DOI: 10.1002/chem.201000988 Isolation of a Zwitterionic DienegoldACHTUNGTRENNUNG(III) Complex Intermediate in the Direct Conversion of Enyne–Amines to Cyclopentadienes Michele Melchionna, [a] Martin Nieger, [b] and Juho Helaja* [a] Gold-mediated homogeneous catalysis has shown, recent- ly, its power, uniqueness, and versatility especially in electro- philic activation of alkynes for nucleophilic attack. [1] Cycloi- somerization of enynes, and other alkynes carrying nucleo- philic groups represents a large category in gold mediated catalysis. Due to the fact that this chemistry is in its infancy, only a few mechanistic studies have been reported. [1 f, 2] The recent findings in gold and other late transition-metal catalysis have provided effective alternatives to classical Pauson–Khand cyclocarbonylation [3] and the Nazarov cycli- zation [4] for the synthesis of cyclopentadienes (Cp). Toste et al. have shown that the cationic Au I species Ph 3 PAuCl/ AgSbF 6 catalyses the cycloisomerazation of 1,2,4-triene (enallene; 2) to cyclopentadiene (Cp; 3 ; Scheme 1) with high yields and turnover numbers. [5] In line with these find- ings, Iwasawa et al. have shown that the same conversion can be catalyzed by PtCl 2 with equal performance. [6] An evi- dent restriction in these methods is that the synthesis of the prerequisite starting material (allenes) proceeds in many cases with modest or low yields when traditional methods are applied. Nevertheless, Che)s group has recently intro- duced a facile method to access (asymmetric) allenes, 2, from aminoalkynes, 1, either by KAuCl 4 or AgNO 3 cataly- sis. [7, 8] Inspired by these achievements we set out to investigate the possibility of a direct conversion of enyne–amine 1 to Cp 3, thus reducing a two-step sequence (synthesis of allene followed by cycloisomerization) to a single catalytic event. For the gold-catalyzed allene formation from propargyla- mines, deuterium labeling experiments have proven that a 1,5-hydride migration from the amine moiety controls the progress and direction of the reaction (Scheme 2). [8] For sys- tems in which intramolecular migrations occur by other nu- cleophiles than hydride, there are examples showing how the migration mode, and therefore the direction of reaction can be controlled, for example by the choice of cationic Au I or neutral AuACHTUNGTRENNUNG(I/III) catalysts, as well as by slight modifica- tions of the substrate. These two factors can both have a [a] Dr. M. Melchionna, Dr. J. Helaja Department of Chemistry, University of Helsinki A.I.Virtasen aukio 1, P.O. Box. 55, University of Helsinki 00014 Helsinki (Finland) Fax: (+ 358) 919150466 E-mail: juho.helaja@helsinki.fi [b] Dr. M. Nieger Laboratory of Inorganic Chemistry, Department of Chemistry University of Helsinki, A.I.Virtasen aukio 1 P.O. Box. 55, 00014 Helsinki (Finland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000988. Scheme 1. Transition-metal catalyzed stepwise conversions from enyne– amine, 1 via enallene, 2 to Cp 3. Scheme 2. Mechanistic pathways from enyne–amines by a) proven 1,5-hy- dride migration to enallenes, or b) hypothesized 1,4-hydride migration to Cps. # 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 8262 – 8267 8262