Zinc halide

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

Of these reductant metals, the most commonly used is zinc. The reason is that the zinc halides are more volatile than the parent metal and the chances of codeposition of the halides are minimized. Either chloride or iodide is used, although the iodide, being the most volatile, is usually preferred. The volatility of these halides decreases as one goes from the iodides to the chlorides to the fluorides. The reaction is as follows ... 

Zinc halides detonate strongly in the presence of potassium. 

C. C. Shin. Corrosion inhibiting composition for zinc halide-based clear, high density fluids. Patent WO 8802432,1988. 

Zinc halide complexes with acetophenone have been structurally characterized and form monomeric or polymeric structural motifs.346 The bromide and iodide derivatives are monomeric and the chloride derivative is a coordination polymer with bridging chlorides. 

The presence of halides in the coordination sphere with aldehydes is important as zinc halides are typically used in organic synthesis as organic carbonyl activators. Large excesses of aldehydes and anhydrous zinc halides were necessary. Both monomeric and polymeric structures with halide bridges were observed. Tetrahedral geometries were observed for mixed ligand, aldehyde, and halide complexes.353... 

A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

Heterocyclic nitrogen donors and their adducts with zinc chloride have been studied.623,624 A large number of other ligand systems have also been characterized, for example, zinc halide adducts of 2,2-dimethylpropane-1,3-diamine and hexamethylphosphoramide have been studied.625,626 The formation of mixed ligand complexes with chloride and substituted pyridines has been studied.627 The zinc tris(pyridyl) chloride anion has also been structurally characterized.628 Manganese(II) ions have been used to probe the stereochemistry in reactions of zinc halides with pyrazine.629... 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

Polymeric or monomeric complexes are formed in the reaction between zinc halides and dimethyl(aminomethyl)phosphine oxide dependent on the ratio of reactants. The ligand can bind as bidentate to one metal center or bridge two metal centers through the amine N and phosphine oxide O atoms.851... 

What has long been sought and badly needed is a soluble weighting agent less corrosive than, and equally non-damaging as, the popular calcium and zinc halides. Preliminary laboratory and field data indicate TKPP may be the solution. 

Although potassium chloride fluids performed better than the calcium and zinc halides, damage was still measurable. These results, confirmed in triplicate, were unexpected since it is well accepted that even 0.5 weight % KC1 should protect Berea cores against permeability damage. The most plausible explanation lies in variation between the test procedures. 

At the request of an international petroleum company, a major manufacturer and supplier of down-hole equipment performed tests of the various elastomers commonly used in the construction of packers and other oil field tools. Seven of the nine most commonly used thermoplastic materials were found to be completely inert to TKPP solutions. The test included continual immersion in saturated TKPP for 21 days at 280°F. Only two elastomers, Vi-ton and Fluorel, showed any adverse reaction. O-rings made from these two elastomers showed minor cracking at the termination of the test. A listing of the elastomers that tested inert to TKPP solutions include nitrile, saturated nitrile (HNBR), Aflas, Kalrez, PEEK, Glass-filled Teflon, and Ryton. Several of these elastomers are attacked or degraded by conventional clear completion fluids containing calcium and zinc halides. The inertness of commonly employed elastomers to TKPP is an important advantage for TKPP fluids in normal operations. 

The limitation to electron-rich alkenes in Rh(II)-catalyzed cyclopropanation with phenyldiazomethane leaves untouched the great versatility of zinc halides for this purpose with this, catalyst, efficient and very mild cyclopropanation of 1,3-dienes and unactivated alkenes has been reported 46). 

Zinc/copper couple (Zn/Cu), produced by the treatment of zinc with HC1, followed by the addition of copper(n) sulfate, has been known since the seminal work of Simmons and Smith.67 A less reactive form of activated zinc, but one that is sufficiently active for most applications, is produced by the treatment of the metal with 1,2-dibro-moethane, followed by Me3SiCl.68 The most reactive form of activated zinc, the so-called Rieke zinc (Zn ), is finely divided metallic zinc produced by the homogenous reduction of zinc halides in THF.69... 

Primary and secondary alkylzinc iodides and benzylic zinc halides also undergo Ni-catalyzed reactions with various primary alkyl iodides and bromides.407-409 According to the procedure by Knochel and co-workers, the transformations with alkylzinc iodides, which are less reactive than the corresponding dialkylzincs, require the presence of two additives Bu4NI and 4-fluorostyrene (Scheme 155).407,408... 

Zinc forms numerous organometallic compounds, with the dialkyls being the most important. Although these compounds do not associate to give aggregates, they are spontaneously flammable. The reaction of a zinc halide with a Grignard reagent can be used to prepare the compounds. 

Since the early disclosure by Negishi that zinc halide salts accelerate Pd(0) -catalyzed crosscouplings between vinyl zirconocenes and various halides [78], several methods have been developed that extend the utility of this metathesis process from a zirconium chloride to a zinc chloride (79 Scheme 4.47). Alternatively, routes to more reactive diorganozinc intermediates, e. g., using Me2Zn, convert readily available zinc derivatives to mixed species 80, which selectively couple with various electrophiles [14]. 

Methylenative dimerization takes place when terminal alkynes are treated with the tita-nocene/methylidene/zinc halide complex generated from titanocene dichloride and CH2(ZnI)2. The process is believed to involve the formation of a titanacyclobutene intermediate [75],... 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

In addition, the mechanism of the zinc-catalyzed [3+2] dipolar cycloaddition of azides and nitriles to form tetrazoles was examined <2003JA9983>. The energy barrier of the reaction is lowered by 5-6kcalmol 1 which corresponds to an acceleration of 3 1 orders of magnitude. The source of the catalytic activity seems to be the coordination of the Lewis acidic zinc halide to the nitrile, which is supported by model calculations. Also AICI3 was examined as another Lewis acid which catalyzes the reaction to a greater extent than ZnBr2-... 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brpnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(II) acetate [640-642], rhodium(II) carboxylates [626,643] and copper(I) triflate [636]. Copper(II)... 

Insertion of zinc dust into aryl or heteroaryl iodides is also possible, but polar cosolvents are required in some cases [48, 49]. The use of highly activated zinc (Rieke zinc) prepared by reduction of zinc halides with lithium results in faster insertion (Scheme 2.24) [50-52]. 

Scheme 10.7 Total synthesis of (S)-ibuprofen using aryl zinc halides.

Violent reactions can occur with many metal hahdes. For example, with zinc halides or iron halides, single replacement reactions take place. Such potassium-metal halide mixtures can react violently when subjected to mechanical shock. 

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