Use of ionic liquids in the platinum- and gold-catalyzed cycloisomerization of enyne systems

Article abstract of DOI:10.1016/j.jorganchem.2008.10.054

The platinum- and gold-catalyzed cycloisomerization of enyne systems has been carried out in various ionic liquids (ILs). In some cases, better selectivities and shorter reaction times have been observed compared to conventional conditions.

Full text of DOI:10.1016/j.jorganchem.2008.10.054

Journal of Organometallic Chemistry 694 (2009) 561–565
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Use of ionic liquids in the platinum- and gold-catalyzed cycloisomerization
of enyne systems
*
Xavier Moreau, Alexandra Hours, Louis Fensterbank , Jean-Philippe Goddard,
*
Max Malacria , Serge Thorimbert
UPMC – Univ. Paris 06, Laboratoire de Chimie Organique (UMR CNRS 7611), Institut de chimie moléculaire (FR 2769), 4 Place Jussieu, C. 229, 75005 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The platinum- and gold-catalyzed cycloisomerization of enyne systems has been carried out in various
ionic liquids (ILs). In some cases, better selectivities and shorter reaction times have been observed com-
pared to conventional conditions.
Received 25 July 2008
Received in revised form 15 September
2008
Accepted 29 October 2008
Available online 7 November 2008
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquid
Gold
Platinum
Cycloisomerization
Enyne
1. Introduction
Generally speaking, ILs are one of the leading alternatives to the
traditional organic solvents [36,37].
Cycloisomerization of polyunsaturated acyclic molecules is
highly desirable due to the increase of complexity it produces in
the final product from generally readily accessible precursors
[1,2]. Among the various precursors, the enyne moiety associated
with electrophilic metals species such as PtCl2, AuCl3, and AuCl
derivatives allowed the preparation of a large variety of polycyclic
compounds [3–11].
For instance, we [12–17] and others [18–30] have shown that
different acyl groups at a propargylic position of an enyne can mi-
grate in a 1,2-fashion with concomitant intramolecular cycloprop-
anation onto the ene part of the substrate. We demonstrated that
acyclic propargylic enyne acetates such as 1 are useful precursors
of bicyclic compounds 2 [12,15,31]. Taking into account that
charged intermediates are generally involved in these processes,
we have been interested in the use of alternative solvents such
as ionic liquids (ILs) [32]. Indeed, such organic salts have been
largely used over the last two decades due to their original
physicochemical properties such as high thermal stability, low
vapor pressure, non-inflammability and polar properties [33–35].
Catalytic reactions performed in ILs have been the subject of in-
tense research. An attractive feature is that most of the active com-
plexes are stable in such media allowing the possibility to reuse
the catalyst several times with negligible decrease in activity
[38–44]. In comparison to palladium-, rhodium- and ruthenium-
catalyzed transformations, which have been extensively studied,
platinum and gold complexes have been much less used in associ-
ation with molten salts [45] or ILs. Doherty et al. reported efficient
enantioselective platinum-catalyzed carbonyl ene alkylations [46].
Hydrosilylations of alkenes [47,48] or direct oxidation of methane
[49] have been performed into ionic liquids. The first thiazolium
gold(III) compound that qualifies as an ionic liquid has been
prepared in 2002. In association with other imidazolium ILs, this
complex as well as other gold salts were efficient catalysts for
the hydration of phenyl acetylene [50]. Electrophilic activation of
the alkyne function of an enyne precursor to give skeletal rear-
rangement products could also be accomplished through PtCl2
catalysis in ILs, as disclosed by Chatani in 2004 [51]. In the same
vein, gold-catalyzed intramolecular nucleophilic attacks of oxygen
or nitrogen atoms onto alkynyl functions have been reported to
proceed in good to excellent yields in various ILs giving the corre-
sponding furans and indoles, respectively [52,53]. Finally, a recent
report by Krause on the Au-catalyzed synthesis of 2,5-dihydrofu-
rans from allene precursors in ILs prompted us to report our own
findings in this area of investigation [54].
* Corresponding authors.
E-mail addresses: louis.fensterbank@upmc.fr (L. Fensterbank), max.mala-
cria@upmc.fr (M. Malacria).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.054

