Phosphine gold(I) bis-(trifluoromethanesulfonyl)imidate complexes as new highly efficient and air-stable catalysts for the cycloisomerization of enynes

Article abstract of DOI:10.1021/ol0515917

(Chemical Equation Presented) A study concerning the synthesis of new phosphine gold(I) complexes using the bis(trifluoromethanesulfonyl)imidate moiety as a weakly coordinating counteranion is described. These new air-stable complexes are more convenient to prepare, store, and handle and are exceedingly active for the catalysis of a wide range of enynes cycloisomerizations.

Full text of DOI:10.1021/ol0515917

ORGANIC
LETTERS
2005
Vol. 7, No. 19
4133-4136
Phosphine Gold(I)
Bis-(trifluoromethanesulfonyl)imidate
Complexes as New Highly Efficient and
Air-Stable Catalysts for the
Cycloisomerization of Enynes
Nicolas Me´zailles, Louis Ricard, and Fabien Gagosz*
Laboratoire de Synthe`se Organique and Laboratoire He´te´roe´le´ments et Coordination,
Ecole Polytechnique, 91128 Palaiseau, France
gagosz@dcso.polytechnique.fr
Received July 7, 2005
ABSTRACT
A study concerning the synthesis of new phosphine gold(I) complexes using the bis(trifluoromethanesulfonyl)imidate moiety as a weakly
coordinating counteranion is described. These new air-stable complexes are more convenient to prepare, store, and handle and are exceedingly
active for the catalysis of a wide range of enynes cycloisomerizations.
Phosphine gold(I) complexes have recently emerged as
valuable catalysts for the conversion of various types of
enynes into a range of useful structural motifs.1 However,
despite its efficiency, the catalytic system generally employed
in these transformations,2 which corresponds to the combined
use of a phosphine gold(I) chloride complex and a silver
salt as a cocatalyst (Scheme 1), suffers some limitations. The
silver salts are all very hygroscopic, causing difficulties in
properly weighing the reagent and in keeping the reaction
medium nonacidic. Moreover, the active gold(I) species is
quite unstable, especially when a fluorine-based counteranion
is used,3 and may therefore not be isolable.4
As part of our work on gold(I) catalysis,5 we were
particularly interested in developing efficient catalysts that
would not require the use of a cocatalyst and would therefore
be more convenient to use. We herein report the results of
our endeavors, which led to the discovery of a new class of
highly active and air-stable phosphine gold(I) complexes.
(1) (a) Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am. Chem.
Soc. 2005, 127, 6178-6179. (b) Mun˜oz, M.; Adrio, J.; Carretero, J. C.;
Echavarren, A. M. Organometalics 2005, 24, 1293-1300. (c) Nevado, C.;
Echavarren, A. M. Chem. Eur. J. 2005, 11, 3155-3164. (d) Nieto-
Oberhuber, C.; Mun˜oz, M.; Bun˜uel, E.; Nevado, C.; Ca´rdenas, D. J.;
Echavarren, A. M. Angew. Chem., Int. Ed. 2004, 43, 2402-2406. (e) Shi,
X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 5802-5803.
(f) Luzung, M. R.; Markham, J. P.; Toste, F. D. J. Am. Chem. Soc. 2004,
126, 10858-10859. (g) Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2004,
126, 15978-15979. (h) Staben, S. T.; Kennedy-Smith, J. J.; Toste, F. D.
Angew. Chem., Int. Ed. 2004, 43, 5350-5352. (i) Kennedy-Smith, J. J.;
Staben, S. T.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 4526-4527. (j)
Zhang, L.; Kozmin, S. A. J. Am. Chem. Soc. 2004, 126, 11806-11807. (k)
Zhang, L.; Kozmin, S. A. J. Am. Chem. Soc. 2005, 127, 6962-6963. For
a review of Au-catalyzed reactions, see: Hashmi, A. S. K. Gold Bull. 2004,
37, 51-65.
Scheme 1. General Formation of Active Gold(I) Species
10.1021/ol0515917 CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/18/2005

