Nitration Lewis-acid catalyzed

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

The direct nitration of calix[4]arene, obtained by Lewis acid catalyzed dealkylation of ferf-butylcalix[4]arene, with HNO3/ACOH in benzene, however, has been reported to give 88% yield of / ara-nitrocalix[4]arene (equation 52) . The method has been extended to other calix[n]arenes (n = 4, 6, 8), providing a convenient one-step method for the preparation of / ara-nitrocalix[n]arenes . Calix[n]arenes have also been directly nitrated with KNO3/AICI3 to give / ara-nitrocalix[n]arenes in good yields . 

Both protic- or Lewis-acid-catalyzed nitration of aromatics can be carried out with a/ky/ nitrates (i.e., alkyl esters of nitric acid). Acid catalysts are assumed to form nitronium ion from alkyl nitrates or strongly polarized complexes. 

There is always a certain amount of ring-chlorinated by-product formed in the nitrations. Reactions carried out either by using an excess of aromatics as solvent (TiCU is miscible with many aromatics) or in carbon tetrachloride solution, always contain chlorinated by-products. The amount of chlorinated by-products can be decreased by using solvents with higher dielectric constants. Tetramethyiene sulfone (sulfolane) was found to be a suitable solvent for the TiCL and also for most of the other Lewis-acid-catalyzed nitrations. It has excellent solvent properties for aromatics and the catalysts as well as for nitryl halides. It is superior to other solvents that can be used, such as nitromethane. As it is completely miscible with water, the work-up of the reaction mixtures after the reactions are completed is very easy. 

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

A reaction in which an electrophile participates in het-erolytic substitution of another molecular entity that supplies both of the bonding electrons. In the case of aromatic electrophilic substitution (AES), one electrophile (typically a proton) is substituted by another electron-deficient species. AES reactions include halogenation (which is often catalyzed by the presence of a Lewis acid salt such as ferric chloride or aluminum chloride), nitration, and so-called Friedel-Crafts acylation and alkylation reactions. On the basis of the extensive literature on AES reactions, one can readily rationalize how this process leads to the synthesis of many substituted aromatic compounds. This is accomplished by considering how the transition states structurally resemble the carbonium ion intermediates in an AES reaction. 

Nitryl chloride reacts with many organics forming their nitro derivatives. Such Friedel-Crafts nitration is catalyzed by a Lewis acid, such as AICI3. An example is nitration of benzene to nitrobenzene ... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

N,N-Methyltosylhydrazones. Ketones react only slowly with N,N-methyl-tosylhydrazine (1) but hydrazones of this type can be prepared by N-alkylation of tosylhydrazones with phase-transfer catalysis. The more reactive thioketones also react sluggishly with 1, but in this case, the reaction can be catalyzed by soft Lewis acids. Thus the reaction of 2 with 1 proceeds in high yield at room temperature in the presence of 1 equiv. of silver nitrate. Mercuric acetate also promotes this reaction, but the yield of 3 is only 50% because of formation also of 4 in 44% yield. 

Similarly, the A -tosylaziridine 14 is smoothly cleaved by aniline in the presence of bismuth trichloride acting as the Lewis acid to give the diamine 15 <2003SC547>. Ceric ammonium nitrate (CAN) catalyzes the ring opening of 14 with water to afford the amino alcohol 16 in 88% yield (Scheme 6) <2003CL82>. 

As for surfactants, they have uncertain, sometimes contradictory, consequences on reaction rates [45], but the main advantage of using surfactants as additives lies in their solubilizing effect. Special attention has been paid to the rate-accelerating effect of Lewis acid catalysts. The first study deals with the Diels-Alder reaction between cyclopentadiene and a bidentate dienophile a large acceleration can be achieved by the combined use of copper(II) nitrate as catalyst and water as solvent. The rate enhancement imposed on the catalyzed Diels-Alder reaction is much less pronounced than that for the uncatalyzed reaction... 

Additives that can be added to aqueous reactions include Lewis acids, which have roles as catalysts in organic transformations, mainly in Diels-Alder reactions [24]. A number of Lewis acids which can be used in water have been described, such as nitrates, for example, Cu(N03)2 and Zn2 +, Ni2 +, Co2+ analogs [25], lanthanide triflates, Ln(OTf)3 [26], and others, including indium trichloride [27]. Increased yields and product selectivities have been observed in several systems. A typical example is the three-component hetero-Diels-Alder reaction catalyzed by lanthanide triflate (Equation 4.14). Lanthanide triflates were used in the pH range 5-7, and when no Ln (OTf)3 was added, the product was isolated in only 4% yield however, with added lanthanide catalyst the yield was increased to 64% [28]. 

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

In Lewis acid halide catalyzed nitrations with nitryl chloride the question arises are these reactions nitronium salt nitratiors according to the ionization... 

LEWIS ACID HALIDE CATALYZED FRIEDEL-CRAFTS NITRATION OF BENZENE AND TOLUENE WITH NITRYL CHLORIDE AT 25°... 

Nitration with nitric acid in the presence of strong protic acids such as H2SO4, FSO3H, and CF3SO3H or Lewis acids such as boron trifluotide requires subsequent separation of spent acid (due to water formed in the reaction) and neutralization of acid left in the product. One is generally left with a large amount of dilute acid for disposal, which is neutralized in the case of sulfuric-acid-catalyzed nitrations to a mixture of ammonium nitrate and ammonium sulfate. By using a solid acid catalyst most of these environmental problems can be eliminated. The solid acid catalyst is simply separated and recycled for subsequent use. 

In order to study this problem, Olah and Lin carried competitive studies of nitration of benzene and toluene with nitry chloride, catalyzed by Lewis acid halides. When carbon tetrachloride or excess aromatics were used as solvent, the data summarized in Table XII were obtained. The data show that the ortho para ratios are smaller than in nitrations with nitio-nium salts. The observed changes point to the fact that the nitrating agents arc the conesponding donor acceptor complexes and not the nitronium ion itself. The lower orthojpara ratios than those obtained in case of NOj, particularly point to bulkier nitrating agents. 

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