Communication pubs.acs.org/OPRD
Exceptionally Stable and Efficient Solid Supported Hoveyda-Type Catalyst Krzysztof Skowerski,*,† Jakub Pastva,† Stefan J. Czarnocki,† and Jana Janoscova‡,§ †
Apeiron Synthesis SA, Duńska 9, 54-427 Wrocław, Poland J. Heyrovský Institute of Physical Chemistry, Department of Structure and Dynamics in Catalysis, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, 182 23 Prague 8, Czech Republic § Czech Republic and University of Pardubice, Department of Inorganic Technology, Doubravice 41, 532 10 Pardubice, Czech Republic ‡
S Supporting Information *
this topic was recently reviewed.9 All possible ligands around the ruthenium were modified in order to obtain heterogeneous material, but this has not led to an efficient catalyst. It seems obvious that the disappointing results of terminal alkenes metathesis originates mainly from the low stability of ruthenium methylidenes formed during the catalytic cycle.10 Heterogeneous materials which are expected to work during a long period of time makes them especially sensitive to the limited stability of active, propagating species. Analysis of the literature results for both homo- and heterogeneous catalysts and our own experiments allowed us to identify the crucial elements that, properly integrated, should provide a heterogeneous catalyst able to transform terminal dienes with high TONs. To avoid (i) the release of active species into the liquid phase which can result in bimetallic decomposition and/or ruthenium leaching (in the case of heterogenization through benzylidene or phosphine ligands)11 and (ii) the decrease of catalyst activity (modification of anionic ligands),12 the heterogenization should be realized through the NHC ligand. The N-substituents in the NHC ligand should be aromatic rings,13 and these rings should not be used as a heterogenization site in order to maintain the original bonding properties of NHC14 and to avoid possible interaction of active species with support.15 Thus, attachment of a tag to the NHC backbone appears to be the most beneficial way of preparation of heterogeneous material.16 Most importantly the ortho substituents in the aromatic rings of NHC should be bulky to stabilize active species and decrease intramolecular decomposition.10 Moreover, it should be beneficial for heterogeneous material to work, at least to some extent, as a catalyst reservoir; thus, the benzylidene ligand should stay unmodified or, alternatively, be substituted with a weakly electron-donating group.17 Finally, the support should ensure fast diffusion of alkenes to and from catalytic centers and be characterized by a large surface area for proper site isolation. The foregoing describes the synthesis and catalytic activity of the first solid supported catalyst that satisfies all of the abovementioned criteria.
ABSTRACT: The ammonium tagged Hoveyda-type catalyst bearing sterically enlarged N-heterocyclic carbene ligand was synthesized and supported on SBA-15. The obtained heterogeneous olefin metathesis catalyst forms unprecedentedly stable Ru-methylidenes and provides products of ring-closing and cross metathesis with turnover numbers (TONs) up to 35 000 and turnover frequencies (TOFs) up to 1590 min−1. The catalyst proved to be truly recyclable and effective in a continuous flow mode.
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INTRODUCTION Extensive research in the field of olefin metathesis over the past 20 years resulted in the development of several rutheniumbased complexes which can satisfy almost any metathetic transformation (Figure 1).1 Most of these catalysts are now commercially available which has led to the widespread growth of metathesis technology throughout academia and industry.2 Excellent functional group tolerance and high stability are the main advantages of Ru-complexes, especially those containing a N-heterocyclic carbene (NHC) ligand. TONs up to 1000 were obtained for the ring-closing metathesis (RCM) of multifunctional dienes3 and ≫10 000 for easier transformations4 proving the practical usefulness of homogeneous Ru-based catalysts. Heterogeneous Ru-catalysts were also extensively developed over the past decade, as their application can bring several advantages. Low contamination of products with residual ruthenium, no scale-up effect, and a possible increase in reaction selectivity thanks to the support properties and/or the characteristic of the continuous flow process are the main benefits of heterogeneous systems.5 Unfortunately, while a few heterogeneous catalysts proved to be effective in the transformation of internal alkenes (TONs up to 10 000),6 none of them showed satisfactory efficiency in reactions which proceed with the evolution of ethylene. In the latter case even simple dienes were usually cyclized with TONs
