Immobilisation of the Grubbs III Olefin Metathesis Catalyst with Polyvinyl Pyridine (PVP)

Abstract

A new concept for immobilising Grubbs III catalyst by direct coordination of ruthenium to polyvinyl pyridine (PVP) is presented. PVP was prepared by precipitation polymerisation, which led to small bead sizes (0.2-2 µm) and large surface areas. Compared to commercial resins, this phase showed superior properties when employed in model ring-closing metathesis (RCM) and in representative RCM, enyne and CM reactions with various substrates. The concept of immobilisation was also applied to Raschig rings made from a glass polymer composite material, which can be incorporated into devices for continuous flow processes.

catalysis - immobilisation - olefin metathesis - microwave assistance - Ru-catalyst - polymer support

References

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12

Part of the work was described by K. Mennecke in his Diploma thesis (Hannover 2004).

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14 Kunz U, Kirschning A, and Hoffmann U. inventors; EP  1268566 B1. 

15

Preparation of Polyvinyl Pyridine Phase by Precipitation Polymerisation. First, a mixture of 4-vinylpyridine (43.20 g, 410.8 mmol) and divinylbenzene (3.86 g; purity 65% besides ethyl benzene) was prepared. This mixture was filled up with an n-alkane (C14-C17 fraction) to a total volume of 300 mL, AIBN (327 mg, 2 mmol) was added and the temperature was raised to 70 °C. The reaction mixture is kept at this temperature for 24 h. Then the solid material formed was filtered, rinsed with CHCl₃ and further purified by extraction in a Soxhlet extractor with CHCl₃ and finally dried under reduced pressure to yield PVP (4.85 mmol/g capacity).

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18

Indeed, this idea has been shown to be powerful for the immobilisation of enzymes using nickel NTA-linkers on sepharose for coordinatively trapping enzymes tagged with a His-tag.

19

Preparation of Functionalised Polyvinyl Pyridines 6a and 6b. A suspension of ruthenium catalyst 5 (140 mg, 0.16 mmol; prepared according to ref. 16) and PVP (6a: 400 mg, 1.84 mmol; 6b: 373 mg, purchased from Acros) in toluene (3 mL) was shaken under argon at r.t. for 72 h. The polymer was filtered and washed with five portions of toluene (2 mL) to yield functionalised polymer 6 (polymer obtained by precipitation polymerisation: 510 mg, 0.15 mmol ruthenium; 96% and polymer from Acros: 477 mg, 0.09 mmol ruthenium; 80%).

20

In comparison, treatment of catalyst 5 with pyridine yielded a new material which from mass spectrometric analysis does not contain bromine but which turned out to be highly unstable and quickly degraded under nitrogen even at -20 °C.

21

General Procedure for Metathesis Reactions with Polymer 6a. To a suspension of polymer 6a (5 mol%) in dry toluene (0.02 M, 10 mL) under nitrogen was added the substrate (0.25 mmol). The resulting mixture was shaken for 4-7 h at 100 °C. At the end of the reaction (GC monitoring) the polymer was filtered off and washed with several portions of CH₂Cl₂. The solution was concentrated under reduced pressure and in most cases the crude material was sufficiently pure. In order to obtain analytically pure samples the crude material was purified by flash column chromatography (mixture of PE-EtOAc as eluent). Studies on the Stability of Polymer-Bound Catalysts 6a. Repeated reactions were carried out according to the general procedure given above by dissolving diallyl malonate 7 (55 µL, 0.228 mmol) in toluene (3 mL). After each reaction the polymer was filtered, washed with five portions of toluene (2 mL), dried under vacuum and reused for the next run. The crude products were isolated quantitatively; the yield of the RCM product 8 was determined after purification by flash column chromatography: 1^st run (2 h; 43.5 mg, 0.2 mmol; 89%); 2^nd run (2 h; 31.7 mg, 0.14 mmol; 65%); 3^rd run (2 h; 30.5 mg, 0.14 mmol; 64%); 4^th run (2 h; 13.8 mg, 60 µmol; 29%); 5^th run (6 h; 4.8 mg, 22 µmol; 10%).

22 Recently, Grubbs and coworkers were able to isolate a ruthenium-hydrido complex, formed as a thermal degradation product of catalyst 2 which could be made responsible for double-bond migration: Hong SH. Day MW. Grubbs RH. J. Am. Chem. Soc.  2004,  126:  7414 

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26b

Reaction was carried according to the general procedure given for the metathesis reactions with polymer 6a. Additionally, 1-octene (5 mol%) was added to the reaction mixture to yield 2,5-dihydro-1-tosyl-1H-pyrole(15) in 35% instead of 96% without 1-octene.