Effective immobilisation of a metathesis catalyst bearing an ammonium-tagged NHC ligand on various solid supports

Introduction

Over the past decade olefin metathesis has undergone a grand development. The design of stable and active ruthenium-based metathesis catalysts has been the cardinal factor to distribute olefin metathesis in the synthesis of many important compounds [1–4]. Commercially available homogeneous complexes, including phosphine-containing Gru-II, Ind-II or phosphine-free Hov-II and Gre-II are usually employed in such cases (Fig. 1) [5]. However, heterogenisation of these complexes was also extensively tested, as their applications in a solid form can be beneficial [6]. The efficient removal of ruthenium from metathesis products, possibility of catalyst recovery and reuse as well as their potential use in continuous processes are the main benefits of heterogeneous systems [7]. Unfortunately, their application is associated with some drawbacks. These catalysts usually exhibit lower activity than their homogeneous counterparts as reflected by a noticeably lower turnover frequency (TOF). Moreover, their synthesis, due to the need for sophisticated linkers and tags, is significantly more complicated.

Several protocols were developed for heterogenisation of ruthenium catalysts and this topic has been thoroughly reviewed [8–18]. The implementation of such concepts requires the presence of remotely functionalised ligands within the metal coordination sphere. A very efficient covalent immobilisation through anionic ligands was reported by Buchmeiser et al., who synthesised a series of monolith-supported catalysts (such as 1) which gave metathesis products with extremely low residual ruthenium [19–24]. In other contributions originating from the same group, heterogeneous catalysts covalently connected to a monolithic support via NHC ligands were presented [25–26]. These initiators were suitable for continuous metathesis processes and provided products with low residual ruthenium; however, they were less active than complex 1. A very similar idea was explored by Grubbs et al. who obtained catalysts 2 and 3 covalently bonded to silica gel through the NHC ligand [27–29]. This work revealed that, for complexes supported on silica gel, the heterogenization via the NHC backbone is a much better approach than the previously used ones (e.g., via phosphine or benzylidene ligands).

An early example of a non-covalent attachment is complex 4, an activated catalyst deposited on glass polymer Raschig rings, which was tested in various metathesis reactions carried out in batch and circulating flow reactor, as well as in an industrial setup [30–31]. The concept was explored...