4830 Organometallics, Vol. 28, No. 16, 2009
Ito et al.
trialkylstanannes.10 However, to the best of our know-
ledge, there have been no detailed characterizations of gold
hydride complexes that show catalytic activity and little
information on the chemical behavior of gold hydride species
in catalysis.
One major obstacle in the progress of gold chemistry is
the difficulty in preparing gold hydride species. Gold hy-
drides themselves have attracted considerable attention
over the last few decades, primarily due to their unique
structural properties.11 Many experimental and theoretical
investigations have been described on the structure of gold(I)
or gold(III) hydride compounds.12 The simplest gold(I)
hydride molecule (AuH) and the related polyhydride mole-
cules (AuHn, etc.) were generated in the gas phase and have
been subjected to matrix isolation for characterization.13,14
These species are unstable under typical catalytic conditions,
and it is difficult to obtain preparative quantities of the
compounds by gas phase synthesis. Many stable heterome-
tallic complexes that contain Au-H-M bonds15 (M ¼ Au)
were also synthesized on a preparative scale. However, these
complexes are not suitable as models for gold hydride
catalysis. Synthesis of a simple monomeric gold(I) hydride
complex (HAuL, L: ligand) had been desired for a long
time. Very recently, Tsui and co-workers reported the first
synthesis of a monomeric gold(I) hydride complex, utilizing
Figure 1. AuCl(xantphos) (1a) and AuCl(xy-xantphos) (1b)
complexes.
an N-heterocyclic carbene (NHC) ligand, IPr (IPr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene), as the sup-
porting ligand that stabilized the gold(I) hydride structure.16
This compound is stable against exposure to air and moisture
in the solid state but is decomposed slowly in solution.
Although stoichiometric reactions of the gold(I) hydride
with dimethyl acetylenedicarboxylate and ethyl diazoacetate
were reported, catalytic properties of this complex were not
described in the paper.16
We recently reported gold(I)-catalyzed dehydrogenative
silylation of alcohols with hydrosilanes, in which a gold(I)
hydride intermediate was proposed as a key species, despite
the fact that spectroscopic evidence for the formation of the
gold(I) hydride intermediate was not shown.6a Dehydro-
genative silylation of alcohols is a more selective and envir-
onmentally benign process for producing silyl ethers than
conventional chlorosilane/base silylation.17 A number of
transition-metal complexes exhibit catalytic activity in dehy-
drogenative silylation of alcohols, although they often suffer
from low functional group compatibility. This is thought to
be because such complexes typically have catalytic activity
for undesired hydrosilylation of the unsaturated functional-
ities in the substrates. An interesting feature of the AuCl-
(xantphos)-catalyzed reaction is that it tolerates a wide range
of functional groups including alkenes, alkynes, alkyl halides
(RCl, RBr), ketones, aldehydes, conjugated enones, esters,
and carbamates. Furthermore, this reaction can be carried
out in numerous solvents (Scheme 1). In addition to these
unique chemoselectivities, it is also noteworthy that the
catalytic activity strongly depends on the phosphine ligand
of the gold(I) catalyst. Of the various combinations of
phosphine ligands and gold(I) salts, only the Xantphos
ligand showed reasonable catalytic activity for this gold(I)-
catalyzed dehydrogenative silylation.6,18 In an effort to
understand these characteristic features of the AuCl-
(xantphos)-catalyzed reaction as well as design new gold(I)
(7) For gold-catalyzed hydroboration, see: Baker, R. T.; Calabrese,
J. C.; Westcott, S. A. J. Organomet. Chem. 1995, 498, 109–117.
(8) For C-H activation reactions, see: (a) Wei, C.; Li, C.-J. J. Am.
Chem. Soc. 2003, 125, 9584–9585. (b) Skouta, R.; Li, C.-J. Angew. Chem.,
Int. Ed. 2007, 46, 1117–1119. (c) Zhang, X.; Corma, A. Angew. Chem., Int.
Ed. 2008, 47, 4358–4361.
(9) For examples of gold-catalyzed aerobic oxidations of alcohols,
´
see: (a) Abad, A.; Almela, C.; Corma, A.; Garcıa, H. Chem. Commun.
2006, 3178–3180. (b) Abad, A.; Corma, A.; García, H. Chem.;Eur. J.
2008, 14, 212–222. (c) Conte, M.; Miyamura, H.; Kobayashi and, S.;
Chechik, V. J. Am. Chem. Soc. 2009, 131, 7189–7196.
(10) For dehyrogenative dimerization of trialkylstannanes, see: Ito,
H.; Yajima, T.; Tateiwa, J.; Hosomi, A. Tetrahedron Lett. 1999, 40,
7807–7810.
(11) For reviews of gold hydride compounds, see: (a) Puddephatt, R.
J. In Comprehensive Coordination Chemistry: The Synthesis, Reactions,
Properties & Applications of Coordination Compounds; Wilkinson, G.,
Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: Oxford, 1987; Vol. 5,
p 869. (b) Crawford, M. J.; Klapotke, T. M. Angew. Chem., Int. Ed. 2002,
41, 2269–2271. (c) Gimeno, M. C.; Laguna, A. In Comprehensive Coordi-
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€
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€
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(16) Tsui, E.; Muller, P.; Sadighi, J. R. Angew. Chem., Int. Ed. 2008,
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(14) For gas phase synthesis of AunH+ and AunH- type compounds,
€
see: (a) Fischer, D.; Wanda, A.; Curioni, A.; Gronbeck, H.; Burkart, S.;
€
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