Acetal interchange reaction

Carboxy-hydroxy reactions, 63 Carboxyl endgroup chemical titration, 94 Carboxylic acid-aryl acetate interchange reactions, 62, 63... 

Competing side reactions in cationic polymerization of carbonyl monomers include cyclotrimerization and acetal interchange. Cyclotrimerization is minimized by low-polarity solvents, low temperatures, and initiators of low acidity. Acetal interchange reactions among different polymer chains do not occur except at higher temperatures. Acetaldehyde and higher aldehydes are reasonably reactive in cationic polymerization compared to formaldehyde. Haloaldehydes are lower in reactivity compared to their nonhalogen counterparts. 

Hill and Carothers (4) investigated the acetal interchange reaction with di-n-butyl formal and glycols (Equation 4). When n was 3 or 4, cyclic formals were the principal products. Fentamethylene glycol (n = 5) gave a sirupy liquid polymer. The reaction with decamethylene gly-... 

The objectives of this study were to extend these synthetic methods to the preparation of low molecular weight, hydroxy-terminated polymers containing acetylenic bonds and evaluating these prepolymers in castable solid propellant formulations. Since 2-butyne-l,4-diol was commercially available, the formation of polyacetals from this glycol seemed to be an attractive route to the desired polymers. It was believed that the rigid triple bond would inhibit the cyclic acetal formation observed by the earlier workers with the lower members of the saturated glycol series. Thus, in an acetal interchange reaction a linear polymer (I) should be favored over the cyclic acetal (II) ... 

Similar acetal interchange reactions were carried out between butynediol and di-n-propyl acetal (Ilia) and butynediol and diethylpropional (Illb), to give low molecular weight, liquid polyacetals (IVa and IVb) ... 

Unlike anionic initiators or anionically growing alkoxide chains which can only grow (or terminate), cationic initiators (Lewis, Bronsted acids or preformed initiators) or the cationically growing chain may cause acetal-interchange reactions. These reactions are also called transacetalization and cause rearrangement in the molecular weight distribution in homopolymers. The rates of transacetalization are relatively slow compared to that of polymerization except at high temperatures. In the presence of cyclic ethers or cyclic formals, for example, dioxolane, polyformaldehyde can incorporate randomly the co-monomer polyoxyethylene units into the polymer under transacetalization conditions. 

Scheme 5.17 Acetal interchange reaction a) and the same principle applied to polymerisation by the acetal metathesis polymerisation mechanism b)...

Carothers was one of the first to exploit the polyacetal-forming reaction in his attempt to prepare polyformals by the reaction of formaldehyde [3] with glycols. He found that diols below 1,5-pentanediol led to cyclic formals whereas the principal product from 1,6-hexanediol and from higher molecular weight diols was a polyformal [3]. Similar products were formed by the acetal interchange reaction when appropriate diols were used [Eq. (4)] [4]. 

Hill and Carothers found that the acetal interchange reaction is carried out easily and that the reaction is reversible [3]. 

Phenolic compounds are weaker nucleophiles and better leaving groups than aliphatic alcohols. They do not yield polyesters when reacted with carboxylic acids or alkyl carboxy lates. The synthesis of polyesters from diphenols is, therefore, generally carried out through the high-temperature carboxylic acid-aryl acetate or phenyl ester-phenol interchange reactions with efficient removal of reaction by-product (Schemes 2.10 and 2.11, respectively). 

Finally, it remained for us to convert the protective group scheme required for glycosidation to a protection scheme for the lipid II endgame. To achieve this, four protective group interchange reactions were carried out in a single operation. Disaccharide 37 was dissolved in acetic... 

Acetal-acetal interchange. Formaldehyde and acetaldehyde acetals can be prepared from methoxymethyl ethers and ethoxyethyl ethers, respectively, by reaction with TsOH in refluxing benzene. 

Several approaches were taken. Capillary-column gas chromatography on a vinylation reaction product showed 24 separate peaks present in the hexenyl acetate fraction however, six of the peaks accounted for about 90% of the total sample. Hydrogenation of the reaction mixture (hydrogen over platinum on carbon) reduced the hexenyl acetates to a mixture of three hexyl acetates and thereby greatly simplified determining the position of oxygen substitution with, however, loss of information on olefin position. We tried to synthesize the specific hexenyl acetate isomers by the ester interchange reactions (Reactions 7a, 7b, and 7c). Mixtures of isomers were obtained, but they corresponded to the main components of the vinylation reaction mixture. For example, the main products isolated from the vinylation of hexene-1 corresponded to the products from Reactions 7a and 7b—i.c., vinyl rather than allyl esters. 

The oxygen-sulfur interchange reaction was extended to an acyclic anhydride system. High conversion of acetic anhydride to a mixture of thioacetic acid and acetic acid was obtained employing alkali metal... 

The reaction of BejjO(OCOCH )g with diacid chlorides (4 ) gives low molecular weight polymers. On heating, interchange occurred and BejjO(OCCH-), could be sublimed at 110-l40 C at 10 torr pressure for tne product derived from adipyl chloride. The use of terephthalyl chloride appears to Increase the stability of the product such that a temperature of 340 C was required to obtain sublimed beryllium acetate. Interchange also slowly occurs at room temperature. 

As shown above, in starch the glucopyranoside ring is present in the a-form, while in cellulose the repeating units exist in the P-form. Because of this difference in structures, the enzymes that catalyze the acetal hydrolysis reactions in the biodegradation reactions of each of these two polysaccharides are different for each and are not interchangeable. 

Ester interchange reactions between glycol acetates or diphenol diacetates and... 

To explore the difference between these two approaches, consider the following esterification reaction (more precisely known as ester interchange reaction) where dimethyl terephthalate (DMT) is reacted with ethylene glycol (EG) in presence of metal acetate catalyst ... 

The basic principle of this polymerisation reaction is based on the acetal interchange illustrated in Scheme 5.17. 

Ester interchange reactions in the melt at high temperatures of either of two different types (a) reaction of a diphenyl ester of the aromatic dicarboxylic acid monomer with the difunctional phenol monomer to liberate phenol, which is removed under vacuum, or (b) reaction of a diacetate ester of the difunctional phenol monomer with the dicarboxylic acid monomer by an acidolysis reaction to liberate acetic acid, which is readily removed under vacuum... 

Several derivatives of cellulose, including cellulose acetate, can be prepared in solution in dimethylacetamide—lithium chloride (65). Reportedly, this combination does not react with the hydroxy groups, thus leaving them free for esterification or etherification reactions. In another homogeneous-solution method, cellulose is treated with dinitrogen tetroxide in DMF to form the soluble cellulose nitrite ester this is then ester-interchanged with acetic anhydride (66). With pyridine as the catalyst, this method yields cellulose acetate with DS < 2.0. 

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