Abstract
Promoted by homogeneous Ru-benzylidene complexes, the olefin metathesis reaction is a powerful methodology for C-C double bonds formation that can find a number of applications in green chemical production. A set of heterogeneous olefin metathesis pre-catalysts composed of ammonium-tagged Ru-benzylidene complexes 4 (commercial FixCat™ catalyst) and 6 (in-house made) immobilized on solid supports such as 13X zeolite, metal-organic framework (MOF), and SBA-15 silica were obtained and tested in catalysis. These hybrid materials were doped with various amounts of ammonium-tagged styrene derivative 5—a precursor of a spare benzylidene ligand—in order to enhance pre-catalyst regeneration via the so-called release-return “boomerang effect”. Although this effect was for the first time observed inside the solid support, we discovered that non-doped systems gave better results in terms of the resulting turnover number (TON) values, and the most productive were hybrid catalysts composed of 4@MOF, 4@SBA-15, and 6@SBA-15.
Highlights
It is well established that the initiation step in olefin metathesis reaction catalyzed by the so-calledHoveyda–Grubbs catalyst (1) involves the dissociation of the 2-(isopropoxy)benzylidene ligand and the release of 2-(isopropoxy)styrene 2 (Figure 1a) [1,2]
In 2008 we executed a study showing that during the metathesis reaction conducted in the presence of one equivalent of deuterium labeled styrene 2, up to 50% of Hoveyda pre-catalyst isolated after the reaction contained the labeled benzylidene ligand, the release-return mechanism may be operational [7]
[18] was deprotonated using potassium tert-amylate to form in situ the free N-heterocyclic carbene (NHC), which later in the sequence of one-pot transmetalation reactions was transformed into Ru complex 11 in 34% overall yield starting from 9
Summary
It is well established that the initiation step in olefin metathesis reaction catalyzed by the so-calledHoveyda–Grubbs catalyst (1) involves the dissociation of the 2-(isopropoxy)benzylidene ligand and the release of 2-(isopropoxy)styrene 2 (Figure 1a) [1,2]. In 2008 we executed a study showing that during the metathesis reaction conducted in the presence of one equivalent of deuterium labeled styrene 2, up to 50% of Hoveyda pre-catalyst isolated after the reaction contained the labeled benzylidene ligand, the release-return mechanism may be operational [7]. Since these experiments used relatively high catalyst loading (5 mol %), the relevance of the boomerang mechanism in truly catalytic reaction conditions
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