pubs.acs.org/Organometallics Published on Web 10/02/2009 r 2009 American Chemical Society 6036 Organometallics 2009, 28, 6036–6043 DOI: 10.1021/om900544u Rates and Mechanism of Rhodium-Catalyzed [2þ2þ2] Cycloaddition of Bisalkynes and a Monoalkyne Anna Dachs, †,‡ Anna Torrent, ‡ Anna Pla-Quintana, ‡ Anna Roglans,* ,‡ and Anny Jutand* ,† † D epartement de Chimie, Ecole Normale Sup erieure, UMR CNRS-ENS-UPMC 8640, 24 Rue Lhomond, F-75231 Paris Cedex 5, France, and ‡ Departament de Quı´mica, Universitat de Girona, Campus de Montilivi, s/n 17071 Girona, Spain Received June 23, 2009 The mechanism of RhCl(PPh 3 ) 3 -catalyzed [2þ2þ2] cycloaddition of alkynes is investigated in the case of the reaction of symmetrical diynes 1a and 1b with the monoalkyne 3 (HOCH 2 -CtC-CH 2 OH), leading to highly substituted benzene derivatives in dichloromethane at 25 °C. The two main steps of the catalytic cycle are characterized. The intermediate rhodacyclopentadiene Rh III complexes 2a and 2b (formed by oxidative coupling after the coordination of the diynes 1a and 1b to RhCl(PPh 3 ) 2 ) are characterized by cyclic voltammetry, conductivity measurements, 31 P NMR, and ESI-MS. The formation of complexes 2a and 2b (step A) and their further reactions with the monoalkyne 3, which deliver RhCl(PPh 3 ) 3 and the final product (step B), are followed by means of electrochemical techniques that deliver kinetic data for the two successive separately investigated steps. From the relative values of the half-reaction times of step A (t A1/2 = 650 and 75 s for 1a and 1b, respectively) and step B (t B1/2 = 130 and 680 s for 2a and 2b, respectively) determined under stoichiometric conditions, it emerges that step A (coordination of the two CtC bonds of 1, followed by oxidative coupling) is rate-determining in the reaction involving 2a, whereas step B (reaction of the intermediate complexes 2 with the monoalkyne 3) is rate-determining in the reaction involving 1b. Kinetic data for the catalytic cycle of a RhCl(PPh 3 ) 3 -catalyzed [2þ2þ2] cycloaddition of alkynes are thus presented for the first time. Introduction Transition-metal-catalyzed [2þ2þ2] cycloaddition reac- tions of three alkynes is one of the most elegant methods for the construction of three new bonds in a one-step process to generate polysubstituted benzene derivatives. 1 Various transition-metal complexes, including those of Ni, Co, Rh, Ru, Mo, Ir, Pd, and Fe, have been shown to be efficient catalysts in this type of process. 1 Our group has recently demonstrated 2 that the [2þ2þ2] cycloisomerization of aza- macrocyclic triynes can be achieved in the presence of catalytic amounts of the Wilkinson catalyst (RhCl(PPh 3 ) 3 ), affording efficiently fused tetracycles with a benzene core. In order to comprehensively establish this methodology, it is necessary to elucidate the mechanistic aspects of the [2þ2þ2] cycloadditions of alkynes. 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