Acids Zinc bromide

E)-a.,fl-Unsaturated acids. Zinc bromide is the most effective Lewis acid for promoting a reaction of C,0,0-tris(trimethylsilyl) ketene acetal (1) with aldehydes (but not ketones) to form a,p-unsaturated acids. The ketene acetal can be prepared as shown in equation (I). 

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 cyclohexylidene protecting group has been employed in several syntheses. A preparation of 2,3-0-cyclohexylidene-4-deoxy-L-threose (445) fi om L-( + )-diethyltartrate (lb) in seven steps illustrates one synthetic application (Scheme 99). Conversion of the monobenzyl protected alcohol 443 to its tosylate followed by reduction with sodium borohydride provides the deoxy intermediate 444, which is reductively deprotected and Swem oxidized to 445 in good overall yield. Treatment with benzylamine provides an imine that undergoes a stereoselective carbon-carbon bond forming reaction with a-lithio-A, A -dimethylacetamide in the presence of the Lewis acid zinc bromide to furnish, after Cbz-amine protection, the j9-aminoamide 446. This is converted in four steps to A -acetyl-L-daunosamine (447), a sugar moiety particularly important as the carbohydrate constituent of the anthracycline antibiotics [149]. 

Sodium Hypochlorite Sodium Hypophosphite Sodium m-Bisulfite Sodium Pyrosulfite Nitric Acid Zinc Bromide Colloidal Sulfur Sulfur Ferric Chloride Ferric Choride Iron (III) Chloride Iron Perchloride Dipentene p-Menthadiene... 

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. 

Brom-wasserstoffsaure, /. hydrobromic acid, -wismut, tn. n. bismuth bromide, -zahl, /. bromine number, -zink, n. zinc bromide, -zinn, n. tin bromide. 

A salt derived from a hydracid is named according to the nonmetal present in the parent acid, and the salt will end in - ide . The metallic part of the salt is named first. The prefix hydro is dropped and suffix - ic (of the acid) is changed to - ide . HC1 HBr HCN H2S Sodium chloride (NaCl) Magnesium chloride (MgCl2) Potassium bromide (KBr) Zinc bromide (ZnBr2) Sodium cyanide (NaCN) Potassium sulfide (K2S)... 

Solutions of TKPP were mixed with aqueous fluids commonly encountered in drilling or completion of wells. Unlike saturated zinc bromide, concentrated TKPP solutions can be mixed in any proportion with fresh water with the only result being a decrease in solution density. Similar results were obtained with conventional oil field brines containing as much as 400 parts per million polyvalent cations, mostly calcium. Saturated solutions of calcium hydroxide also can be added to TKPP in any proportion without promoting precipitation as can concentrated hydrochloric acid solutions, conventionally used for well stimulation. The acid tends to generate a slight haze as the pH is reduced from 11.5 to approximately 8 however, this haze rapidly disappears as the pH is lowered by further addition of acid. 

Zinc plus hydrobromic acid yields zinc bromide plus hydrogen... 

Baldwin and coworkers82 studied the Diels-Alder reactions between dihydropyri-dinium ions and diene 77 with the aim to synthesize functionalized hydroisoquinolines. The reaction of diene 77 with dihydropyridinium ion 79, which was prepared in situ by treating 78 with zinc bromide, afforded 80. After acidic work-up, a mixture of methoxyke-tone 81 and enone 82 was obtained (equation 25). The reaction proceeded with complete exo selectivity. Without the addition of zinc bromide, no Diels-Alder reaction was observed. 

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. 

Zinc dust is frequently covered with a thin layer of zinc oxide which deactivates its surface and causes induction periods in reactions with compounds. This disadvantage can be removed by a proper activation of zinc dust immediately prior to use. Such an activation can be achieved by a 3-4-minute contact with very dilute (0.5-2%) hydrochloric acid followed by washing with water, ethanol, acetone and ether [/55]. Similar activation is carried out in situ by a small amount of anhydrous zinc chloride [156 or zinc bromide [157 in alcohol, ether or tetrahydrofuran. Another way of activating zinc dust is by its conversion to a zinc-copper couple by stirring it (180g) with a solution of 1 g of copper sulfate pentahydrate in 35 ml of water [/55]. 

Kinetic studies using the water-soluble nitrile li revealed first-order dependence in both nitrile and azide and one-half order dependence for zinc bromide. The mechanism of the addition of hydrazoic acid/azide ion to a nitrile to give a tetrazole has been debated, with evidence supporting both a two-step mechanism (Scheme 1, eq 2) and a concerted [2 + 3] cycloaddition (Scheme 1, eq 3). Our mechanistic studies to date imply that the role of zinc is not simply that of a Lewis acid a number of other Lewis acids were tested and caused little to no acceleration of the reaction. In contrast, Zn exhibited a 10-fold rate acceleration at 0.03 M, which corresponds to a rate acceleration of approximately 300 at the concentrations typically used. The exact role of zinc is not yet clear. 

The prominent role of alkyl halides in formation of carbon-carbon bonds by nucleophilic substitution was evident in Chapter 1. The most common precursors for alkyl halides are the corresponding alcohols, and a variety of procedures have been developed for this transformation. The choice of an appropriate reagent is usually dictated by the sensitivity of the alcohol and any other functional groups present in the molecule. Unsubstituted primary alcohols can be converted to bromides with hot concentrated hydrobromic acid.4 Alkyl chlorides can be prepared by reaction of primary alcohols with hydrochloric acid-zinc chloride.5 These reactions proceed by an SN2 mechanism, and elimination and rearrangements are not a problem for primary alcohols. Reactions with tertiary alcohols proceed by an SN1 mechanism so these reactions are preparatively useful only when the carbocation intermediate is unlikely to give rise to rearranged product.6 Because of the harsh conditions, these procedures are only applicable to very acid-stable molecules. 

Zinc bromide also may be prepared by the action of zinc with hydrobromic acid followed by crystaUization. 

Allylic zinc bromides also generally couple with ketones and acid anhydrides. In the case of trifluoroprenyl bromide, the attempts were only successful with ethyl pyruvate. However, it appears that this reaction provides the transposition product in 50% yield (equation 20). 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

Prepare zinc bromide and iodide in the same way, using hydro-bromic and hydroiodic acids instead of hydrochloric acid. 

Notes This alcohol protecting is easily attached and readily removed by Lewis acids such as zinc bromide and titanium tetrachloride. Phenols can be protected (reaction of the sodium salt with MEMC1) and deprotected with TFA. More easily removed than the MOM group. 

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... 

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