Zinc bromide catalyst

Zinc chloride was used as a catalyst in the Friedel Crafts benzylation of benzenes in the presence of polar solvents, such as primary alcohols, ketones, and water.639 Friedel-Crafts catalysis has also been carried out using a supported zinc chloride reagent. Mesoporous silicas with zinc chloride incorporated have been synthesized with a high level of available catalyst. Variation in reaction conditions and relation of catalytic activity to pore size and volume were studied.640 Other supported catalytic systems include a zinc bromide catalyst that is fast, efficient, selective, and reusable in the /wa-bromination of aromatic substrates.641... 

Technical grade zinc cyanide was used as supplied by Matheson, Coleman and Bell. Other Lewis acids, notably aluminum chloride, zinc bromide, and zinc iodide may be used as catalysts for the reaction. 

Note. With silyl ketene acetals, anhydrous zinc bromide (lmol%) is the preferred catalyst. 

Alkylzinc halides have also been prepared under microwave irradiation. The Reformatsky reagents (2-t-butoxy-2-oxoethyl)zinc bromide and [(2-dibenzylamino)-2-oxoethyl]zinc bromide were synthesized from the corresponding bromides via reaction with zinc in THF (Scheme 5) [24], The oxidative addition was executed at 100 °C in 5 min. The obtained reagents were subsequently used in Negishi reactions on 2-bromopyridine, 3-bromopyridine, 2-bromo-5-nitropyridine, and 2-bromo-5-trifluoromethyl-pyridine using Pd(PPh3)4 as a catalyst (Scheme 5). 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

Russian chemists [228] found that trimethylsilyl groups protect adjacent triple bonds against hydrogenation with poisoned Pd-catalysts. A similar effect is shown in reductions of trimethylsilylated 1,3-diynes with (activated) zinc powder [226]. One disadvantage of the zinc method is that the zinc salts present in the reaction mixture can cause cleavage of the =C-Si bond (this was shown in a separate experiment in which a trimethylsilylated 1,3-diyne was heated with a solution of zinc bromide or chloride in ethanol [2]). It seems therefore important to keep reaction times of the reductions with zinc as short as possible and to activate the zinc powder with a limited amount of dibromoethane. 

Hydrolysis of the enamine 14 furnishes citronellal (15) in high optical purity (ca. 99% ee) which gives 17 via ene cyclization with zinc bromide as catalyst. The diastereoselectivity of this step is the result of simple diastereoselection in a trans-decalin-like transition state 16. Catalytic hydrogenation converts the olefin 17 into (—)-menthol (18). Despite its elegance this novel route has not been able to replace the older resolution-based procedure described earlier in this section. 

Under these standard reaction conditions, the acetonitrile/pyridine mixture can replace the DMF/pyridine one. This solvent mixture is also quite convenient for running the preparation of arylzinc halides. Yields are good to excellent (60-90%) and even higher than those obtained in DMF. However, with bromophenol, no organozinc species was formed in acetonitrile as observed in DMF. The formation of arylzinc species is also effective in a mixture of solvents of acetonitrile-DMF-pyridine (8/1/1) in the presence of C0CI2 (13%) as the catalyst precursor and zinc bromide (30%). This method has also been applied to the formation of organozinc halides from alkyl and alkenyl halides. So far, only low yields have been obtained using the standard reaction conditions in DMF-pyridine. Results are reported in Table 8. 

Methanol can be converted to hydrocarbons over acidic catalysts. However, with the exception of some zeolites, most catalysts deactivate rapidly. The first observation of hydrocarbon formation from methanol in molten ZnCl2 was reported in 1880, when decomposition of methanol was described to yield hexamethylbenzene and methane.414 Significant amounts of light hydrocarbons, mostly isobutane, were formed when methanol or dimethyl ether reacted over ZnCl2 under superatmo-spheric pressure.415 More recently, bulk zinc bromide and zinc iodide were found to convert methanol to gasoline range (C4-C13) fraction (mainly 2,2,3-trimethyl-butane) at 200°C with excellent yield (>99%).416... 

Zinc chloride is much less reactive than aluminum chloride and usually requires higher reaction temperatures. However, it has the advantage that, unlike aluminum chloride, it is less sensitive to moisture and can sometimes even be used in aqueous media.88 Concentrated aqueous solutions of ferric chloride, bismuth chloride, zinc bromide, stannous chloride, stannic chloride, and antimony chloride are also alkylation catalysts, particularly in the presence of hydrochloric acid.89... 

Alicyclic hydrocarbons, Six-membered rings (Continued) aromatic rings Palladium catalysts, 230 Raney nickel, 265 Cyclohexanes by other routes /-Butyllithium, 58 Menthol, 172 Potassium /-butoxide, 252 Zinc bromide, 349... 

