CommunicationMetal-centered oxidations facilitate the removal of ruthenium-based olefin metathesis catalysts
Graphical abstract
The redox processes intrinsic to commercially-available Ru-based olefin metathesis catalysts were used to switch between two different states of activity in ring opening metathesis polymerizations and ring closing metathesis reactions, and also used to facilitate the removal of the catalysts.
Introduction
The removal and reuse of precious metal based catalysts from reaction media has been a long-standing challenge in the field of homogeneous catalysis. Indeed, catalysts have been attached to various types of phase tags [1], [2] including fluorine containing derivatives [3], [4], [5], photoresponsive groups [6], [7], redox active groups [8], [9], and ionic liquids [10] to facilitate separation. The use of redox active phase tags to recover and reuse catalysts is a particularly attractive approach as redox processes are often diffusion controlled and operate in a manner that is orthogonal to other stimuli, such as light or heat. One area where redox active phase tags [8], [9], [11] have found utility is in the recovery of ruthenium-based olefin metathesis catalysts [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. For example, Plenio and co-workers [9] elegantly designed a Ru-based olefin metathesis catalyst featuring two ferrocenyl groups attached to an N-heterocyclic carbene ligand [22], [23], [24], [25], [26], [27], [28], [29]. Upon the addition of two equivalents of [FcCOCH3][CF3SO3], the ferrocenyl moieties underwent oxidation to their ferrocenium derivatives and caused the catalyst to precipitate from solution. The oxidized catalyst could then be recovered via filtration and reused upon reduction to its neutral form. A related catalyst which featured a ferrocenyl group appended to the 2-isopropoxybenzylidene moiety of a Hoveyda–Grubbs type catalyst was recently reported by Wang and co-workers [11]. Similar to the above mentioned system, exposing the catalyst to iodine resulted in the oxidation of the ferrocenyl group and facilitated extraction of the corresponding complex from non-polar media into an ionic liquid; subsequent reduction with decamethylferrocene (Fc*) restored the complex's solubility as well as its catalytic activity [11]. Alternatively, a number of other approaches have been employed to remove ruthenium residues upon completion of metathesis reactions [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], including attachment to silica gel [30], [31], [32], [33], [34], [35], [36] or other heterogeneous supports [37], [38], [39], [40], [41], [42], selective chemical degradation [43], [44], [45], [46], conversion to water soluble derivatives through ligand exchange [47], [48] or quaternization of a pendant amino groups [49], and selective extraction [49], [50], [51], [52].
The primary drawbacks to the aforementioned redox-based approaches are that either specially designed redox-active ligands are needed or the metal is recovered in a form that is not easily reusable. Since Ru-based olefin metathesis catalysts contain redox active metal centers, we envisioned finding suitable conditions that utilizes the RuII/III couple to induce solubility changes and to facilitate catalyst removal. An ability to selectively oxidize the metal center without irreversible chemical degradation of the complex should also facilitate reuse upon subsequent recovery and reduction, or enable external control over polymerizations [55] and other types of redox-mediated applications [56].
Section snippets
General comments
Toluene and CH2Cl2 were dried and degassed using a Vacuum Atmospheres Company solvent purification system and then subsequently stored over 3 Å molecular sieves. Benzene-d6 was distilled from sodium and benzophenone ketyl under an atmosphere of nitrogen then degassed by three, consecutive freeze-pump-thaw cycles. CD2Cl2 and toluene-d8 (99.9%) were purchased from Cambridge Isotope Laboratories and stored over 3 Å molecular sieves. (PCy3)2Cl2RuCHPh (Cy = cyclohexyl) (1), (SIMes)(PCy3)Cl2Ru(CHPh) (
Electrochemistry
The electrochemical properties of three commercially available catalysts (1–3; Fig. 1) were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). While the bisphosphine complex 1 was found to display a quasi-reversible redox couple (at E1/2= 0.63 V; scan rate = 100 mV s−1) in CH2Cl2 containing [nBu4N][PF6] as the electrolyte, the redox couples of 2 (E1/2 = 0.51 V) and 3 (E1/2 = 0.91 V) were reversible (Fig. 2 and Fig. S1 in the Supporting information) and dependent on
Conclusions
In summary, the ability to use metal centered oxidation processes to recover commercially available olefin metathesis catalysts was explored. The addition of DDQ to 2 or 3 was shown to significantly reduce catalytic activity in representative RCM and ROMP reactions, a result that was attributed to a RuII → RuIII oxidation process concomitant with precipitation of the catalyst; activity was subsequently restored through the addition of Fc*. The removal of the residual Ru species from solution
Acknowledgment
We are grateful to the U.S. Army Research Laboratory under grant number W911NF-09-1-0446 and the National Science Foundation (CHE-1266323) for their generous financial support.
References (65)
Tetrahedron
(2004)- et al.
Tetrahedron
(1998) - et al.
Preface
J. Mol. Catal. A: Chem.
(1998)- et al.
Tetrahedron Lett.
(1999) - et al.
Tetrahedron
(2013) - et al.
Tetrahedron Lett.
(2001) - et al.
Tetrahedron Lett.
(2010) - et al.
Tetrahedron Lett.
(1999) - et al.
Tetrahedron
(2009)