylic alcohols of low reactivity was facilitated by use of excess Simmons-Smith reagent and distillation of ether to a halfvolume followed by heating in a stainless steel tube 182). The Simmons-Smith reaction was successfully applied to vinyl trimethylsilyl ethers to prepare, after methanolysis, cyclopropyl alcohols unobtainable by other methods 109). Here, the zinc-silver couple was used instead of the zinc-copper couple, and the quenching of the reaction mixture of the Simmons-Smith reaction was improved by using pyridine under nonaqueous conditions. 

Improved reaction. Conia et al. have reported two modifications of the Simmons-Smith reaction which ave improved yields. One is the use of a zinc-silver couple in place of the zinc copper couple. This couple is prepared by adding granular zinc to a stirred hot solution of silver acetate in acetic acid. The mixture is stirred for 30 sec. and the zinc-silver couple formed is isolated by decantation and washed with acetic acid and ether. It is then stabilized by addition of a small amount of silver wool. The second improvement is that the reaction mixture is not subjected to acid hydrolysis. Instead an amine, for example pyridine, is added. This forms the insoluble complexes Znlj-C HsN and ICH jZnl -(C, 115N)j the cyclopropane products are then isolated from the filtrate. 

DIBAL-H/n-butyllithium, in cyclic and acyclic systems with iron pentacarbonyl, in cyclodecanes with lithium dihydrodimethoxyaluminate(III)/copper(I) iodide, and in cyclohexane and cyclopentane systems with NaH/sodium r-butylpentyl/Ni(OAc)2. ° The monoreduction of 1,3-diketones can be carried out under similar conditions, as illustrated by the reaction of a substituted cyclohexane-1,3-dione with oxalyl chloride to give the corresponding 1-chlorocyclohexenone, which was subsequently reduced to the enone with zinc-silver couple (Scheme 45). Kropp et al. have reported the photolytic reduction of vinyl iodides in acyclic systems however, when an a-hydrogen is present, formation of the diene product is a limiting side reaction (Scheme 46). For a more extensive discussion of vinyl halide reductions, see the preceding chapter in this volume. 

Thioketones, unlike ketones, undergo a coupling reaction on heating with certain powdered metals such as copper, iron, zinc or silver thus, the diaryl thioketone (6) yields the tetraarylalkene (31) (Scheme 18). 

As mentioned in Section 4.2, zinc couples such as zinc-copper and zinc-silver couples have frequently been used in organic synthesis due to their increased reactivity. Their use in the preparation of dialkyl zinc compounds has already been pointed out. To try to summarize all the reactions in which the use of zinc couples has been reported, even for just the past few years, would be much to ambitious for this review. Therefore, we shall just present some representative examples and key papers reporting the latest advances in new types of couples. 

Further examples can be found in ref470 (no details) 465,471 (Mg) 465 (Zn) 466 (Zn/Ag). Dispiro[2.1.2.1]oetane-4,8-dione derivatives are also obtained when mixed anhydrides between diethylphosphoric acid and 1-bromocyclopropanecarboxylic acids are exposed to zinc-silver couple and zinc-copper couples in tetrahydrofuran. ... 

The first report of the oxidative addition of zinc metal to organic halides dates back to the work of Frankland [1-4] around 1850. He discovered that dialkylz-inc compounds could be prepared from zinc metal and methyl iodide or ethyl iodide. However, the reaction did not proceed with alkyl bromides or chlorides. Also, no aryl halides were found to undergo the oxidative addition reaction. Several approaches have been reported since that time to increase the reactivity of the zinc metal. The majority of these modifications have employed zinc-copper couples [5-8] or zinc-silver couples. However, all of these procedures still only worked with alkyl iodides. Noller used a mixture of alkyl iodides and bromides but found that the mixture must contain a large percent of alkyl iodide [9]. 

DL-Mannitol (a-acritol) has been obtained by the reduction of a-acrose (see p. 104). Divinylglycol from acrolein was the starting point of Lespieau and Wiemann (4S). The glycol was obtained from acrolein by reduction with the zinc-copper couple and was then oxidized with silver chlorate and osmium tetroxide to DL-mannitol. Allitol was obtained simultaneously. It is apparent, therefore, that this divinylglycol must be a mixture of diols in which the hydroxyls are cis and trans. 

A zinc-silver rather than a zinc-copper couple is recommended and, rather than a final hydrolysis, addition of an amine is used to remove zinc salts. 

The detection and determination ot the perchlorates.—The perchlorates give no precipitates with silver nitrate or barium chloride soln. cone. soln. give a white crystalline precipitate with potassium chloride. Unlike all the other oxy-acids of chlorine, a soln. of indigo is not decolorized by perchloric acid, even after the addition of hydrochloric acid and they do not give the explosive chlorine dioxide when warmed with sulphuric acid unlike the chlorates, the perchlorates are not reduced by the copper-zinc couple, or sulphur dioxide. Perchloric acid can be titrated with —iV-alkali, using phenolphthalein as indicator. The perchlorates can be converted into chlorides by heat and the chlorides determined volumetrically or gravimetrically they can be reduced to chloride by titanous sulphate 28 and titration of the excess of titanous sulphate with standard permanganate they can be fused with zinc chloride and the amount of chlorine liberated can be measured in terms of the iodine set free from a soln. of potassium iodide and they can be... 

The most important activators for sulfide phosphors are copper and silver, followed by manganese, gold, rare earths, and zinc. The charge compensation of the host lattice is effected by coupled substitution with mono- or trivalent ions (e.g., Cl or Al3+). In addition, disorders, such as unoccupied sulfur positions, can also contribute to charge compensation. 

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