An ionic liquid-tagged second generation Hoveyda–Grubbs ruthenium carbene complex as highly reactive and recyclable catalyst for ring-closing metathesis of di-, tri- and tetrasubstituted dienes

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Abstract

A second generation Hoveyda–Grubbs ruthenium carbene complex bearing an ionic liquid tag was prepared and shown to be a highly reactive catalyst for the ring-closing metathesis of di-, tri- and tetrasubstituted diene and enyne substrates in minimally ionic solvent systems ([Bmim]PF6/CH2Cl2, 1:9–1:1 v/v). Both the catalyst and the ionic liquid can be conveniently recycled and repeatedly reused (up to 17 cycles) with only a very slight loss of activity. The ionic liquid tag is crucial to the high level of recyclability of the catalyst since the original second generation Grubbs and Hoveyda–Grubbs catalysts rapidly lose their activity when recycled in the ionic liquid layer.

Graphical abstract

A Ru catalyst bearing an ionic liquid tag was prepared and shown to be highly reactive for the RCM of a variety of diene and enyne substrates in minimally ionic solvents ([Bmim]PF6/CH2Cl2, 1:9–1:1 v/v). Both the catalyst and the ionic liquid can be conveniently recycled and repeatedly reused (up to 17 cycles).

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Introduction

The advent of well-defined Ru carbene catalysts, most notably, the Grubbs-type Ru benzylidenes 1 [1a] and 2 [1b], and the closely related Ru complex 3 [1](c), [1](d), has fueled the widespread application of olefin metathesis [2] (Fig. 1). Another significant advance in this area is the discovery by Hoveyda and co-workers of the Ru catalysts 4 [3a] and 5 [3b] that bear an isopropoxy ether ligand tethered to the benzylidene group. The unique bidentate nature of this ligand renders catalysts 4 and 5 a number of interesting and useful properties [3d]. For example, both 4 and 5 exhibit enhanced stability compared to catalysts 1 and 2, allowing them to be retrieved from the crude reaction mixture by silica gel chromatography. These catalysts also show reactivity profiles different from those of 4 and 5. The robustness and recyclability of 4 and 5 provide an excellent opportunity for the development of immobilized and supported Ru catalysts for olefin metathesis [4]. Our contribution in this field includes the synthesis and catalytic application of the air stable poly(ethylene glycol) monomethyl ether (MeO-PEG)-bound first generation Ru catalyst 6 [5a], the more reactive poly(ethylene glycol) (PEG)-bound second generation Ru catalyst 7 [5b], and the poly(fluoroalkyl acrylate)-bound second generation Ru complex 8 [6] (Fig. 2). We have established that the soluble polymer-bound catalysts 6 and 7 are highly reactive toward the ring-closing metathesis (RCM) of various diene and enyne substrates, and in the case of catalyst 7, cross metathesis (CM) and ring-opening/cross metathesis (RO-CM). Meanwhile, these catalysts can be conveniently recycled by precipitation with diethyl ether and repeatedly reused without significant loss of their activity, demonstrating the practical advantage of using a soluble polymer to immobilize Ru catalysts. The air stable fluorous Ru carbene complex 8 was shown to be highly reactive in effecting the RCM of a broad spectrum of diene and enyne substrates in minimally fluorous solvent systems and can be readily separated from the reaction mixture by fluorous extraction with FC-72 [7], [8] and repeatedly reused.

The recently emerged room-temperature ionic liquids have gained increasing popularity as innovative and environmentally benign reaction media, and as new vehicles for the immobilization of transition metal-based catalysts [9]. This new concept of catalyst immobilization and recycling has also been applied to several Ru-catalyzed olefin metathesis reactions [10]. Although the olefin metathesis reactions generally proceed smoothly if an appropriate ionic liquid is chosen as the solvent, the recovered catalysts in the ionic liquid layer rapidly lose their activity in subsequent runs. We have recently reported that both the first generation Grubbs catalyst 1 and the more stable and recyclable catalyst 4 experienced poor recyclability when the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim]PF6 was used to immobilize these catalysts [11]. This is attributed to the inherently poor recyclability in the case of catalyst 1, and for the recyclable catalyst 4, its poor retention in the ionic liquid phase, and hence significant leaching during extraction of the reaction product with diethyl ether. To solve this problem, a designer ionic liquid tag is attached to the isopropoxy styrenyl ligand and the resulting catalyst 9 (Fig. 3) was found to exhibit an extremely high level of recyclability in RCM reactions that lead to the formation of disubstituted cyclic olefins. We now wish to report on our extension of this strategy to the second generation Ru catalyst 5. The ionic liquid-bound second generation Ru catalyst 10 maintains a high level of activity similar to that of 5 but can be conveniently recycled and repeatedly reused in the RCM of a wide variety of di-, tri-, and even tetrasubstituted diene and enyne substrates. A similar concept was used, independently, by Mauduit and co-workers in the development of the ionic liquid supported-Ru catalysts 11 [4p] and 12 [4q].

Section snippets

Result and discussion

The ionic liquid-tagged Ru catalyst 10 was assembled by treatment of the functionalized ionic liquid 13 [11] with an equimolar amount of the second generation Grubbs catalyst 2 in CH2Cl2 at 45 °C as shown in Scheme 1. After removal of the volatiles under vacuum, 1H NMR (CDCl3, 500 MHz) spectroscopic analysis revealed a conversion of greater than 90% of the styrene ligand. The formation 10 was indicated by its characteristic carbene signal at δ 16.50 ppm, which is the only carbene peak observed, as

General

Unless otherwise noted, all reactions were performed under an atmosphere of dry Ar with oven-dried glassware and anhydrous solvents. CH2Cl2 was dried over CaH2 and distilled prior to use. Diethyl ether was distilled from sodium/benzophenone under a nitrogen atmosphere. Catalyst 2 [1b] was obtained from Strem and was used as received. The ionic liquid [Bmim]PF6 was purchased from Acros and used as received. 1H NMR spectra were acquired in CDCl3 at 500 MHz. Diethyl diallylmalonate 14 was purchased

Acknowledgements

The National Institutes of Health (GM-63522) is gratefully acknowledged for support of this work. We thank Mr. Yiliang Zhang for providing samples of ligand 13 and of some diene substrates.

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