Angewandte
Chemie
DOI: 10.1002/anie.201407055
Reactive Intermediates
Mechanistic Studies on the Rearrangement of 1-Alkenyl-2-
alkynylcyclopropanes: From Allylic Gold(I) Cations to Stable
Carbocations**
Eva Tudela, Jairo Gonzꢀlez, Rubꢁn Vicente, Javier Santamarꢂa, Miguel A. Rodrꢂguez, and
Alfredo Ballesteros*
Abstract: An allylic gold(I) cation, proposed as key inter-
mediate in the gold-promoted rearrangement of 1,5-enynes
bearing a fixed conformation, has been detected and charac-
terized by NMR spectroscopy. Moreover, its participation in
the overall transformation was confirmed. Computational
studies indicate that the gold-catalyzed transformation occurs
through an uncommon rearrangement. Additionally, this study
led us to isolate and characterize a stable homoantiaromatic
carbocation.
Scheme 1. a) Resonance structures in gold(I)-promoted enyne cyclo-
G
old-catalyzed cycloisomerization of 1,n-enynes represents
isomerizations. b) Coordination of gold(I) to 1,n-enynes. c) NMR
evidence of gold(I) intermediates in cycloisomerizations.
a paradigm in gold catalysis.[1] From these substrates, a great
variety of products with an increased molecular complexity
can be easily accessed by skeletal rearrangements. These
transformations involve the nucleophilic attack of the alkene
onto the gold-activated alkyne, thus leading to an intermedi-
ate which can be depicted with multiple resonance structures
(Scheme 1a).[2] Thus, the reaction outcome has been related
to the contribution of these resonance structures, which can
be strongly influenced by the enyne structure, catalyst, or
reaction conditions. Furthermore, the high reactivity of the
proposed intermediates makes their trapping a remarkably
challenging task. In this sense, NMR studies have shown that
gold(I) coordinates to both the alkene and alkyne to give an
equilibrium mixture (Scheme 1b),[3] while unique experimen-
tal evidence of a gold(I) intermediate has been recently
reported by Widenhoefer and co-workers in the cycloisome-
rization of a 1,6-enyne (Scheme 1c).[4]
We have recently studied the reactivity of a specific class
of 1,5-enynes, that is, the alkynylcyclopropanes 1, which
Scheme 2. Gold(I)-catalyzed rearrangement of the enyne 1 into the
cyclohexadienes 2 and 3 and previously proposed mechanism.
[*] Dr. E. Tudela, J. Gonzꢀlez, Dr. R. Vicente, Dr. J. Santamarꢁa,
Prof. Dr. A. Ballesteros
undergoes a gold-catalyzed rearrangement to the alkynylcy-
clohexadienes 2 and 3 (Scheme 2).[5] At that stage, a tentative
pathway for the formation of 2 and 3 involving a 3-exo-dig
attack via the intermediates I and I’, respectively, was
suggested. The formation of 2 and 3 was justified assuming
an isomerization of 1 through a reversible 6-endo-dig attack of
the olefin involving intermediate II, a formal allylic gold(I)
cation. To gain further proof on the existence of intermediates
such as II, as well as to unravel the reaction pathway, we
carried out experimental and computational studies, the
results of which are described herein.
Instituto de Quꢁmica Organometꢀlica “Enrique Moles” and
Departamento de Quꢁmica Orgꢀnica e Inorgꢀnica, Universidad de
Oviedo, c/Juliꢀn Claverꢁa 8, 33007, Oviedo (Spain)
E-mail: abg@uniovi.es
Prof. Dr. M. A. Rodrꢁguez
Departamento de Quꢁmica, Centro de Investigaciꢂn en Sꢁntesis
Quꢁmica, Universidad de la Rioja, 26006 LogroÇo (Spain)
[**] We are grateful to the MINECO (Spain) (Projects CTQ-2010-20517-
C02, CTQ2013-41511-P and CTQ2011-24800) for financial support.
R.V. is a Ramꢂn y Cajal fellow. We thank Dr. I. Merino and Dr. E.
Rubio for the assistance in NMR experiments, Dr. A. L. Suꢀrez-
Sobrino for the X-Ray analysis, and Prof. Dr. J. Barluenga and
Prof. Dr. M. Tomꢀs for their suggestions.
Preliminary indications on the participation of II were
gained by its trapping with alcohols as nucleophiles,[5] and led
to the isolation of the compounds 4 and 5 (see Scheme 3)
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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