Highly Active Ammonium-Tagged Olefin-Metathesis Catalyst for Simplified Purification

Abstract

When an 2-isopropoxy-substituted benzylidene ligand of a Hoveyda-type Ru-metathesis complex is modified by an additional 4-diethylamino group and treated wit PTSA it yields a highly active precatalyst suitable for RCM, cross metathesis, and enyne metathesis. The important property of this ammonium tagged Ru-complex is associated with the possibility of its easy removal by a simple filtration protocol.

Key words

catalysis - olefin metathesis - ruthenium catalyst - tag

References and Notes

• For selected reviews on olefin metathesis, see:
• 1a Trnka TM. Grubbs RH. Acc. Chem. Res.  2001,  34:  18 
• 1b Handbook of Metathesis   Grubbs RH. Wiley-VCH; Weinheim: 2003. 
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• 1g For an industrial perspective: Thayer AM. Chem. Eng. News  2007,  85:  37 
• 2 Review: Clavier H. Grela K. Kirschning A. Mauduit M. Nolan SP. Angew. Chem. Int. Ed.  2007,  46:  6786 
• 3 Hong SH. Day MW. Grubbs RH. J. Am. Chem. Soc.  2004,  126:  7414 
• 4 Review: SledŸ P. Mauduit M. Grela K. Chem. Soc. Rev.  2008,  in press; DOI: 10.1039/b711482f
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  J. Am. Chem. Soc.  2004,  126:  9318 
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• 7b For a review on aqueous olefin metathesis, see: Burtscher D. Grela K. Angew. Chem. Int. Ed.  2008, in press; DOI: 10.1002/anie.200801451h
• 8 Bieniek M. Michrowska A. Guajski Ł. Grela K. Organometallics  2007,  26:  1096 
• 10 Complex 2b has been previously used for preparing of Hoveyda catalyst 3b, albeit with moderate 40% yield, see: Randl S. Gessler S. Wakamatsu H. Blechert S. Synlett  2001,  430 
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9

Synthesis of Complex 7
Carbene complex 2b (94.9 mg, 0.10 mmol), CuCl (14.9 mg, 0.15 mmol), and toluene (4 mL) were placed in a Schlenk flask equipped with a condenser. A solution of styrene (27.2 mg, 0.12 mmol) in toluene (2 mL) was added and the resulting solution was stirred under argon at 80 ˚C for 1 h. The reaction mixture was concentrated in vacuum and the resulting material was purified by column chromatography on SiO₂. Elution with c-hexane-EtOAc-Et₃N (4:1:0.1, v/v/v) yielded 7 as a dark brown microcrystalline solid (49.6 mg, 71%) after removal of the solvent, washing with a minimal amount of cold n-pentane and drying under vacuum. ^¹H NMR (500 MHz, CDCl₃): δ = 1.14 (t, J = 7.1 Hz, 6 H), 1.42 (d, J = 6.1 Hz, 6 H), 2.38 (s, 6 H), 2.49 (s, 12 H), 3.31 (t, J = 7.1 Hz, 4 H), 4.15 (s, 4 H), 4.86 (hept, J = 6.1 Hz, 1 H), 6.00 (d, J = 1.6 Hz, 1 H), 6.12 (dd, J = 8.8, 2.2 Hz, 1 H), 6.68 (d, J = 8.8 Hz, 1 H), 7.04 (s, 4 H), 15.65 (s, 1 H) ppm. ^¹³C NMR (125 MHz, CDCl₃): δ = 12.8, 20.0, 21.5, 25.8, 45.5, 55.0, 74.4, 96.3, 104.3, 115.0, 125.6, 129.7,130.5, 138.3, 138.3, 138.8, 150.8, 155.8, 216.2, 298.0 ppm.
Synthesis of Complex 10
Carbene complex 7 (27.9 mg, 0.04 mmol) was placed in a flask and dissolved in CH₂Cl₂ (2 mL). Next, PTSA˙H₂O was added, and this mixture was stirred for 30 min. During that time the reaction mixture changed its colour from brown to green. The reaction mixture was concentrated in vacuum and precipitated from CH₂Cl₂-n-pentane and dried under vacuum to afford the title compound 10 (34.8 mg, 99%) as a green microcrystalline solid. ^¹H NMR (500 Hz, CDCl₃): δ = 1.16 (t, J = 6.7 Hz, 6 H), 1.21 (d, J = 6.1 Hz, 6 H), 2.34 (s, 3 H), 2.39 (s, 6 H), 2.45 (s, 12 H), 3.28 (s, 3 H), 3.71 (s, 3 H), 4.18 (s, 4 H), 4.90 (hept, J = 6.1 Hz, 1 H), 6.92 (d, J = 8.5 Hz, 1 H), 7.06 (s, 4 H), 7.07 (dd, J = 4.9 Hz, 1 H), 7.16 (d, J = 7.9 Hz, 1 H), 7.29 (d, J = 15.8 Hz, 1 H), 7.75 (d, J = 7.9 Hz, 1 H) ppm. ^¹³C NMR (500 MHz, CDCl₃): δ = 10.4, 21.2, 21.5, 21.8, 29.8, 51.0, 53.9, 77.5, 109.7, 115.2, 123.4, 126.0, 129.0, 129.2, 129.5, 136.1, 139.2, 140.7, 141.2, 145.6, 152.9, 208, 291 ppm. ESI-HRMS: m/z calcd for C₄₂H₅₄N₃O₄C^¹0²Ru [M - 2 Cl - H⁺]: 798.2879; found: 798.2896.

12

We observed that after filtration of the reaction mixture the material deposited on SiO₂ is still green which is a clear indication that active catalyst has been removed. The catalyst decomposition products are commonly brown-black.

13

General Procedure for Metathesis Reactions
A reaction tube equipped with a magnetic stirring bar was charged with CH₂Cl₂ (10 mL), precatalyst 10 (5 mol%), and alkene (0.2 mmol). The reaction mixture was stirred at 25 ˚C. After complete conversion (monitored by TLC), the reaction mixture was passed through a cartridge containing activated acidic alumina (pH ca. 9.5, 1-2 g). The cartridge was washed with an additional portion of CH₂Cl₂ (10-20 mL). The CH₂Cl₂ fraction was concentrated under reduced pressure to yield the crude product.

16

Kirschning, A.; Harmrolfs, K.; Knobloch, T. C. R. Chim., 2008, in press, DOI: 10.1016/j.crci.2008.02.006.