Homogeneous benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride: Hammett correlation, mechanism and regioselectivity

Abstract

Homogeneous benzoylation of cellulose with a series of substituted benzoyl chlorides, in which substituents varied from electron donating to electron withdrawing groups, was investigated in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). The electronic effect of substituents had a considerable effect on the reaction. A plot of Hammett parameters of substituents vs. degree of substitution (DS) of resultant cellulose esters exhibited a V-shaped graph: a negative slope for negative Hammett parameters, and a positive slope for positive Hammett parameters. Such a Hammett plot indicated the mechanism of cellulose benzoylation: the reaction underwent a unimolecular ionization mechanism with a carbocationic intermediate when benzoyl chlorides were with electron-donating substituents, and a bimolecular addition-elimination mechanism with a tetrahedral intermediate when benzoyl chlorides were with electron-withdrawing substituents. In addition, 13 C-NMR analysis showed that most of the substituted benzoyl chlorides exclusively preferred the primary hydroxyl at the C-6 position. This unusually high regioselectivity was attributed to the synergistic effect of appropriate reaction rate, moderate steric effect and reaction mechanism of benzoylation.

Novel Thermoplastic Cellulose Esters Containing Bulky Moieties and Soft Segments

“…In particular, ionic liquids (ILs) have provided a new method for the homogeneous synthesis of cellulose derivatives at the molecular level. − As a new kind of aprotic solvent, ILs are thermally stable, chemically inert, and highly polar; hence, they are ideal media for the homogeneous derivatization of cellulose. In fact, many conventional cellulose esters and a variety of novel and functional cellulose derivatives have been successfully synthesized in ILs. − Compared with traditional heterogeneous derivatization, the homogeneous synthesis of cellulose esters in ILs has several obvious advantages, such as high efficiency, high yield, mild reaction conditions, no or few byproducts, simple separation, and easy solvent recovery. More importantly, the structure of the cellulose derivatives can be precisely controlled with different reaction conditions.…”

Parametrization of Catalytic Organic Reactions with Convex Hammett Plots

“…Figure B shows a representative example of a convex Hammett plot reaction, i.e., the nucleophilic substitution reaction of benzyl derivatives. − Here, the transition state is composed by either benzylic carbocations stabilized by electron donor groups (EDGs) or poorly charged benzylic derivatives activated by electron-withdrawing groups (EWG), to which the nucleophile is finally added by a substitution reaction. This convex Hammett plot does not come from the action of any catalyst and is thus inherent to the substrates in play and unmodifiable.…”

Electronic effects of group fragments on the XPS of Fe 2p and 3p photoelectron lines of ferrocenyl-containing chalcones

“…V-shape correlations obtained with Hammett constants are not uncommon, there are many reports of V-or U-shaped curves linking Hammett constant with other physical parameters, including kinetic constants and degree of substitution. [38][39][40][41] The Hammett constant is a value composing of a mixture of electronic effects including polar and resonance effects. 42 Since polar effects which is incorporated in the Hammett constant represents a similar effects as electronegativity of the Gordyscale group electronegativity, it is proposed that the V-shape presumably comes from the resonance effect incorporated in the Hammett constant.…”