The zinc cation gives by far the most active catalyst. Iron, cobalt, and nickel cations also gave salts with considerable catalytic activity. Cadmium, because of its chemical similarity to zinc, and aluminum, because of its use in other epoxide polymerization catalysts, were considered as likely candidates to give active catalysts. However, complexes of the salts of these cations were only slightly catalytic. The salts used as cation sources in catalyst preparations also affected catalytic activity. Zinc salts, especially zinc chloride and zinc bromide, were retained in considerable amounts in the finished complexes, and the use of these salts gave the most active catalysts. 

Xie H, Li S, Zhang S (2006) Highly active, hexabutylguanidinium salt/zinc bromide binary catalyst for the coupling reaction of carbon dioxide and epoxides. J Mol Catal A Chem 250(l-2) 30-34... 

Chelating agents such as triethanolamine borate also behave as latent catalysts at elevated temperatures. Other boron-containing compounds, cadmium or zinc bromide dieth-ylenetriamine, and salts of aluminum acetoacetic ester have also been suggested as curing agents for high-temperature epoxy adhesives. 

The reaction time required depends on the catalyst. Zinc iodide, zinc cyanide, and zinc bromide produce essentially complete conversion under these conditions in approximately 16.5, 28 and 30 hr, respectively, probably reflecting solubility differences. When zinc iodide is used, the distilled product is often colored because of the formation of small amounts of Iodine. 

In order to form double reversible reactions in a one-pot process, amine D was added to aminonitrile 29A, followed by zinc bromide as catalyst as shown in Scheme lib. The expected product 29D was formed and detected by NMR... 

The resulting dynamic aminonitrile systems were first subjected to lipase mediated resolution processes at room temperature. A-Methy] acetamide was observed as a major product from the lipase amidation resolution. In this case, free methylamine A was generated during the dynamic transimination process and transformed by the lipase. To avoid this by-reaction, the enzymatic reaction was performed at 0 °C, and the formation of this amide was thus detected at less than 5% conversion. To circumvent potential coordination, and inhibition of the enzyme by free Zn(II) in solution [54], solid-state zinc bromide was employed as a heterogeneous catalyst for the double dynamic system at 0 °C. The lipase-catalyzed amidation resolution could thus be used successfully to evaluate /V-substituted a-aminonitrile substrates from double dynamic systems in one-pot reactions as shown in Fig. 7d. Proposedly, the heterogeneous catalyst interfered considerably less or not at all in the chemo-enzymatic reaction because the two processes are separated from each other. Moreover, the rate of the by-reaction was reduced due to strong chelation between the amine and zinc bromide in the heterogeneous system. 

To obtain the best lipase-catalyzed amidation resolution, the lipase operational conditions, i.e., additives, lipase preparations, acyl donors, and solvents, were further evaluated. When molecular sieve 4 A was added to control the water content in the enzymatic resolution, decompositions of aminonitrile intermediates were observed. Among a range of lipases, the resolution process by lipase PS-C I provided the highest conversion of amide products. Phenyl acetate 37 was chosen as acyl donor because its reaction led to marginal by-reactions. Thus, the lipase-catalyzed amidation resolution of the dynamic aminonitrile systems in the presence of zinc bromide as heterogeneous catalyst was performed by lipase PS-C I and phenyl acetate as acyl donor in dry toluene at 0 °C. 

This reaction has been extended to a similar alkylation with more reactive primary and secondary alkyl halides, such as benzylic and allylic halides. For this purpose the milder Lewis acid zinc bromide is generally preferable to titanium(IV) chloride as catalyst. ... 

Phenylthioalkylation of silyl enol ethers. Silyl enol ethers of ketones, aldehydes, esters, and lactones can be alkylated regiospecifically by a -chloroalkyl phenyl sulfides in fhe presence of a Lewis acid. Zinc bromide and titanium(IV) chloride are the most effective catalysts. The former is more satisfactory for enol ethers derived from esters and lactongs. ZnBr2 and TiCL are about equally satisfactory for enol ethers of ketones. The combination of TiCL and Ti(0-f-Pr)4 is more satisfactory for enol ethers of aldehydes. Since the products can be desulfurized by Raney nickel, this reaction also provides a method for alkylation of carbonyl compounds. Of more interest, sulfoxide elimination provides a useful route to a,B-unsaturated carbonyl compounds. 

The cleavage of tetrahydrofuran and its alkylated derivatives with halogen acids is an excellent method for the preparation of, A-dihalo-alkanes.The reaction of tetrahydrofuran with the less-reactive hydrogen chloride stops at the chlorohydrin stage, whereas the reaction in the presence of zinc chloride catalyst leads to the formation of the dichloride. The crude reaction mixture containing the intermediate chlorohydrin may be treated directly with phosphorus tribromide, yielding tetramethylene chlorobromide. The preparation of dibromides can be accomplished easily with hydrogen bromide or phosphorus and bromine and diiodides, by the action of potassium iodide and orthophosphoric acid. ... 

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