Site-Selective, Organoboron-Catalyzed Polymerization of Pyranosides: Access to Sugar-Derived Polyesters with Tunable Properties

Abstract

A new class of sugar-derived polyesters has been synthesized, using organoboron-catalyzed acylation to generate linear macromolecules having pyranoside units enchained with a uniform 3,6-connectivity. The method provides access to welldefined polymers bearing pendant hydroxyl groups, from commercially available diacid chlorides and readily accessible pyranoside feedstocks. Variation of the configuration of the pyranoside (α-manno, α-galacto, or β-galacto) and the identity of the aglycon has been conducted, enabling studies of structure− property relationships. Flexible, long-chain substituents at the anomeric position behave as intrinsic plasticizers, affording decreased glass-transition temperatures (T g ) relative to their short-chain counterparts. The T g values of this family of polymers were found to vary from −3 to 164 °C. Enzymatic degradation and cytotoxicity studies in vitro suggested that the pyranoside-derived polyesters are slow-degrading, cytocompatible materials. After being subjected to urethane cross-linking, the polymers were studied by dynamic mechanical analysis and tensile testing, revealing the impact of stereochemistry and aglycon structure on the elastic and complex moduli, damping, and elongation at failure. The presence of hydrogen-bond-donating OH groups also conferred self-healing behavior to one of the linear polymers. This work demonstrates how organoboron-catalyzed, site-selective activation of OH groups can be employed to generate well-defined, carbohydrate-derived polymers.

Catalytic Approaches to Chemo- and Site-Selective Transformation of Carbohydrates

“…Based on the proposed mechanism, a more electron-rich borinic acid would display higher catalytic activity due to the enhanced nucleophilicity of active borinate species 67 . Screening phenylborinic acid derivatives with a variety of substituents led to the identification of heteraborinine-derived borinic acids ( 70 and 71 ) as more efficient catalysts for regioselective sulfonylation, glycosylation, sulfation, and acylation of carbohydrates (Figure a). − For example, 3- O -benzylation of methyl α- l -rhamnopyranoside ( 36 ) proceeded in similar yield with smaller catalyst loading (from 10 mol % to 5 mol %) in shorter reaction time (from 48 to 24 h, Figure b) than the reaction using 57 as a catalyst. These heteraborinine-derived borinic acids showed lower association constants with diol substrates, but DFT calculation suggested that their borinate adducts had higher nucleophilicity due to incorporation of the borinic acid group into a 6π electron system and the resulting decreased Lewis acidity of the boron center.…”

Emergent Organoboron Acid Catalysts

“…74 Analogous to known reactions using tin reagents, complexation of the borinic acid activated the bound oxygen atoms as nucleophiles (Figure 4), with selectivities based on dipole−dipole interactions along with steric interference to favor one of the two bound oxygen atoms. These reaction conditions have also been applied to polymerization reactions to produce sugar-based polyesters 103 and to the site-selective formation of sulfate esters 104 and sulfamate esters 105 of sugars. Boronic acids displayed significantly reduced catalytic activity and selectivity in these reactions.…”

Progress in the Electrochemical Synthesis of Organoboron Compounds

“…Organoboron is a class of compounds have a broad range of applications in materials science such as dynamic covalent materials, [1][2][3][4] optical and electrochemical sensors, [5][6][7] nanomaterials, [8][9][10][11][12] functionalized biomaterials, [13][14][15][16] natural products, [17][18][19] agrochemicals, [20][21] and pharmaceuticals chemistry as the active drugs molecules [22][23][24][25][26][27][28][29] for instances Velcade, Ninlaro etc. (Figure 1).…”