Zinc bromide, supported

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 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... 

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. 

Naturally occurring (l S, 25, 3i )-4-hydroxymethylcyclopent-4-ene-l,2,3--triol (950) plays a central role in the ability of a non-aristeromycin producing mutant strain of Streptomyces citricolor to support production of both aristeromycin and neplanocin. Swem oxidation of readily available 13 from L-tartaric acid provides the aldehyde 943 which, when treated with an excess of propargyl zinc bromide, leads to a 2.3 1 diastereomeric mixture of acetylenic alcohols 944. Silylation of the hydroxyl group with TBSOTf and subsequent saponification of the ester group yields the carboxylic acid 945 in 74% overall yield from 13. Interestingly, Dess-Martin oxidation of 943 provides the allenic ketone 946, which is unstable to base and cannot be used in the subsequent radical cyclizations. 

Triazole-containing compounds exhibit various biological effects, such as antiviral (Moorhouse and Moses, 2008), antibacterial (Hon et al., 2012), antifungal (Agalave et al, 2011), and anticancer (Jordao et al., 2009). Heravia et al. (2006) developed an easy and eco-friendly method for the synthesis of thiazolo[3,2-fc]l,2,4-triazoles using SSA as an efficient catalyst. This method involved the condensation of 3-mercapto-l,2,4-triazoles with allyl bromide. Keivanloo et al. (2013) also reported silica-supported zinc bromide as an efficient heterogeneous catalyst for the one-pot synthesis of 4,5-disubstituted-l,2,3-(NH)-triazoles. The reaction involved the cross-coupling of 1,3-dipolar cycloaddition with various acid chlorides, terminal alkynes, and sodium azide in the presence of silica-supported zinc bromide under aerobic conditions. The reaction is described in Scheme 6.34. 

Methylenemalonate Esters. In the presence of Zinc Bromide, 1, 2, and 3 react with di-i-butyl methylenemalonate to afford cyclopropane [2+1] and cyclobutane [2 + 2]-cycloadducts (eq 9) in combined yields varying from 54 to 94%. Based on the stepwise mechanistic reasoning presented above, it is thought that bifurcation occurs in the ring closure step. This hypothesis was supported by the fact that treatment of pure cyclobutane under the described reaction conditions shows partial isomerization to the cyclopropane. 

A PRP -1 (Hamilton Reno, NV) reversed phase column was coated with cetylpyridinium and eluted with tetramethylammonium salicylate acetoni-trile water.89 The separation was comparable to that observed on conventional ion exchange. Coated phases were also used to separate oxalate complexes of manganese, cobalt, copper, and zinc.90 Reversed phase silica supports were coated with poly(N-ethyl-4-vinylpyridinium bromide), poly(dimethydiallylammonium chloride), poly(hexamethyleneguanidinium... 

This last electrochemical process is carried out in an undivided electrolysis cell fitted with a sacrificial magnesium anode and a nickel foam as cathode. The reaction is conducted in dimethylformamide in the presence of both NiBr2(bpy) as the catalyst and dried ZnBr2 (1.1 molar equivalents with respect to bromothiophene), which is used both as supporting electrolyte and as a zinc(II) ion source. The other conditions are the same as those described in the section concerning the aromatic halides. The yield of 3-thienylzinc bromide was roughly 80%, as determined by GC analysis after treatment with iodide (equation 34). 

Enholm [26] has reported the first examples of asymmetric radical cy-clizations on soluble polymer supports. The stereocontrol element employed consists of a (+)-isosorbide group attached by a 4-carbon chain to each subunit of a soluble succinimide-derived ROMP backbone. Treatment of the radical cychzation substrate 162 with tributyltin hydride in the presence of zinc chloride followed by hydrolysis of the resulting polymer-supported ester 163 gave the desired product 164 in 80% yield and > 90% ee (Scheme 38). The use of alternative Lewis acids, such as magnesium bromide etherate and ytterbiiun (III) triflate, resulted in lower enantioselectivities, 84% and 72% respectively. No such decrease in selectivity was observed in analogous reactions carried out off-support [27], suggesting that the polymer backbone is somehow responsible for this phenomenon. 

Methylations with methyl iodide were observed to proceed with high yields and stereoselectivities. Longer-chain alkyl iodides failed in most attempts. Allyl bromide reacts smoothly - however, products of low enantioenrichment (see 146g) result. We explain the fact by a single electron transfer (SET) during the alkylation. The intermediate formation of a mesomerically stabilized allyl radical supports the SET pathway [89]. A solution to this problem was most recently published by Taylor and Papillon who converted a lithio carbamate into the corresponding zinc cuprate prior to allylation [90]. Studies on the stereochemistry in a few metal-exchange reactions have been published by Nakai et al. [91]. 

C-metalated species is normally referred to as the Reformatsky reagent. These two species may further aggregate to form dimer by elimination of alkoxyzinc bromide, though the C-metalated species is the active one in this reaction. This assertion is supported first by the partial recovery of ketone during the complete consumption of zinc when an equivalent zinc and ketone are used and second, by the titration experiment on ethyl a-bromoisobutyrate, in which 70% active reagent and 30% dimeric substance were identified. According to these experimental results, an illustrative mechanism is proposed here using acetone and methyl a-bromoacetate as an example. 

Studies of the Zn VZn redox couple in aqueous solutions containing bromide have indicated that the presence and concentrations of other supporting electrolytes strongly affects the kinetics and behavior of zinc cations [20]. If electrodes with high porosity are used on the zinc side, it is necessary to consider the hydration structure of zinc cations in aqueous solutions as it will affect the way electrode... 

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