Organometallics 2000, 19, 2661-2663
2661
Allyl/Alk yn e Cou p lin g Rea ction s Med ia ted by Neu tr a l
Ru th en iu m (II). Isola tion a n d Ch a r a cter iza tion of
Ru th en iu m (II) η3-Allyl η2-Alk yn e Com p lexes
Christina M. Older and J effrey M. Stryker*
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
Received March 30, 2000
Summary: In contrast to the cyclization products ob-
tained from allyl/ alkyne coupling reactions in the iso-
structural cationic series, neutral complexes of the form
(C5Me5)Ru(η3-allyl)(η2-alkyne) react to give acyclic η5-
pentadienyl complexes in high yield.
allyl)(η2-alkyne)]+ (eq 1) have been used to prepare
Investigations of transition-metal-mediated allyl/
alkyne coupling reactions have revealed a variety of
synthetically valuable or potentially exploitable reactiv-
ity patterns.1-8 In particular, metal-mediated allyl/
alkyne cycloaddition reactions using cationic late-
transition-metal complexes of the form [(CnRn)M(η3-
substituted η5-cyclopentadienyl complexes4-7 and, more
recently, η5-cycloheptadienyl complexes.8 To probe the
effects of reducing the electrophilicity of the metal
without substantially altering the molecular orbitals
involved, we have investigated the chemistry of the
isostructural but neutral (C5Me5)RuII template. In this
communication, we report the synthesis of a range of
isolable (C5Me5)Ru(η3-allyl)(η2-alkyne) complexes, which
undergo allyl/alkyne coupling to yield acyclic η5-penta-
dienyl complexes rather than cycloadducts.
Despite the kinetic lability of most allyl alkyne
complexes, (C5Me5)Ru(η3-allyl)(η2-PhCtCPh) (3a ) can
be synthesized by simply treating a THF solution of (C5-
Me5)Ru(η3-allyl)X2 (X ) Br, Cl)9 1 with Rieke zinc10 (10-
12 equiv) in the presence of diphenylacetylene (eq 2).11
Complex 3a was isolated as an air-stable orange mate-
rial by trituration of the crude residue into pentane.
Analytically pure complex 3a was obtained by recrys-
tallization from cold pentane (76%).12 The exo orienta-
tion of the allyl ligand in complex 3a was confirmed both
by difference NOE spectroscopy and by X-ray crystal-
lography (vide infra). While this complex is quite stable
in the solid state, slow conversion to the endo isomer is
observed upon standing in benzene solution at room
temperature (ca. 30% conversion after 24 h).
(1) Catalytic or stoichiometric organic reactions involving coupling
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Tetrahedron 1998, 54, 1063 and references therein. (g) Montgomery,
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(2) Coupling of η1-allyl complexes and alkynes: Welker, M. E. Chem.
Rev. 1992, 92, 97.
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J . Organomet. Chem. 1972, 39, C13. (c) Sbrana, G.; Braca, G.;
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The corresponding 2-butyne complex (C5Me5)Ru(η3-
allyl)(η2-MeCtCMe) (3b) can be prepared in a similar
manner, although it is thermally more labile than
diphenylacetylene complex 3a . Prolonged exposure of
3b at room temperature to either vacuum or the
atmosphere results in decomposition to unidentifiable
products, yet this complex is indefinitely stable in the
(9) (a) Nagashima, H.; Mukai, K.; Shiota, Y.; Yamaguchi, K.; Ara,
K.; Fukahori, T.; Suzuki, H.; Akita, M.; Moro-oka, Y.; Itoh, K.
Organometallics 1990, 9, 799. (b) Masuda, K.; Saitoh, M.; Aoki, K.;
Itoh, K. J . Organomet. Chem. 1994, 473, 285.
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Hanson, M. V. In Active Metals; Fu¨rstner, A., Ed.; VCH: New York,
1996; Chapter 1.
(6) Schwiebert, K. E.; Stryker, J . M. Organometallics 1993, 12, 600.
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(11) Freshly acid-washed zinc powder also reduces (C5Me5)Ru(η3-
allyl)X2, but the reaction proceeds much more slowly.
(12) Complete experimental and characterization data are provided
as Supporting Information.
10.1021/om000275n CCC: $19.00 © 2000 American Chemical Society
Publication on Web 06/13/2000