Strategies for organocatalyst heterogenisation and performance in selective transformations

Abstract

Various organocatalysts are successfully heterogenised to robust inorganic frameworks, such as mesoporous silicas, by following a developed methodology of covalent tethering. The structural integrity of the organocatalyst, the nature of the active-sites and verifying the covalent anchoring were characterised through spectroscopic and physical measurements. The benefits of a heterogeneous system were exemplified through recovery and recycle experiments as well as directly measuring improvements in activity and selectivity in catalytic reactions.

The immobilisation technique was applied to covalently graft the amino acid lysine through the side chain functionality and the consequent heterogeneous systems were catalytically tested in the aldol reaction between acetone and 4-nitrobenzaldehyde. Lysine functionalised silicas offered improved activity and chemoselectivity than that of the homogeneous system due to the immobilisation method creating well-defined, isolated sites. Maximising this isolated nature was achieved through increasing the spatial resolution of sites by solid-phase deprotection and governing the degree of catalytic loading on the mesoporous support.

A similar methodology of immobilisation was applied to naturally occurring cinchona alkaloids (cinchonidine, cinchonine, quinidine and quinine) through the radical initiated formation of C-S bonds that covalently link the alkaloid to the surface of the silica support. All heterogeneous alkaloids were proven to be active catalysts in the Michael reaction of ethyl-2-oxocyclopentanecarboxylate (1,3 dicarbonyl) with N-benzylmaleimide, exhibiting the impressive stereoselectivities that are achieved in the homogeneous state. Furthermore, under solvent-free conditions the reaction could be completed within 2 hours with no detrimental effect on stereoselectivity.

It was established in the heterogeneous organocatalytic systems that minimising spatial constraints, having a precise balance of substrate-catalyst-pore wall interactions and the polar, hydrophilic character of the mesoporous silica surface provides a favourable environment for facilitating asymmetric organocatalytic transformations. The versatility of organo-functionalised mesoporous silicas can be extended to hosts for coordinating transition-metals that create amino-acid complexes, which can functionally mimic the active site in metalloenzymes. This was achieved by covalently heterogenising Cu2+-valine and Fe3+-proline complexes onto inorganic (mesoporous silica) and organic (polystyrene) supports which acted as highly selective catalysts in the oxidation of benzyl alcohol, dimethyl sulfide and cyclohexene using benign oxidants such as O2 from air. The covalent strategies offer a more stable, well-defined active site, which is demonstrated through improved catalytic turnovers of the covalent analogues in the various oxidation reactions. Through controlling the degree of catalytic loading the creation of a more isolated single-site nature of the bio-derived complexes can be achieved which increases catalytic efficiency. Correlating the different catalyst environments upon immobilisation onto varying supports while concurrently measuring the catalytic activity, leads to the development of structure-property relationships for support optimisation.

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Identifiers

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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