Acid-catalyzed Synthesis Of 1,1-supported Diacetate

Acylals are carbonyl ester compounds and exhibit important applications in organic synthesis. Acylals have been used as a valuable intermediate in various organic syntheses, can be converted to many useful compounds after nucleophilic substitution reaction; they can be used as building blocks for the synthesis of dienes for Diels-Alder cycloaddition reactions, and as substrates in many organic reactions. In addition, acylals have been applied as cross-linking reagents for cellulose in cotton and serve as activators in the composition of the bleaching mixture used for the treatment of wire-strained fibers. It should be noted that, in recent years, aylals are frequently used to protection of aldehydes. The common method of protect carbonyl is generate acetals and ketals, thioacetal and thioketal by the reaction between adehydes/ketones and alcohols or thioalcohol. However the deprotection of acetals and ketals need in the presence of acids, and the deprotection of thioacetal and thioketal need the catalysis of metal salts or oxidizing agents. But acylals can transfer to adehydes in mild conditions, and acylals are very robust to aqueous acid, neutral, and basic media. Therefore, this method of protect carbonyl have attracted considerable attention for organic chemists.Generally, acylals are prepared from aldehydes and acetic anhydride in the presence of strong protonic acids, lewis acids, heteropoly acids and some non-homogeneous catalysts such as SiO2, zeolite, montmorillonite and macroporous resin. But these procedures are often accompanied by longer reaction times, low product yields, stringent conditions, high catalyst loading, corrosive reagents, high temperature and require the use of toxic metal ions and solvents. Therefore, mild reaction conditions that can overcome the shortcomings of previous methods are necessary. This research in the thesis also foucs on the catalytic synthesis of acylals. The main content presents as follows:1. CAT600is a strong acidic polystyrene cation exchange resin, its advantages are cheap and have higher density of sulfonic acid groups (4.9-5.1mmol/g). In this paper, we investigate its catalytic effect for the conversion of aldehydes to acylals. The preparation of acylal from benzaldehyde as model reaction, the optimized reaction conditions were obtained as:1mmol aldehyde,2mmol acetic anhydride,15mg CAT600, at room temperature and solvent-free conditions. Under this conditions, all aromatic aldehydes or aliphatic aldehydes are able to generate the corresponding1,1- diacetate in a shorter time (5-30min) with a higher yield (84-94%). Comparing to other reported catalysts, in the presence of this reusable catalyst, the reaction exhibited short time, a small dose of acetic anhydride and mild condition. Finally, to evaluate the selectivity of this method, we investigated competitive reaction for acylation of aldehydes in the presence of ketones using CAT600as catalyst. With this catalytic system the highly selective conversion of aldehydes in the presence of ketones was observed.2. Sulfonated cage-type mesoporous carbon (S-CKT) was prepared with the reported methods previously established in our laboratory. Initially we explored the acylation reaction of benzaldehyde with acetic anhydride in the presence of sulfonated cage-type mesoporous carbon. The optimized reaction conditions were obtained as:1mmol aldehyde,2.5mmol acetic anhydride,15mg S-CKT, at room temperature and solvent-free conditions. Moreover, it should be noted that the catalyst was recycled seven times without a distinct loss of activity by simple filtration and washing. Meanwhile, to evaluate the selectivity of this method, we investigated competitive reactions for acylation of aldehydes in the presence of ketones using S-CKT as catalyst. With this catalytic system, the highly selective conversion of aldehydes in the presence of ketones was observed. Finally, we employed the optimized reaction conditions for the conversion of various aldehydes to the corresponding acylals, the results showed that both aromatic and aliphatic aldehydes react smoothly with acetic anhydride to afford the corresponding1,1-diacetates at short time (5-12min) in good to excellent yields (89-98%). In conclusion, S-CKT is a chemoselective and highly efficient catalyst for acylal formation from aldehydes. The advantages of this methodology over the reported methods is the reuse of the catalyst, simplicity of acylation procedure, a clean work-up, a short reaction time, and high yields.