Elimination acid catalysed

A low-temperature study in superacid media of mono-, di-, and tri-protonated thiourea has been carried out. The experimental results were confirmed by theoretical calculations. Monoprotonation occiu s at sulfin and, whereas the mono- and di-protonated forms are thermodynamically stable, the triprotonated ion is only kinetically stable. The pyrolysis of A-acetylthiourea and iV,A -diacetylthiourea (335) are unimolecular first-order eliminations. Acid-catalysed ethanolysis of Nfl -dd- and tri-substituted aryl- and alkylaryl-thioureas gives O-ethyl 7V-aryl thiocarbamates and amines. The acid-catalysed hydrolysis of thiourea was first order in thiourea and acid. ... 

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

Phosphonoformic acid (85) decarboxylated in acid solution, and it was proposed that the uncatalysed reaction involved a simple decarboxylation of the zwitterion. The acid-catalysed reaction showed some kinetic similarity to that of mesitoic acid and an elimination of carbon dioxide as trihydroxymethylcarbonium ion was preferred. Participation of the trans vicinal phosphonyl group in the solvolysis of the halides (86) and (87) has been deduced from rate measurements. In the norbornene derivatives, the relative rates of loss of chloride from (87a) and (87b) were 5 x 10 1. 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

Recently organosilicon compounds are being used for the synthesis of olefines by elimination reactions both in acidic and basic conditions. Thus P hydroxysilanes give defines. These reactions have been shown to be highly stereospecific. The acid catalysed elimination taking place by an anti pathway and the base induced elimination taking place by a syn pathway. 

Several examples of aromatic hydrocarbon sensitized additions of NNP to the same arenes were demonstrated to occur if an acid is present this is in contrast to the failure of benzophenone to sensitize the photoreactions. Irradiation of anthracene in the presence of NNP and hydrochloric acid gives 308 in 70% yield and a small amount of 309 derived from the acid-catalysed elimination of piperidinium ion and addition of ethanol165 (equation 147). Anthracene possesses Es = 76.3 kcal mol 1,

[Pg.813]

The allylic cation (40), formed in a specific acid-catalysed process, is relatively stable thermodynamically, stable enough towards trapping by nucleophiles that the reaction product obtained is almost invariably the naphthalene elimination product. di-Enediynes (42) are formed regiospecifically when the allylic cation (41) is trapped as shown. The walking of methanol around optically active l-methyl-3-ethylallyl... 

The specific acid-catalysed solvolysis of l-methoxy-l,4-dihydronaphthalene or 2-methoxy-l,2-dihydronaphthalene has been subjected to kinetic and product studies. The elimination product, naphthalene, predominates. 

Specific acid-catalysed solvolysis of l-methoxy-l,4-dihydronaphthalene or 2-methoxy-l,2-dihydronaphthalene in 25% acetonitrile in water has been found to yield mainly the elimination product, naphthalene, along with a small amount of 2-hydroxy-1,2-dihydronaphthalene, there being no trace of either the 1-hydroxy-1,4-dihydronaphthalene or the rearranged ether. The nucleophilic selectivity, ns/ hoh = 2.1 X 10", between added azide ion and solvent water has been estimated for the relatively stable = 1 x 10 s ) intermediate benzallylic carbocation for which the barrier to dehydronation is unusually low k = 1.6 x 10 ° s ), as evidenced by the large elimination-to-substitution ratio with solvent water as base/nucleophile. The kinetics of acid-catalysed solvolysis of 1-hydroxy-1,4-dihydronaphthalene and 2-hydroxy-1,2-dihydronaphthalene have also been studied. 

Results of a study of acid-catalysed epimerization of indolo [2,3-a]quinolizidine derivatives support a mechanism involving nitrogen lone pairs in an eliminative ring opening-ring closure. ... 

