Investigating cobalt zeolitic imidazolate frameworks as catalysts for cross-dehydrogenative couplings; exploring mesoporous silicas as scaffolds for an organocatalyst

Abstract

One of the greatest challenges for homogeneous catalysis is the difficulty in recovering and reusing the catalyst following a reaction. In order for applications to be carried out industrially, it is highly desirable to choose a system with a tuneable design, higher stability, and easy recyclability. This report presents novel applications of heterogeneous catalysts in reactions which have previously relied upon homogeneous systems. The heterogeneous alternatives reported here look to incorporate the growing fields of visible-light photocatalysis and organocatalysis in an ever more environmentally focused world.

Two zeolitic imidazolate frameworks (ZIFs) have been identified as solid photocatalysts for the cross-dehydrogenative coupling (CDC) of isoquinolines and nitromethane. There are few reports of Co-ZIFs in catalysis, with fewer still utilising photoactivation, here high yields have been achieved with ZIF-9 and ZIF-67 in the aza-Henry reaction using a solar simulator. A combination of UV-Vis and XAS spectroscopy, alongside theoretical simulations, was used to elucidate the nature of the framework’s activity. The catalysts were then trialled in C-O bond formations to produce functionalised benzoxazoles. Comparing and contrasting their mode of action, oxidation state transitions, and applicability to wider research.

The immobilisation of organic catalysts onto mesoporous supports is an exciting evolution of the recent insurgence of organocatalysis. The aim was to adopt highly attractive properties of the homogeneous parent catalysts, such as high enantioselectivity and low toxicity, with a solid support to aid recyclability. By harnessing pendant silanols, a pyrrolidine silyl ether was successfully anchored onto a range of mesoporous silicas, as confirmed by elemental analysis, NMR and BET. The materials were then trialled in the cyclopropanation of a benzyl halide with an α,β-unsaturated aldehyde, revealing the support has a significant effect on the yield and enantioselectivity achieved.

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Identifiers

ORCID for Ramon Rios Torres: orcid.org/0000-0002-3843-8521

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