Chemically modified sporopollenin as a solid support for heterogeneous catalysts

Abimbade, Sunday Felix

Abstract

Application of the principles of ‘Green Chemistry’ has been a focus in recent years in the synthesis of fine chemicals, and solid-phase catalysts are useful tools in that endeavour compared to stoichiometric reagents. Another important principle of ‘Green Chemistry’ is in the choice of renewable feedstocks. With those two aspects in mind, the plant-derived polymer sporopollenin, isolated in the form of sporopollenin exine capsules (SpECs), has been extracted from the spores of Lycopodium clavatum. Modification of these polymer capsules with chlorosulfonic acids gave rise to sulfonated SpECs (SpEC- SO3H), which have been investigated as a heterogenous acid catalyst in a range of organic reactions. Other modifications of the SpECs investigated were zinc (SpEC-Zn), brominated (SpEC-Br) and thiolated (SpEC-SH) forms of the SpECs.

In this work, sulfonated SpECs were used as a heterogeneous catalyst for the: (i) synthesis of disubstituted benzimidazole derivatives; (ii) protection of carbonyl compounds to form spiro ketals; (iii) formation of tetrahydropyranyl ethers from alcohols; (iv) formation of ketals from monosaccharides; (v) Fischer glycosylation of monosaccharides; (vi) O-glycosylation of phenols and alkyl alcohols; (vii) N- glycosylation of urea and urethane; (vii) dehydration of monosaccharides; (ix) Beckmann rearrangement of benzophenone oximes; (x) transesterification of fatty acid triglycerides.

In these reactions, the sulfonated sporopollenin exine capsules (SpEC-SO3H) proved to be an efficient catalyst in reactions that were either carried out in water, in aqueous solvent mixtures, in various organic solvents, or in some cases in solvent-free processes. For example, the synthesis of disubstituted benzimidazoles from o-phenylenendiamine and substituted benzaldehydes was investigated in water at 70 °C, under solvent-free melt conditions at 70 °C, in aqueous ethanol or in ethanol at room temperature. The catalytic action of SpEC-SO3H resulted in a good yield of the benzimidazoles (43% - 90 %) in all of these processes except in the reaction of o-phenylene diamine with 2- methoxybenzaldehyde in aqueous ethanol that gave 6 %. Likewise, similar yields were obtained when the reaction of o-phenylene diamine with benzaldehyde or 4- hydroxybenzaldehyde catalysed by SpEC-SO3H using microwave heating was compared to the conventional heating. The reaction of aliphatic aldehydes and o-phenylenediamine to form alkylated benzimidazoles in the presence of SpEC-SO3H was performed at room temperature to avoid the formation of aldol adducts. In the formation of benzimidazole from aliphatic aldehyde, the reaction time was shorter and with minimal by-products resulting from competing aldol side reactions.

SpEC-SO3H also proved effective in catalysing glycosylation reactions of alcohols; however, O-glycosylation of phenols and N-glycosylation of benzamide, acrylamide, purine, imidazole and derivatives were unsuccessful. However, N-glycosylation of urea and urethane was successful and the O-glycosylation and N-glycosylation reactions that worked, yielded high products using SpEC-SO3H catalysis.

The protection of carbonyl compounds (as ketals) and alcohols (as THP ethers) also revealed the effectiveness of the catalytic performance of SpEC-SO3H as a bio-based heterogeneous catalyst. Application of SpEC-SO3H to dehydration of fructose led to the formation of a high yield of hydroxymethylfurfural (HMF). Also, SpEC-SO3H was effective in the transesterification reaction of sunflower fatty acid triglycerides with methanol, and in the Beckmann rearrangement of benzophenone oxime.

Another aspect that portrayed potential usefulness of the SpEC-SO3H catalyst is its recyclability. In most of the reactions studied, the SpEC-SO3H catalysts was able to be isolated, recycled and re-used.

Citation

Abimbade, S. F. (2019). Chemically modified sporopollenin as a solid support for heterogeneous catalysts. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4222418

• Thesis Type: Thesis
• Deposit Date: Jun 11, 2020
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4222418
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Dec 1, 2019