Homogeneous acylation

Homogeneous catalysis by lin compounds is also of great indusirial importance. The use of SnCU as a Friedel-Crafts catalyst for homogeneous acylation, alkylation and cyclizaiion reactions has been known for many decades. The most commonly used industrial homogeneous tin catalysis, however, are the Sn(ll) salts of organic acids (e.g. acetate, oxalate, oleale, stearate and ocToate) for the curing of silicone elasloniers and, more importantly, for the production of polyurethane foams. World consumption of tin catalysts for the.se Iasi applications alone is over 1000 tonnes pa. 

Dextran usually dissolves in water, DMSO, DMA/LiCl, DMF/LiCl, formamid, aqueous urea and glycerin. The solubility may be restricted by a high crystallinity of the polymer (Sect. 2.3). The most versatile solvents for homogeneous acylation reactions are DMSO or mixtures of a polar aprotic solvent and a salt. The broadest application found the combination of substituted amide with LiCl. Thus, dextran dissolves easily in the mixture DMF/LiCl upon heating to 90-100°C. This solvent can be exploited for the esterification of dextran as displayed in Fig. 19. 

Davies and Liebeskind independently prepared chiral aluminum enolates from enantiomerically homogeneous acyl-iron complexes (137) and recorded the first aluminum-mediated asymmetric aldol reactions. Although the lithium enolate of the chiral iron complex (CHIRAC) provides aldol products with... 

The whole argument is organized in four sections stoichiometric acylations, catalytic homogeneous acylations, catalytic heterogeneous acylations, and phenol acylations. It is structured according to the role played by the catalyst in the activation of reagents as well as in the different modes of regioselectivity encountered in the acylation of arenes, aromatic ethers, and phenols. 

As already underlined in the introduction to this book, catalytic homogeneous acylation reactions represent a remarkable improvement in the preparation of aromatic ketones because, in the conventional Lewis-acid-promoted reactions, formation of a stable complex between the ketone product and the catalyst implies that at least a stoichiometric amount of catalyst must be utilized. This drawback prompted a great number of studies aimed at setting up the experimental conditions to make catalytic Friedel-Crafts acylation reactions. Some positive results from fhe homogeneous catalytic Friedel-Crafts acylations are described here, with special attention to crucial economic and environmental advantages such as the recycling of expensive catalysts and the development of solvenf-free and highly selective synthetic processes. 

Esters of cellulose with interesting properties such as bioactivity and thermal and dissolution behavior can be obtained by esterification of cellulose with nitric acid in the presence of sulfuric acid, phosphoric acid, or acetic acid. Commercially important cellulose esters are cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Cellulose esters of aliphatic, aromatic, bulky, and functionalized carboxylic acids can be synthesized through the activation of free acids in situ with tosyl chloride, iV,iV -carbonyldiimidazole, and iminium chloride under homogeneous acylation with DMA/LiCl or DMSO/TBAF. A wide range of cellulose esters that vary in their DS, various substituent distributions, and several desirable properties can be obtained through these reactions. Recently, a number of enzymes that degrade cellulose esters have been reported. Some of them are acetyl esterases, carbohydrate esterase (CE) family 1, and esterases of the CE 5 [169-172] family. 

G.T. Ciacco, TF. Liebert, E. Frollini, TJ. Heinze, Application of the solvent dimethyl sulfoxide/tetrabutyl-ammonium flumde trihydrate as reaction medium for the homogeneous acylation of sisal cellulose, Cellulose 10 (2) (2003) 125-132. 

Homogeneous acylation with in situ activated carboxylic acids... 

In addition to DMA/LiCl, DMSO/TBAF is an appropriate reaction medium for homogeneous acylation of cellulose applying in situ activation with CDI. Results of reactions of cellulose with acetic-, stearic-, adamantane-1-carboxylic-, and furan-2-carboxylic acid imidazoUdes are summarized in Table 16.7. 

Homogeneous acylation of cellulose dissolved in dimethyl sulfoxide (DMSO)/tetra-n-butylammonium fluoride (TBAF) with different carboxylic acids, mediated by Af V -carbonyldiimidazole (CDI)... 

In general, by chemical modification reactions of cellulose no equal reactivity of positions 2, 3, and 6 appears. For instance, homogeneous acylation in DMA/LiCl results in products with a preferred functionalization at position 6, while the carboxymethylation in 2-propanol/aqueous NaOH proceeds faster at position 2 compared to 6 and 3. 

The homogeneous acylation of the hemicellulose is mostly carried out in the solvent iV,/V-dimethylformamide (DMF) in combination with LiCl. Hemicellulose extracted from poplar wood chips was acylated with various carboxylic acid chlorides in DMF/LiCl applying triethylamine and 4-N,N-dimethylaminopyridine (DMAP) as base and catalyst (J, Table 1). Depending on... 

The preparation of acylchitin and chitin mixed esters was performed as in [4]. Methanesulfonic acid was found to be a suitable solvent and catalyst for the homogeneous acylation reaction. A mixture of 4 parts of CH3SO3H and 6 parts of carboxylic anhydride for one part of chitin powder was employed. When a smaller amount of CH3SO3H is employed, it is difficult to obtain fully acylated chitin owing to the heterogeneity of the reaction mixture. The temperature for the reaction was kept at 0 - 5°C to prevent decomposition of the chitin by CH3SO3H. In the case of chitin mixed esters the products were obtained by changing the amounts of carboxylic anhydrides. 

---