Zinc-lead couple

Not only a zinc-copper couple, but also a zinc-lead couple forms a gem-dimetal species from diiodomethane, according to the 1975 Nysted patent5. He also insisted that treatment of dibromethane with a zinc-lead couple in THF at 80 °C forms a characteristic gem-dizinc species 1 (equation 3). However, there was no further evidence concerning the structure except H NMR data, which was not enough for the complete structural determination. The obtained compound was definitely a gem-dizinc species, but the written structure 1 was not fully characterized. The white solid 1 is obtained as a dispersion in THF, and does not dissolve in DMF and DMI (l,3-dimethyl-2-imidazolidinone). This THF dispersion is commercially available from Aldrich Co. as Nysted reagent. Nysted also showed that this dizinc compound is effective for the methylenation of a-hydroxy ketone moiety in steroid derivatives5. 

The Troc group played an important role in Boger s synthesis of Teichopla-nin.243 Considerable effort was expended in optimising the conditions for its removal An example is the deprotection shown in Scheme 8.109. Various metals were used as reductants such as zinc, cadmium and cadmium-lead couple, but the best results (80% yield) were obtained using zinc-lead couple in M aqueous ammonium acetate in THF at room temperature. Zinc-lead couple is easily prepared. Yellow lead oxide is dissolved in warm aqueous acetic acid (1 1) and the solution added to a vigorously stirred suspension of zinc dust in deionised water. The zinc darkens as lead deposits on its surface, and forms clumps that... 

Insertion of zinc into the carbon-halogen bond of organozinc carbenoids is accelerated by the presence ofa catalytic amount of Pb(II) salts [40]. By this means, several l,l-(biszincio)alkane reagents such as 14,15 [41], or 16 [42] can be prepared (Scheme 4.4). The commercially available Nysted reagent 17 is also prepared through zinc insertion with zinc-lead couple [43]. In the case of aUcyl-substituted l,l-(biszincio)aIkanes, P-hydride elimination is prevented with the adjunction of TMEDA (N,N,N, N -tetramethylethylenediamine) [44]. 

Most electrodes are metallic. Sometimes the metal of an electrode can also be one component part of a redox couple. Good examples include metallic iron, copper, zinc, lead or tin. A tin electrode forms a couple when in contact with tin(IV) ions, etc. Such electrodes are called redox electrodes (or non-passive). In effect, a redox electrode has two roles first, it acts as a reagent and, secondly, it measures the energy of the redox couple of which it forms one part when connected to a voltmeter. 

Reduction of quinolines in acid solution at a lead cathode or by dissolving zinc leads to attack on the heterocyclic ring with the formation of 4,4-coupled products, together with the tetrahydroquinoline [82,83]. In the case of 2- and 4-methyl substituted quinolines, dimeric products are obtained in 10 90 % yields. In these processes, dimerization of the one-electron addition product is in competition with further reduction to give the 1,4-dihydroquinoline, The latter is an enamine and it... 

DeCamp et al.t19l synthesized the lactone intermediate of the 1-hydroxyethylene isostere with high yields and stereoselectivity. As summarized in Scheme 10 (Section 10.6.2), the titanium homoenolate is prepared from ethyl 3-iodopropionate. The iodide is metalated with zinc/copper couple to give the iodozinc homoenolate species. The alkyltitanium homoenolate is then generated by transmetalation of the iodozinc precursor with one of the several chlorotitanium isopropoxide species. The resulting titanium homoenolate reacts with a N-protected a-amino aldehyde, leading to a mixture of 45-diastereomers. In the last step, the product is lactonized. 

Phosphomolybdic acid-Potassium di-chromate-Copper(II) sulfate, 248 Tin(II) trifluoromethanesulfonate, 301 Other unsaturated carbonyl compounds Lead tetraacetate, 155 Pyrylium perchlorate, 263 Zinc-copper couple, 348 a-Substituted-a, p-unsaturated carbonyl compounds... 

Initial reports leading to speculation about non-linear or branched polymeric connectivity were provided by researchers such as Zincke,121 who isolated an insoluble hydrocarbon-based material upon the treatment of benzyl chloride with copper. Later, in 1885, reaction of benzyl chloride with aluminium chloride led Friedel and Crafts131 to report similar observations. When benzyl chloride was subjected to the action of a zinc-copper couple analogous materials were obtained.141... 

Adamantylideneadamantane has been prepared by (1) photolysis of 2-adamantylketene dimer,2 (2) reduction of 4< -chloroadaman-tylideneadamantane with sodium in liquid ammonia,3 (3) rearrangement of spiro[adamantane-2,4 -homoadamantan-5 -ol] with Lewis acids,4,5 (4) reduction of 2,2-dibromoadamantane with magnesium6 or zinc-copper couple,7 and (5) treatment of the azine of 2-ada-mantanone with hydrogen sulfide, followed by oxidation with lead tetraacetate and heating with triphenylphosphine.8... 

Methylene ( CH2) addition to a double bond has been achieved by use of the Simmons-Smith reagent (CH2I2 and a zinc-copper couple) f 70J. The reaction gives a cyclopropane derivative by stereospecific cis addition, and in allylic or homo-allylic alcohols leads to methylene addition ci to the hydroxyl function. Examples in the steroid field include a s-methylene addition to the double bonds of the ga-hydrdxy-A dO), 3j8-hydroxy-AJ- and 3 a- and 3/9-hydroxy-A4 systems... 

Many other 1,3-dipolar cycloadditions are known, amongst which is the addition of diazomethane (CH2N2). Expulsion of nitrogen from the adduct leads to the formation of a cyclopropane ring. Another way of achieving the same result involves the addition of a carbene such as the Simmons Smith reagent. This is generated from methylene iodide (CHjIj) and a zinc/copper couple. 

One synthesis of cyclopentenone [80], requiring a resolution, involved initial ring contraction of phenol when treated with alkaline hypochlorite (49). Resolution of the resulting cis acid [85] was effected with brucine. The desired enantiomer [86] formed the more soluble brucine salt and was thus obtained from the mother liquors of the initial resolution. Oxidative decarboxylation with lead tetracetate, partial dechlorination with chro-mous chloride, and alcohol protection gave chloro enone [87]. Zinc-silver couple (50) dechlorinated [87] to the desired cyclopentenone [80]. 

Intramolecular Reformatsky-type reactions a-methylene-t-lactones. Reaction of the (Z)-bromoaldehyde (1) in a dilute solution of THF with zinc dust at 65° or with zinc-copper couple at 25 leads to the c/i -fused a-methylene-y-lactone (2) in 60-62% yields. The reaction involves an intramolecular Reformatsky-type reaction followed by spontaneous lactonization. The same product (2) is also obtained from the (E)-isomer of (1) in 46% yield together with the transfused isomer of (2). The reaction of (1) with bis(l,5-cyclooctadicnc)nickel (4, 33-35 5, 34-35) also yields (2) in 527 yield. It is obtained in comparable yield... 

PRACTICE PROBLEM 8.19 Treating cyclohexene with 1,1-diiodoethane and a zinc—copper couple leads to two... 

E. Davy discovered nitrosyl chloride, which he called chloronitrous gas and prepared from aqua regia. He thought it was formed by the combination of equal volumes of nitric oxide and chlorine without condensation , which would not agree with the formula NOCl, and the density (1-759 air = i, or 25 33 for H = i, instead of 32-7) he found was too low. He described an electrolytic method (deposition on platinum foil by a zinc-platinum couple) for the detection of arsenic, mercury, lead, and copper in poisoning, worked on corrosion of iron by sea water and on nicotin , which had been dis-... 

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