Scheme 2. Synthesis of (PPh3)AuNTf2
Scheme 4. Cycloisomerization of Enynes 4, 6, and 8
By analogy with the numerous studies reporting the use
of the bis(trifluoromethanesulfonyl)imidate moiety (Tf2N-)
for the synthesis of highly electrodeficient cationic com-
plexes,6 we envisaged this weakly coordinating counteranion7
as a potentially valuable candidate for the synthesis of stable
gold(I) catalysts.8 To this end, treatment of (Ph3P)AuCl with
9
1 equiv of AgNTf2 was attempted (Scheme 2).
We were pleased to see that this metathesis reaction led
to the quantitative formation of the desired (Ph3P)AuNTf2
complex (1a), which was isolated as an air-stable crystalline
compound.10 This new complex, whose synthesis may be
performed on a multigram scale, was then tested in a range
of previously reported gold(I)-catalyzed transformations.
Simple enyne 2 was first chosen as a model substrate
(Scheme 3).
Scheme 3. Model Cycloisomerization of Enyne 2
We were delighted to observe a rapid and highly exother-
mic conversion of 2 into metathesis product 3 when 1% 1a
was used as the catalyst. Moreover, this new catalyst turned
out to be highly efficient since compound 3 was formed in
(2) Protonation of an alkylgold(I) complex with a strong acid may also
be used to generate the cationic species; however, these conditions are not
always compatible with the substrates.
the same 97% yield when only 0.01% 1a was used (TON )
9700). These results contrast with those reported by Echa-
varren and co-workers who described the formation of 3 in
91% yield after 25 min of reaction and the use of 2% (PPh3)-
AuSbF6.1d Then, 1/100-fold less catalyst was needed to
accomplish the same transformation.
Complex 1a also efficiently catalyses the Conia-ene,1h,i
the Rautenstrauch,1e and the propargyl Claisen1g rearrange-
ments recently reported by Toste and co-workers (Scheme
4). It is interesting to note that a simple change in the nature
of the counteranion (TfO- for Tf2N-) led to a much faster
conversion of enyne 6 into cyclopentenone 7, which was
isolated in an improved 81% yield when catalyst 1a was
used instead of (PPh3)AuOTf. A remarkable temperature
effect was also observed for the transformation of enyne 8
into allene 9. Product 10 derived from competing [1,3]
rearrangement, which was isolated in 12% yield when the
(3) Weakly coordinating anions such as BF4-, PF6-, or SbF6- are Lewis
-
acid/base conjugates of a superior nucleophile (e.g., MFn+1 f MFn
+
F-). Their ability to act as inert counterions is always limited by a
competition reaction for that nucleophile (e.g., F-), and the free Lewis acid
MFn can act as an oxidizing agent and thus cause unwanted side reactions.
See: Raabe, I.; Krossing, I. Angew. Chem., Int. Ed. 2004, 43, 2066-2090.
Raabe, I.; Krossing, I. Chem. Eur. J. 2004, 10, 5017-5030.
(4) Preliminary 31P NMR studies of preformed solutions of (PPh3)AuBF4,
(PPh3)AuPF6, and (PPh3)AuSbF6 complexes in CH2Cl2 from (PPh3)AuCl
and the corresponding silver salt showed their instability. All attempts to
isolate these complexes failed.
(5) Gagosz, F. Org. Lett. 2005, 7, 4129-4132.
(6) Numerous metal triflimides are known as strong Lewis acid
catalysts: For lanthanides, see: (a) Ishihara, K., Kubota, M.; Yamamoto,
H. Synlett 1996, 265-266 and 839-841. For silicium, see: (b) Mathieu,
B.; Ghosez, L. Tetrahedron 2002, 58, 8219-8226. For tin, see: (c) Vij,
A.; Wilson, W. W.; Vij, V.; Corley, C. R.; Tham, F. S.; Gerken, M.; Haiges,
R.; Schneider, S.; Schroer, T.; Wagner, R. I. Inorg. Chem. 2004, 43, 3189-
3199. For other metals, see: (d) Sibi, M. P.; Petrovic, G. Tetrahedron:
Asymmetry 2003, 14, 2879-2882.
-
- 6b
(7) NTf2 is a weaker coordinating anion than OTf- and ClO4
.
4134
Org. Lett., Vol. 7, No. 19, 2005

