Regioselectively substituted cellulose esters and efficient methods of preparing them

Abstract

Embodiments of the invention provide regioselectively substituted carbohydrate and polysaccharide derivatives, such as cellulose esters, and methods for preparing them. Particular methods of the invention include deacylation of esters using tetrabutylammonium fluoride to obtain selective substitution at desired hydroxyl position(s) of the ester. Preferred is deacylation of an ester, which shows selectivity for removal of acyl groups from the ester of the secondary alcohols at C-2 and C-3, and which affords cellulose-6-O-esters with high regioselectivity by a simple one-step process employing no protective groups. Inventive regioselectively substituted cellulose esters can be prepared by such methods to obtain esters with the following anhydroglucose repeating units:wherein R1, R2, and R3 are each independently chosen from a hydrogen atom, and, whether substituted or unsubstituted, branched or unbranched, an alkanoyl group, an aroyl group, and a heteroaroyl group. In embodiments, the alkanoyl, aroyl and heteroaroyl groups can comprise from 1-20 carbon atoms.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application relies on the disclosure of and claims priority to and the benefit of the filing date of U.S. Provisional Application No. 61 / 550,952, filed Oct. 25, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety.BACKGROUND OF FR INVENTION[0002]1. Field of the Invention[0003]The present invention relates to the field of chemistry. More particularly, embodiments of the invention provide regioselectively substituted carbohydrate and polysaccharide derivatives, such as cellulose esters, and methods for preparing them.[0004]2. Description of Related Art[0005]Cellulose is one of the most abundant natural polymers on earth, consisting of unsubstituted, unbranched β(1→4) linked D-glucose units:Cellulose[0006]Cellulose comprises several thousand glucose units linked in a linear fashion. The chains are stabilized by intramolecular and intermolecular hydrogen bonds (shown as the dashed lines in the structur...

AI Technical Summary

Benefits of technology

The invention provides a simple method for making high-quality cellulose esters with high regioselectivity using common solvents and without the need for complex protecting group chemistry. This method can be used to make a wide range of products with specific applications. The efficiency and economy of the reaction between R4NOH and secondary esters make it a practical and industrial method for making regioselectively substituted cellulose derivatives.

Problems solved by technology

The ability to prepare cellulose esters having a high degree of control over the position of substitution is, however, a very difficult problem in organic chemistry, polymer science, analytical chemistry, and materials science.

The paucity of general solutions to this problem is a limiting factor in the development of novel materials from renewable cellulose, which are an important part of a biorefinery-based economy.

In light of the low reactivity of cellulosic hydroxyl groups resulting from the hydrogen and, hydrophobic bonding, poor solubility, and steric hindrance, it is quite challenging to synthesize regioselectively substituted cellulose esters.

The relatively small reactivity differences between the 2-, 3-, and 6-OH groups, however, make selectivity very difficult to achieve, especially for esterification reactions, in which direct esterification of cellulose with sterically demanding acylating reagents provided only modest selectivity on 6-OH.

It is believed that the difficulty of regioselective substitution arises because of the low reactivity of cellulosic hydroxyls towards electrophiles, due to the restricted steric access, mobility, and wetting imposed by the linear, hydrophilic cellulose polymer structure.

The resulting necessity of using strong catalysts, high temperatures, and / or large molar excesses of reagents is not conducive to selectivity.

Prior attempts to synthesize cellulose esters with high regioselectivity have involved the use of protective groups, which can significantly reduce overall yield, require high chemo- and regioselectivity in each of several steps, and can themselves reduce the reactivity of cellulose and impede succeeding reactions, as well as increase overall cost.

Cellulose ether synthesis in DMSO / TBAF is effective, but several researchers have noted that the synthesis of cellulose esters in this solvent is of limited scope due to the low degree of substitution (DS) of the products obtained.

What is more, the TBAF trihydrate is difficult to break by drying processes and attempts to do so result in β-elimination reactions of the TBA (tetrabutylarnrnonium) moiety, degrading the TBAF salt.

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