Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

In this study, we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support

A functionalised macrobicycle complex available for surface immobilisation and protein grafting

The conversion of both the racemic and resolved forms of the Co(III) complex of an hydroxymethyl-substituted tripodal hexamine to macrobicycles via reaction with formaldehyde and nitromethane (and subsequent reduction of the product) has been conducted. The prospect is that it will provide cage complexes in which the hydroxyl substituent is sufficiently remote from the metal ion centre for its nucelophilicity to be largely unaffected. X-ray structure determinations have been used to characterise these new cage species as well as some complexes of the precursor hexamine and its mono-aminal. The electrochemistry and optical activity of the complexes have also been studied in detail

Polyamines from polyols - Pathways to azamacrocycles with hydroxymethyl pendent groups

Several pathways to macromonocylic polyamine ligands with pendent hydroxymethyl substituents have been explored. The new ligands have all been characterised by single-crystal, X-ray structure determinations on their complexes with Co(III) (one case) and Cu(II). As in some related systems, four-membered ring species, here oxetanes rather than azetidines, appear to be involved as reaction intermediates and can be quite readily isolated, providing reactants of potential for the construction of even more complicated multidentate ligands. (C) 2005 Elsevier Ltd. All rights reserved