Organometallics 2010, 29, 3589–3592 3589
DOI: 10.1021/om100507r
Exploring the Scope of Nitrogen Acyclic Carbenes (NACs) in
Gold-Catalyzed Reactions
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Camino Bartolome, Domingo Garcıa-Cuadrado, Zoraida Ramiro, and Pablo Espinet*
´
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IU CINQUIMA/Quımica Inorganica, Facultad de Ciencias, Universidad de Valladolid,
E-47071 Valladolid, Spain
Received May 24, 2010
The catalytic activity of the recently reported nitrogen acyclic carbene (NAC) complexes of gold(I)
has been investigated and compared with the reported activity of other gold(I) and gold(III)
complexes. The complexes studied, [AuCl{C(NEt2)(NHTol-p)}], [AuCl{C(NEt2)(NHXylyl)}], and
[Au(NTf2){C(NEt2)(NHXylyl)}], are very active in processes such as the rearrangement of homo-
propargylsulfoxides, the intramolecular hydroamination of N-allenyl carbamates, the intramole-
cular hydroalkoxylation of allenes, the hydroarylation of acetylenecarboxylic acid ester, and the
benzylation of anisole. Although the NAC ligands have not been optimized for the reactions tested,
the yields obtained are usually similar and sometimes better than those reported with other catalysts,
showing that the presence of N-H bonds and the wider N-C-N angle in the NAC (as compared to
the NHC) complexes are not detrimental for the catalysis. For the hydroarylation reaction (where
two competing products can be formed), the NAC complexes allow favoring one over the other. For
the benzylation of anisole the selectivity is complementary to that obtained using H[AuCl4] as
catalyst, and depending on the substrate, the NAC gold(III) complexes outperform the activity of
H[AuCl4]. On average, the reactivity found suggests that the basicity of NACs toward gold(I) is very
similar to that of NHCs and higher than that of phosphines.
Introduction
attention,2 because, as an advantage on phosphines, they
are not prone to oxidation. Moreover, recently, Bertrand
et al. have successfully used very interesting cationic gold(I)
complexes with five-membered cyclic (alkyl)(amino)-
carbene (CAAC) ligands as catalysts in some organic
transformations.3
In contrast to the scarce attention paid just twenty years
ago to gold complexes as catalysts, they are now recognized
as very active compounds in many organic transformations.1
Much research in the topic has been carried out on classical
complexes with phosphine ligands, but in the last years
nitrogen heterocyclic carbene ligands (NHCs) have gained
To the widely used NHC catalysts and the much less
exploited CAACs, we recently added two other carbene
types, the so-called hydrogen-bonded heterocyclic com-
plexes (HBHCs)4,5 and the nitrogen acyclic complexes
(NACs),6 both shown in Scheme 1. Although gold(I) com-
plexes of the NAC type have been long known,7-9 they had
never been applied in catalysis previous to our works show-
ing their high catalytic activity in the skeletal rearrangement
and in the alkoxycyclization of 1,6-enynes.5,6,10
*To whom correspondence should be addressed. E-mail: espinet@
qi.uva.es.
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Bull. 2004, 37, 1–2. (f) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108,
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~
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(10) With the present manuscript finished, Hashmi et al. have pub-
lished following our previous papers the use of NAC gold carbenes in
phenol synthesis and in the hydration of alkynes: Hashmi., A. S. K.;
€
Hengst, T.; Lothschutz, C.; Rominger, F. Adv. Synth. Catal. 2010, 8,
1307–1314.
r
2010 American Chemical Society
Published on Web 07/22/2010
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