Acid controlled alkyne dimerisation initiated by a Ru  /carbene precursor Karen Melis a , Dirk De Vos b , Pierre Jacobs b , Francis Verpoort a, * a Department of Inorganic and Physical Chemistry, Division of Organometallic Chemistry and Catalysis, Ghent University, Krijgslaan 281 (S-3), 9000 Ghent, Belgium b Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Received 31 May 2002; received in revised form 15 July 2002; accepted 15 July 2002 Abstract The Grubb’s catalyst Cl 2 (PR 3 ) 2 Ru Ä/CHPh (1) is an excellent precursor for the dimerisation of terminal alkynes. Thermal treatment and addition of two equivalents of phenylacetylene to complex 1 generates a new Ru  /vinylidene (3). Complex 3 catalyses a selective product formation for trans -tail-to-tail enynes. Addition of acetic acid enhances the yield and the reaction rate dramatically and a reversed stereoselectivity for the formation of Z -isomers is obtained. The stereoselectivity can be easily tuned by addition of acetic acid. The influence of the acid on the reaction pathway has been revealed. A simple one-pot preparation of tail-to- tail enynes is established. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Homogeneous catalysis; Dimerisation; Ruthenium; Carbene ligands; Vinylidene ligands 1. Introduction Metal vinylidenes (M Ä/C Ä/CHR) have emerged as useful precursors with an unusual reactivity for a variety of organic reactions of industrial importance [1]. The direct simple formation of metal vinylidene intermedi- ates from commercially available terminal alkynes and a metal complex made the transfer from stoichiometric to catalytic reactions possible. Based on the fundamental mechanism metal vinylidenes are involved in catalytic reactions of the type: dimerisation of alkynes, [2 /2] cycloaddition, nucleophilic addition to alkynes and radical cycloaromatisation. Direct coupling of 1-al- kynes, i.e. dimerisation, represents an easy route to unsaturated dimeric species, in particular 1,3- and 1,4- disubstituted enynes [2]. These are valuable precursors for the synthesis of natural products as well as interest- ing building blocks for further organic modifications [3  / 7]. Nucleophilic addition of carboxylic acids to terminal alkynes, also known as vinylation, produces enol esters in a regio- and stereoselective manner. Enol esters are useful intermediates for carbon  /carbon and carbon  / heteroatom bond formation. They have been used for the selective generation of enolates, acylation of car- bonyl compounds and O - and N -acylation under mild conditions [8 /11]. Another important feature is the rearrangement of the vinylidene ligand ([M] Ä/C Ä/CHR) to an alkylidene moiety ([M] Ä/CHR) in the presence of an olefin [12  /14]. This enlarges the application field of vinylidenes to valuable, easy accessible precursors for selective cross-metathesis, ring-closing metathesis and ring opening metathesis polymerisation (ROMP). A well-known stable, well-defined metal  /alkylidene com- plex for olefin metathesis is the Grubb’s ‘first genera- tion’ catalyst (Cl 2 (PR 3 ) 2 Ru Ä/CHPh) [15,16]. The wide spread use of the Ru  /alkylidene is derived from its activity, stability and functional group tolerance [17 / 20]. The catalyst remains active in the presence of a variety of functional groups like carbonyls, amides and alkohols. However, the thermolytic decomposition is a major drawback and limits the use in many challenging reactions [21]. The alkylidene decomposition proceeds predominantly via a second order reaction pathway requiring phosphine dissociation. The nature of the inorganic decomposition products still remains uniden- * Corresponding author. Tel.:  /32-9-264-4436; fax:  /32-9-264- 4983 E-mail address: francis.verpoort@rug.ac.be (F. Verpoort). Journal of Organometallic Chemistry 659 (2002) 159  /164 www.elsevier.com/locate/jorganchem 0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0022-328X(02)01722-9