DOI: 10.1002/chem.201000855 Reactions of Alkynes with [RuCl(cyclopentadienyl)] Complexes: The Important First Steps Barnali Dutta, Basile F. E. Curchod, Pablo Campomanes, Euro Solari, Rosario Scopelliti, Ursula Rothlisberger, and Kay Severin* [a] Introduction Half-sandwich complexes containing the {RuClACHTUNGTRENNUNG(Cp*)} frag- ment (Cp* = h 5 -C 5 Me 5 ), such as [RuClACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(Cp*)] (cod = 1,5-cyclooctadiene), [RuClACHTUNGTRENNUNG(Cp*)ACHTUNGTRENNUNG(PPh 3 ) 2 ], and [RuClACHTUNGTRENNUNG(Cp*)] 4 , are potent and versatile catalysts for organic transformations involving alkynes. [1] The catalytic reactions include the for- mation of arenes by [2+2+2] cyclotrimerizations, [2] the for- mation of triazoles by [2+3] cycloadditions, [3] the formation of cyclobutenes by [2+2] cycloadditions with bicyclic al- kenes, [4] the formation of dienylesters by the coupling of al- kynes with carboxylic acids, [5] the formation of g,d-unsatu- rated aldehydes by reaction with allyl alcohol, [6] and the for- mation of silylated 1,3-dienes by reaction with trimethylsilyl- diazomethane [7] (Scheme 1). Abstract: Cyclopentadienyl–ruthenium half-sandwich complexes with h 2 -bound alkyne ligands have been suggested as catalytic intermediates in the early stages of Ru-catalyzed reactions with alkynes. We show that electronically unsaturated complexes of the formula [RuClACHTUNGTRENNUNG(Cp^)(h 2 -RCCR’)] can be sta- bilized and crystallized by using the sterically demanding cyclopentadienyl ligand Cp^ (Cp^ = h 5 -1-methoxy-2,4- tert-butyl-3-neopentyl-cyclopentadien- yl). Furthermore we demonstrate that [RuCl 2 ACHTUNGTRENNUNG(Cp^)] 2 is an active and regiose- lective catalyst for the [2+2+2] cyclo- trimerization of alkynes. The first ele- mentary steps of the reaction of mono(h 2 -alkyne) complexes containing {RuClACHTUNGTRENNUNG(Cp*)} (Cp* = h 5 -C 5 Me 5 ) and {RuClACHTUNGTRENNUNG(Cp^)} fragments with alkynes were investigated by DFT calculations at the M06/6-31G* level in combina- tion with a continuum solvent model. Theoretical results are able to rational- ize and complement the experimental findings. The presence of the sterically demanding Cp^ ligand increases the activation energy required for the for- mation of the corresponding di(h 2 - alkyne) complexes, enhancing the ini- tial regioselectivity, but avoiding the evolution of the system towards the ex- pected cyclotrimerization product when bulky substituents are present. Theoretical results also show that the electronic structure and stability of a metallacyclic intermediate is strongly dependent on the nature of the sub- stituents present in the alkyne. Keywords: alkynes · cyclotrimeriza- tion · density functional calcula- tions · organometallic chemistry · ruthenium [a] B. Dutta, B. F.E. Curchod, Dr. P. Campomanes, Dr. E. Solari, Dr. R. Scopelliti, Prof. U. Rothlisberger, Prof. K. Severin Institut des Sciences et IngØnierie Chimiques École Polytechnique FØdØrale de Lausanne (EPFL) 1015 Lausanne (Switzerland) Fax: (+ 41) 21-693-9305 E-mail: kay.severin@epfl.ch Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000855. Scheme 1. ACHTUNGTRENNUNG[RuClACHTUNGTRENNUNG(Cp*)(L 2 )] complexes (L = PR 3 , olefin) are versatile cat- alysts for organic transformations of alkynes. # 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 8400 – 8409 8400