Applied Catalysis A: General 433–434 (2012) 214–222 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata A practical ruthenium based catalytic system bearing a switchable selectivity between the dimerization and cyclotrimerization reactions of alkynes Bengi Özgün Öztürk, Solmaz Karabulut ∗ , Yavuz ˙ Imamo˘ glu Hacettepe University, Department of Chemistry, 06800 Beytepe, Ankara, Turkey article info Article history: Received 24 March 2012 Received in revised form 2 May 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Alkyne dimerization Ruthenium Enynes Switchable catalysis abstract In this study, a practical and inexpensive switchable catalytic system (cyclotrimerization vs. dimeriza- tion), [RuCl 2 (p-cymene)] 2 /PR 3 has been developed for the catalytic dimerization of terminal alkynes. Bulky and basic phosphine derivatives, PCy 3 and P( i- Pr) 3 , were used with [RuCl 2 (p-cymene)] 2 and excess of terminal alkyne to in situ formation of vinylidenic intermediates which are active towards dimerization reactions. Effect of phosphine/ruthenium ratio has been investigated. A solvent study was carried out and toluene was found to be the most versatile solvent for both cyclotrimerization and dimerization reac- tions. A set of aryl and alkyl acetylenes were chosen as substrates to investigate the effect of the nature of the substrates on alkyne dimerization reactions catalyzed by [RuCl 2 (p-cymene)] 2 /PR 3 . In conclusion, we have shown that [RuCl 2 (p-cymene)] 2 /PCy 3 can be used as a practical and inexpensive catalytic system which has a switchable selectivity towards cyclotrimerization and dimerization reactions. Best results in means of regioselectivity and yield were observed by using arylacetylene derivatives in these reac- tions. This catalytic system emerges as an economical method for the transformation of arylacetylenes to corresponding enyne and arene derivatives in excellent yields and selectivity. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Transition metal catalyzed carbon carbon bond formation reac- tions have proven to be highly selective, and atom-economical synthetic methods have been used to obtain novel organic inter- mediates which cannot be obtained by other means [1]. Alkynes undergo a great diversity of reactions in the presence of transi- tion metal catalysts, thus making them good candidates as starting materials for the synthesis of both organic and organometallic sub- stances [2]. The catalytic conversion of alkynes yields important organic intermediates like enynes, enol-esters and arene deriva- tives [3]. Alkyne dimerization reactions are of essential importance because the resulting enyne substructures are important build- ing blocks for organic synthesis and are significant components in various biologically active compounds [4]. Enynes are also potential candidates for making conjugated and light-emitting polymers [5]. It has been reported that enynes can also be used for alkyne dimerization reactions form several possible iso- mers, among them, (Z)-RCH CH C CR, (E)-RCH CH C CR and (gem) H 2 C CR C CR are the most common ones. For real and practical applications, one desires a process that exhibits both high ∗ Corresponding author. Tel.: +90 312 297 60 82; fax: +90 312 299 21 63. E-mail address: solmazk@hacettepe.edu.tr (S. Karabulut). regio- and stereoselectivity towards one isomer. To date, several transition metal complexes have been reported to catalyze the dimerization reactions of alkynes in a selective manner [6]. Among these transition metals, recent studies have focused on ruthenium complexes because they exhibit high functional group tolerance and a wide range of oxidation states and coordination geometries, providing unique properties for catalysis [2a]. A wide range of very different (mechanistically) processes are catalyzed by ruthenium complexes using these unique properties. Each carbon carbon bond forming reaction has a unique intermediate and mechanism. For example, alkyne dimerization reactions are known to be cat- alyzed by the formation of vinylidene or acetylide intermediates [7], whereas inter- or intramolecular [2 + 2 + 2] cyclotrimerization reactions of alkynes proceed by the formation of a metallocyclopen- tadiene intermediate [8] or in the presence of a metal alkylidene moiety, the reaction proceeds via cascade metathesis reactions [9]. Due to the growing interest in this field, several research groups have focused their attention on the dimerization reactions of ter- minal alkynes catalyzed by various ruthenium complexes [10]. Pavlik et al. reported the activity of neutral ruthenium vinyli- dene -alkynyl complexes on alkyne dimerization reactions [11]. Katayama and Ozawa [12] reported an efficient protocol to convert [RuCl 2 (p-cymene)] 2 , in the presence of basic and bulky phosphine ligands and terminal alkynes, to corresponding ruthenium vinyli- dene complexes with the general formula RuCl 2 { C C(E)R}(L) 2 , where E = H; R = Ph, and Fc, t-Bu and L = PCy 3 and P( i- Pr) 3 . These 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.05.020