Scheme 5. Cycloisomerization of Enynes 11 and 13
Table 1. Au(I)-Catalyzed Methoxycyclization of Enyne 2
entry
cat
mol %
time
2 h
24 h
3 h
yield (19)
1
2
3
4
5
6
7
8
1a
1a
1
0.1
2
1
1
1
1
2
94%
77% a
84% b
94%
(PPh3)AuSbF6
1b
1c
1d
1e
1f
2 h
75 min
75 min
20 min
15 min
97%
96%
97%
89% b
reaction was performed at room temperature, was nearly
absent when the temperature was lowered to -10 °C and
the catalyst loading reduced to 0.1%.
a 18% of unreacted 2 were also isolated. b See ref. 1a.
Complex 1a was also effective in catalyzing the cyclo-
isomerization of 1,5-enynes1f as depicted in Scheme 5. Using
only 0.1% 1a, bicyclo[3.1.0]hexene 12 and cyclopentadiene
145 were isolated in 88 and 78% yield, respectively.
Less favored intramolecular hydroarylation of alkynes
were also tested.1c Even if these transformations required a
longer reaction time, 2H-chromene 16 and dihydroquinoline
18 were obtained in excellent yield starting from alkynes
15 and 17 (Scheme 6).
AuNTf2 catalysts compared to the corresponding AuSbF6
complexes. It is interesting to note that the reaction time
decreases with bulkier and more basic phosphine gold(I)
complexes. Moreover, the transformation can even be
performed using only 0.1% of the simplest (PPh3)AuNTf2
catalyst (entry 2).
Scheme 6. Hydroarylation of Alkynes 15 and 17
Finally, to further highlight the potential of these new
catalysts, complex 1a was tested in the cycloisomerization
of enynes 20 and 22.1a As shown in Scheme 7, 1a possesses
Scheme 7. Cycloisomerization of Enynes 20 and 22
Moreover, by analogy with the recently reported work of
Echavarren and co-workers,1a,b we prepared several other new
phosphine AuNTf2 complexes 1b-e11 and evaluated their
catalytic activity in the kinetically disfavored methoxycy-
clization of enyne 2. Our results, presented in Table 1,
contrast with those previously reported.
Thus, compound 19 was isolated in nearly quantitative
yield regardless of the catalyst (1a-e) used, and the
transformations only differ in the time necessary to reach
completion. Such results may reflect a higher stability of
(8) By analogy with the stability of AgNTf2 compared to the hygroscopic
salts AgOTf, AgBF4, AgPF6, or AgSbF6.
(9) AgNTf2 is readily available from the commercially available HNTf2
and Ag2CO3.
(10) 1a is soluble in most commonly used solvents (DCM, CH3CN, THF,
MeOH, ether, toluene...). The crystal structure of 1a was established by
X-ray diffraction (CCDC 273834); see Supporting Information.
(11) 1b-e, which are all air-stable compounds, were synthesized as
described for 1a.
Org. Lett., Vol. 7, No. 19, 2005
4135

a better activity than the corresponding (PPh3)AuSbF6
catalyst1a and is at least as efficient as the more complex
and expensive (2-(dicyclohexylphosphino)biphenyl)AuSbF6
catalyst 1g.1a
In summary, we have developed a new class of phosphine
gold(I) catalysts based on the use of the bis(trifluoromethane-
sulfonyl)imidate moiety as a weakly coordinating counter-
anion. These air-stable complexes are more convenient to
prepare, store, and handle and are exceedingly active for the
catalysis of a wide range of enyne cycloisomerizations. They
open new possibilities in homogeneous gold(I) catalysis and
may allow transformations that were hitherto difficult to
achieve.
Acknowledgment. The author wishes to thank Prof. S.
Z. Zard, Dr. B. Quiclet-Sire, and Dr. I. Hanna for helpful
discussions.
Supporting Information Available: Experimental pro-
cedures and spectral data for new compounds and X-ray data
for 1a (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
OL0515917
4136
Org. Lett., Vol. 7, No. 19, 2005