DOI: 10.1002/chem.201101749 Nature of the Intermediates in Gold(I)-Catalyzed Cyclizations of 1,5-Enynes Verónica López-Carrillo, [a] Nfflria Huguet, [a] ngeles Mosquera, [a] and Antonio M. Echavarren* [a, b] Introduction In contrast to 1,6-enynes, which usually react by 6-exo-dig pathways with gold(I) catalysts, [1] 1,5-enynes 1 almost invari- ably undergo 5-endo-dig cyclizations through intermediates of type 2 to form derivatives 3, [1–9] since formation of inter- mediates with a bicycloACHTUNGTRENNUNG[3.1.0]hexane structure is more favor- able than formation of the strained bicycloACHTUNGTRENNUNG[2.1.0]pentane system resulting from exo cyclization (Scheme 1). [10, 11, 12] Ligands play a major role in the control of the regio or site selectivity in gold-catalyzed reactions of 1,6-eny- nes. [1b, 13, 14] Thus, reactions of gold complexes with highly electron-donating N-heterocyclic carbene (NHC) ligands proceed via intermediates with more carbene-like character, whereas reactions with phosphite–gold catalysts can be better interpreted as proceeding via carbocationic inter- mediates. [15, 16, 17, 18] However, the role of ligands in cycliza- tions of 1,5-enynes has not been examined systematically. Herein, we report the results of a study on the effect of li- gands in cyclizations of 1,5-enynes 1. Of particular mecha- nistic relevance, we have found the first examples of intra- molecular cyclopropanation of endo-gold carbenes formed in the cyclization of 1,5-enynes to give products of type 5 (Scheme 1), which proceeded in a concerted manner, ac- cording to DFT calculations. We also report the intramolec- ular opening of intermediates 2 by benzyl groups to give tri- cyclic compounds 6. Results and Discussion DFT calculations on a series of 1,5-enyne models show that the nature of intermediates 2 depends on the ligand on gold(I) (Figure 1). For comparison, Pt II intermediates 7 and free carbocations 8a (R = H) and 8b (R = Me) are also in- cluded for this study. Calculations show that metal-stabilized intermediates are significantly different from single homoal- lylic carbocations 8a,b. Intermediate 2 (L = PACHTUNGTRENNUNG(OPh) 3 ) shows the longest bond length (1.688 ) of the Au I complexes, but this distance is still 0.3  shorter than a classic carbocation, such as 8b (R = Me). In contrast, intermediate 2 with a strongly electron-donating ligand, such as Cl À , presents the shortest bond (1.606 ), which corresponds more closely to a cyclopropyl gold(I) structure. Although the effect is not Keywords: cyclization · density functional theory · enynes · gold · reaction mechanisms Abstract: The carbene or carbocationic nature of the intermediates in the gold-cat- alyzed cycloisomerization of 1,5-enynes can be revealed, depending on the ligands on the gold catalysts. Gold complexes with highly electron-donating ligands pro- mote reactions that proceed via intermediates with carbene-like character, leading to products with a bicycloACHTUNGTRENNUNG[3.1.0]hexene skeleton. The intermediate cyclopropyl endo-gold carbenes formed in this cyclization have been trapped, for the first time, to give biscyclopropane derivatives in a reaction that proceeds in a concerted fash- ion, according to DFT calculations. [a] V. López-Carrillo, N. Huguet, . Mosquera, + Prof. A. M. Echavarren Institute of Chemical Research of Catalonia (ICIQ) Av. Països Catalans 16, 43007 Tarragona (Spain) Fax: (+ 34) 977920225 E-mail : aechavarren@iciq.es [b] Prof. A. M. Echavarren Departament de Química Analítica i Química Orgµnica Universitat Rovira i Virgili, C/Marcel·li Domingo s/n 43007 Tarragona (Spain) [ + ] Summer Visiting Graduate Student from the Departamento de Quí- mica Fundamental, Universidade da CoruÇa (Spain). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101749. Scheme 1. Gold-catalyzed cyclization of 1,5-enynes via cyclopropyl gold(I) carbenes 2.  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 10972 – 10978 10972