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Synlett 2003(3): 0314-0320
DOI: 10.1055/s-2003-37106
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Enol Esters and Dimerization of Terminal Alkynes Catalyzed by Neutral and Cationic Vinylidene Ruthenium Complexes

Tom Opstal, Francis Verpoort*
Department of Inorganic and Physical Chemistry, Laboratory of Organometallics and Catalysis, Ghent University, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
Fax: +32(9)2644983; e-Mail: Francis.Verpoort@rug.ac.be;
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Publication History

Received 7 January 2003
Publication Date:
07 February 2003 (online)

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Abstract

In the current study Ru(II) vinylidene complexes of the general type: Cl2Ru{=C=C(H)R}(PR′3)L (R = Ph, SiMe3, R′ = Ph, Cyclohexyl (Cy) and L = phosphine or N-heterocyclic carbene) are synthesized and tested for the addition of carboxylic acids to terminal alkynes. A careful choice of the catalytic system, substrate and carboxylic acid gives access to alk-1-en-2-yl esters, alk-1-en-1-yl esters or enyne dimerization products.

Furthermore, an extension was made to synthesize an analogous 14-electron species by treating one of the complexes with AgBF4 and its influence on the catalytic activity and selectivity are investigated.

Key words

enol esters - homogeneous catalysis - NHC-ligands - ruthenium - vinylidene

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Synthesis of Complexes 2 and 4: To a suspension of [RuCl2(p-cymene)]2 (0.72 g, 1.17 mmol) in toluene (45 mL), the phosphine PCy3 (1.32 g, 4.7 mmol) or PPh3 (1.23 g, 4.7 mmol) was added and stirred at r.t. The mixture instantly changed into a reddish brown solution. Me3SiCCH (3.34 mL, 23.5 mmol) was added and the solution was stirred for 1 h at r.t. The solution instantly darkened to a dark red solution. After 1 h the temperature was gradually risen to 60 °C and stirred during one night. The analytical pure compound was obtained (65% yield) after washing the crude product with methanol. Cl2Ru{=C=C(H)SiMe3}(PPh3)2(2): 1H NMR (299.89 MHz, C6D6, 25 °C): δ = 7.82-7.64 and 7.00-6.65 (each m, 30 H, PPh3), 0.33 (s, 9 H, SiCH 3), 0.01 (dt, J (RuH) = 2.1 Hz, J (PH) = 2.5 Hz, 1 H, =CHSiMe3).
13C NMR (75.41 MHz, C6D6, 25 °C): δ = 279.33 (dt, J (RuH) = 57.0 Hz, J (PC) = 15.4 Hz, Ru=C=C), 86.45 (dt, J (RuH) = 15.7 Hz, J (PC) = 5.1 Hz, Ru=C=C), 150.16 (s, C1 of PPh), 129.77, 128.00, 126.87 (s, PPh). 31P NMR {1H} (121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ = 26.3 (s). IR (KBr): ν = 1625 (C=C) cm-1. Anal. Calcd for C41H40Cl2P2SiRu: C, 61.96; H, 5.07. Found: C, 62.30; H, 5.82. Cl2Ru{=C=C(H)SiMe3}(PCy3)2(4): 1H NMR (299.89 MHz, C6D6, 25 °C): δ = 2.68-2.59, 2.13-1.97, 1.89-1.64, 1.26-1.16 (each m, 66 H, PCy3), 0.29 (s, 9 H, SiCH 3), 0.023 (dt, J (RuH) = 1.9 Hz, J (PH) = 2.8 Hz, 1 H, =CHSiMe3). 13C NMR (75.41 MHz, C6D6, 25 °C): δ = 274.30 (dt, J (RuH) = 57.2 Hz, J (PC) = 15 Hz, Ru=C=C), 81.20 (dt, J (RuH) = 16 Hz, J (PC) = 5 Hz, Ru=C=C), 35.46 (pseudo triplet, J = 8.7 Hz, C1 of PCy), 30.14 (s, C3,5 of PCy), 27.83 (pseudo triplet, J = 4.2 Hz, C2,6 of PCy), 26.35 (s, C4 of Pcy). 31P NMR {1H} (121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ = 31.50 (s). IR (KBr): ν = 1630 (C=C) cm-1. Anal. Calcd for C41H76Cl2P2SiRu: C, 59.26; H, 9.58. Found: C, 59.52; H, 10.10.

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A Typical Procedure for the Preparation of Complex 6 is as follows. A solution (0.1 M in THF) of the 4,5-dihydroimidazol-2-ylideen tetrafluoro-borate salt (300 ml) (STREM), a magnetic stirring bar and toluene (1 mL) were added to a glass vessel. t-BuOK was added (30 µL 1 M in Et2O, Aldrich) to the rapidly stirred suspension at r.t., resulting in the immediate dissolution of the salt to form a light yellow solution. After 5 min, a 0.1 M solution of the vinylidene (3 or 4, 250 µL) or alkylidene [Cl2(PCy3)2Ru
(= CHPh)] complex in toluene were added via cannula. The mixture was heated to 70 °C for 1 h and subsequently cooled to r.t. Complex 6: 1H NMR (299.89 MHz, C6D6, 25 °C):
δ = 7.03-6.96 (br, 2 H, Mes), 6.92-6.86 (br, 2 H, Mes), 3.49 (s, 4 H, imidazolium), 2.44-2.10, 1.99-1.84 (br,18 H, Mes), 2.12 (m, 3 H, C1 PCy3), 1.55-1.52, 1.39, 1.02-0.96 (m, 32 H, PCy3) 0.05 (s, 9 H, SiCH 3), -0.15 (dt, J (RuH) = 1.9 Hz, J (PH) = 2.8 Hz, 1 H, =CHSiMe3). 13C NMR (75.41 MHz, C6D6, 25 °C): δ = 267.27 (dt, J (RuH) = 56.8 Hz, J (PC) = 15 Hz, Ru=C=C), 194.46 (s, J (CP) = 80 Hz, Ru-CNN), 144.82, 141.00, 137.64, 134.95, 131.93 (all s, Mes), 129.01 (d, J (CH) = 150 Hz, Mes), 128.82 (d, J (CH) = 130Hz, Mes), 71.05 (dt, J (RuH) = 15 Hz, J (PC) = 5.5 Hz, Ru= C=C), 31.30 (pseudo triplet, J = 9 Hz, C1 of PCy3), 29.96 (s, C3,5 of PCy), 27.87 (pseudo triplet, J = 4 Hz, C2,6 of PCy3), 26.36 (s, C4 of PCy3) 21.96, 21.13, 19.54, 18.61 (all s, Mes). 31P NMR {1H} (121.40 MHz, C6D6, 25 °C, ref. H3PO4): δ = 28.03 (s). IR (KBr): ν = 1634 (C = C) cm-1. Anal. Calcd for C44H69N2Cl2PSiRu: C, 61.66; H, 8.11; N, 3.27. Found: C, 62.98; H, 9.23; N, 4.03.