Glycals, for example D-glucal (23), being vinyl ethers are susceptible to acid catalysed additions of alcohols which result in the formation of 2-deoxyglycosides, but elimination reactions accompany additions of this type 86) go such products are better prepared by way of halogen adducts or by the alkoxymercuration reaction (see below). On the other hand, the addition procedure can be more suitable than the alcoholysis... 

Aldehyde (106) and the hydroxamic acids (107) were generated together in an acid-catalysed elimination reaction (Scheme 20, pathway (i)). 

Alkenes are obtained hy dehydration of alcohols via elimination reactions (see Section 5.4.3), and esters are prepared conveniently by the acid-catalysed reaction of alcohols and carboxylic acids (see Section 5.5.5). 

Bimolecular dehydration is generally used for the synthesis of symmetrical ethers from unhindered 1° alcohols. Industrially, diethyl ether is obtained by heating ethanol at 140 °C in the presence of H2SO4. In this reaction, ethanol is protonated in the presence of an acid, which is then attacked hy another molecule of ethanol to give diethyl ether. This is an acid-catalysed Sn2 reaction. If the temperature is too high, alkene is formed via elimination. 

Squalene is an important biological precursor of many triterpenoids, one of which is cholesterol. The first step in the conversion of squalene to lanosterol is epoxidation of the 2,3-douhle bond of squalene. Acid-catalysed ring opening of the epoxide initiates a series of cyclizations, resulting in the formation of protesterol cation. Elimination of a C-9 proton leads to the 1,2-hydride and 1,2-methyl shifts, resulting in the formation of lanosterol, which in turn converted to cholesterol by enzymes in a series of 19 steps. 

Two mechanisms (i.e. direct hydrolysis and alternatively a path via an unstable acyl phosphate intermediate) are involved in the hydrolysis in phosphate buffer of N-arylsulfonyl / -lactams such as (130).107 The acyl phosphate intermediate can be trapped with hydrazine. The alkaline hydrolysis of some torsionally distorted lactams, i.e. the bridged benz[rfe]isoquinolin-l-ones (131), in 70% (v/v) DMSO-water has been compared under the same conditions with the hydrolysis of AvA-dimethyl-1 -naphthamide (132). The relative rates of reaction and activation parameters indicate the effect of torsional distortion.108 The reaction of the tricyclic azetidinones (133) with trifluoroacetic acid gives the bicyclic thioesters (135). The mechanism may involve acid-catalysed elimination of methanethiol to give an azetinone intermediate (134) which, after nucleophilic attack of the thiol, is converted into (135).109... 

The nitrosation of phenol and cresols in buffer solutions involves a diffusion-controlled C-nitrosation followed by rate-limiting proton loss. /r-Crcsol is much less reactive than the other substrates.79 Nitrosation in trifluoroacetic acid or in acetic-sulfuric acid mixtures is regioselective (e.g. 4-nitroso-m-xylene is fonned from m-xylene) and possible non-selective nitrous acid-catalysed nitration can be eliminated by purging reaction solutions with nitric oxide.80... 

The stereochemistry of /1-elimination reactions catalysed by D-galactonate dehydratase (GalD) and D-glucarate dehydratase (GlucD) enzymes is apparently not dictated by the pKas of the 7-protons of the carboxylate anion substrates.74 It had been observed previously that enzyme-catalysed dehydration initiated by abstraction of the a-proton (P a > 29) from a carboxylate anion substrate usually proceeds via anti elimination, whereas syn elimination occurs when the proton is a- to an aldehyde, ketone, or thioester and correspondingly more acidic (pKa < 25). 

A direct comparison of exo-3-trimethylsilyl-ewrfo-2-norbomyl mesylate 14 with 12 and 13 could not be obtained because of solubility problems. However, measurements of the acid catalysed elimination of the corresponding alcohols suggested that 14, which has an anticlinal stereochemistry (dihedral angle = 120°), has a /i-effect of a similar or slightly larger magnitude to that in 12. 

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