Note Co(I)- versus Ru(II)-Catalyzed [2þ2þ2] cycloadditions involving alkynyl halides Laura Iannazzo a,1 , Naoko Kotera a, 1 , Max Malacria a, 1 , Corinne Aubert a, ** , Vincent Gandon a, b,1 , * a Institut Parisien de Chimie Moléculaire (UMR CNRS 7201)-FR 2769 UPMC univ Paris 06, C. 229, 4 place Jussieu, 75005 Paris, France b ICMMO (UMR CNRS 8182), Université Paris-Sud 11, 91405 Orsay cedex, France article info Article history: Received 22 June 2011 Received in revised form 18 August 2011 Accepted 25 August 2011 Keywords: Alkynes Catalysis Cobalt Cycloadditions Ruthenium abstract Alkynyl chlorides, bromides, and iodides have been tested as [2 þ 2 þ 2] cycloaddition partners using CpCo(CO)(dimethylfumarate) and Cp*Ru(cod)Cl as precatalysts. A series of cocyclizations between diynyl dihalides and alkynes, as well as intramolecular cycloadditions of triynyl dihalides, has been carried out. While this study confirmed the versatility of the ruthenium complex with all kinds of halides, the cheap air-stable cobalt complex proved nonetheless efficient with alkynyl bromides. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The transition-metal-catalyzed [2 þ 2 þ 2] cycloaddition reaction has become an increasingly popular way to form functionalizable six-membered rings in the organic chemists’ community [1]. Ever since the first total synthesis involving this reaction in 1980 [2], more than 30 natural products have been prepared owing to this strategy [3]. For instance, Nicolaou’s approach to Sporolide B relies on a Ru(II)-catalyzed [2 þ 2 þ 2] cycloaddition between three alkyne partners including an alkynyl chloride [4]. Alkynyl chlorides [4,5], bromides [5,6], and iodides [6,7] have been only scarcely employed in transition-metal-catalyzed [2 þ 2 þ 2] cycloadditions, and these reactions have been strictly limited to the use of Cp*Ru(cod)Cl as precatalyst. Due to their higher tendency to insert into the CeX bond of alkynyl halides, materialized by the formation of homocoupling products (i.e. 1,2-diynes) [5,8], cobalt-based catalysts are notably absent in this chemistry. There is yet one exception in the bromide series: we have recently synthesized the air-stable cobalt complex 1 which allowed the cyclization of a triyne into the corresponding 1,2- dibromobenzene derivative in excellent yield (Scheme 1) [9]. Due to the potential importance of this methodology for the synthesis of complex molecules, we decided to study in detail the behavior of 1 toward alkynyl halides and to compare the results with those ob- tained with Cp*Ru(cod)Cl. The comparison between cobalt and ruthenium is particularly worthwhile since complex 1 is about 30 times cheaper than Cp*Ru(cod)Cl [10]. Besides, we have explored a new type of cyclization between diynyl dichlorides and alkynes. 2. Results and discussion We started our investigation by a brief examination of the reac- tivity of monoalkynes. Because Ru-catalyzed cyclotrimerizations of alkynyl chlorides and bromides have been previously described [5], and because alkynyl chlorides are prone to homocoupling under cobalt-catalysis [8], we focused on the reactivity of a bromide and an iodide in the presence of 1 (Scheme 2). Homocoupling of iodide 2b occurred highly selectively to afford 1,2-diyne 3 quantitatively. In contrast, neither cyclotrimerization nor homocoupling took place when using bromide 2a. The fact that alkynyl bromides apparently do not autotrimerize with 1 makes them potential candidates for cocyclization with other alkyne partners. We first focused on diynyl dihalides and compared cobalt with ruthenium catalysis (Table 1). In addition to the halide, the nature of the tether Z (carbon-, oxygen-, nitrogen- linkers) and of the R 1 and R 2 substituents (H, vinyl, aryl, alkyl, and ester) at the alkyne was also varied. Two experimental conditions were used: A (DCE, rt, 12 h) which are standard ones for Ru- * Corresponding author. ICMMO (UMR CNRS 8182), Université Paris-Sud 11, 91405 Orsay cedex, France. Tel.: þ33 (0) 1 69 15 39 31; fax: þ33 (0) 1 69 15 4747. ** Corresponding author. Tel.: þ33 (0) 1 44 27 70 68; fax: þ33 (0) 1 44 27 73 60. E-mail addresses: corinne.aubert@upmc.fr (C. Aubert), vincent.gandon@u-psud. fr (V. Gandon). 1 Tel.: þ33 (0) 1 44 27 70 68; fax: þ33 (0) 1 44 27 73 60. Contents lists available at SciVerse ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.08.041 Journal of Organometallic Chemistry 696 (2011) 3906e3908