Acid-catalyzed cleavage

The principal mechanism of polymer degradation during aging is the acid-catalyzed cleavage of the ether linkage in the backbone. The acid acceptor. 

Acid catalyzed cleavage of aromatic methyl or ethyl ethers Quantitative methoxy group determination Also ether cleavage with tnmethylsilyl Iodide ... 

The cleavage proceeds by initial reduction of the nitro groups followed by acid-catalyzed cleavage. The DNB group can be cleaved in the presence of allyl, benzyl, tetrahydropyranyl, methoxy ethoxy methyl, methoxymethyl, silyl, trityl, and ketal protective groups. 

An 6>-nitrobenzyl ether.can be cleaved by photolysis. In tyrosine this avoids the use of acid-catalyzed cleavage and the attendant conversion to S-beirayltyrosine. (Note that this unwanted conversion can also be suppressed by the addition of thioanisole see section on benzyl ether cleavage.)... 

SiCl4, TFA, anisole. SiCl4 serves to reduce the sulfoxide prior to acid-catalyzed cleavage. Other sulfoxide reducing agents could probably be used. 

The conjugated enone (177) is treated withp-toluenesulfonic acid in refluxing toluene to form the more stable product (178). The A -17-keto-system is formed by acid catalyzed cleavage of the A -17-ketal (see page 304), but the conditions are not drastic enough to cause equihbration to the more stable A " -compound. (The latter may be ketalized to form the A -17-ketal.) ... 

The acid-catalyzed cleavage of acetals and ketals is greatly influenced by the substitution on the acetal or ketal carbon atom. The following values for k illustrate the magnitude of the effect ... 

The mechanisms for hydrolysis of 0,5-acetals have been reviewed. The following acid-catalyzed cleavage rates show that the 0,5-acetals have a stability that lies between thioacetals and acetals ... 

Write the mechanism of the acid-catalyzed cleavage of tert-butyl cyclohexyl ether to yield cyclohexanol and 2-methylpropene. 

Ethers undergo an acid-catalyzed cleavage reaction when treated with the Lewis acid Blh at room temperature. Propose a mechanism for the reaction. 

Table 1. Rate constants k and equilibrium constants K for the acid-catalyzed cleavages of (XMe2Si)20 with ROH...

Under conditions similar to those for (XMe2Si)20, the acid catalyzed cleavages with alcohols of linear and cyclic methylsiloxanes have been investigated. The rate constants of the primary reaction, the cleavage of the first Si-O-Si-bond, determined by GLC and HPLC, are given in Table 2. 

Values of pA"R for the addition of water to carbocations to give the corresponding alcohols. The equilibrium constants KR (m) were determined as the ratio Hoh/ h> where fcHOH (s 1) is the first-order rate constant for reaction of the carbocation with water and H (m 1 s ) is the second-order rate constant for specific acid-catalyzed cleavage of the alcohol to give the carbocation.9,12 13... 

Table 2 gives rate and equilibrium constants for the deprotonation of and nucleophilic addition of water to X-[6+]. These data are plotted as logarithmic rate-equilibrium correlations in Fig. 5, which shows (a) correlations of log ftp for deprotonation of X-[6+] and log Hoh for addition of water to X-[6+] with logXaik and log KR, respectively (b) correlations of log(/cH)soiv for specific-acid-catalyzed cleavage of X-[6]-OH (the microscopic reverse of nucleophilic addition of water to X-[6+]) and log( H)aik for protonation of X-[7] (the microscopic reverse of deprotonation of X-[6+]) with log Xafc and log XR, respectively. 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Anodic oxidation of alkyl substituted cyclopropanes and spiroalkanes in methanol/TEATos (tetraethyl ammonium tosylate) affords monomethoxy and dimethoxy products in yields ranging from 6 to 86% [30, 31]. The products result from the cleavage of the most highly substituted C,C bond. In contrast to the anodic cleavage the acid-catalyzed cleavage occurs selectively at the less substituted carbon. The cleavage of hetero-substituted cyclopropanes is reported in Ref [32-35]. 

For phloroglucinolysis, a solution of 0.1 N HCl in MeOH, containing 50 g/L phloroglucinol and 10 g/L ascorbic acid, is prepared. The PA of interest is reacted in this solution at 50°C for 20 min and then combined with 5 volumes of 40 mM aqueous sodium acetate to stop the reaction. After acid-catalyzed cleavage in the presence of phloroglucinol, the fraction is depolymerized and the terminal subunits released as flavan-3-ol monomers and the extension subunits released as phloroglucinol adducts of flavan-3-ol intermediates. These products are then separated and quantified by HPLC [25]. 

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