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X. Moreau et al. / Journal of Organometallic Chemistry 694 (2009) 561–565
CH2Cl2 solution reaction. An obvious benefit from the utilization
Bu
Bu
Bu
of IL notably with gold catalysis resides in the rapidity of the reac-
tion: 1 min instead of over 1 h (entry 5 vs. 6)! This probably can be
rationalized by a concentration effect since these reactions are con-
ducted at 0.57 M in IL while they are generally run at much lesser
concentration in typical solution conditions (0.025 M). Consistent
with this, it has been reported that platinum-catalyzed cycloizo-
merisation reactions in neat conditions allowed to significantly de-
crease the reaction time [17]. Importantly this finding suggests
high-throughput procedures can certainly be devised.
In the same vein, we tested the recyclability of the catalyst solu-
tion. As shown in Table 2, up to 5 cycles could be operated that still
gave a decent yield (80%) of the 2a:2b mixture at the fifth iteration
(Table 2).
N
+
N
N
+
N
N
+
N
-
-
-
PF 6
NTf 2
BF 4
Me
Me
Me
[BMIM][PF6]
Hydrophobic
[BMIM][NTf2]
Hydrophobic
[BMIM][BF4]
Hydrophilic
Fig. 1. Structure of ionic liquids.
2. Results and discussion
We also examined other substrates in order to test the general-
ity of the acetate migration based cycloisomerization reaction.
Thus, precursors 3 and 5 provided the expected polycyclic products
4 and 6 in fair yields, but not better than the typical solution reac-
tions (Scheme 1).
Thus, it was clear from these results that the formation of cyclo-
propane, presumably involving a metalla-carbene intermediate,
was possible. We then wanted to look at the possibility of having
a bis-cyclopropanation reactivity from a dienyne precursor as
Thus, we started our study with precursor 1 and used the fol-
lowing ILs (Fig. 1). We initially investigated the platinum(II) catal-
ysis. Among all the solvents used (Table 1, entries 2–4),
[BMIM][NTf2] proved to be the most reliable since 99% yield of a
9:1 mixture of 2a and 2b was obtained while the other ones gave
more complex intractable mixtures.
Gold catalysis provided contrasted results. A more rapid and
selective reaction than the regular CH2Cl2 solution one (entry 5)
was observed with the use of AuCl3 in [BMIM][PF6] and in
[BMIM][NTf2] (entries 6 and 7) giving a mixture of only 2a and
2b. However, in sharp contrast, but not unexpectedly, methylke-
tone 2d was the major product when the hydrophilic [55–57]
[BMIM][BF4] IL was used (entry 8). The same trend was observed
with the use of AuCl. A fast and more selective reaction was ob-
served in [BMIM][PF6] or [BMIM][NTf2] (entries 10 and 11) and
the [BMIM][BF4] IL resulted also in the major formation of the
hydration product. Finally, cationic gold(I) catalysis proved to be
in our hands inapplicable in IL (see entries 14 and 15) yielding in
each case a lot of hydration product.
Table 2
Recycling experiment of platinum-catalyzed cycloisomerization in ionic liquids.
Cyclea
2a:2bb
Yield (%)
1
2
3
4
5
90:10
90:10
90:10
88:12
87:13
99
96
94
90c
80d
a
Run with PtCl2 in [BMIM][NTf2], 1 h at 80 °C.
Determined by 1H NMR.
7% S.M. recovered.
To draw some simple conclusions from this study, it appears
that hydrophobic IL can provide good yields of cycloisomerization
products, generally in an altered selectivity compared to the
b
c
d
15% S.M. recovered.
Table 1
Distribution of O-acyl migration products in usual solvents and ionic liquids.
OAc
[M]
OAc
+
+
+
conditions
OAc
AcO
AcO
2c
O
2a
2b
1
2d
Entry
[M]
Conditionsa
Toluene, 80 °C, 2 h
[BMIM][PF6], 80 °C, 1 h
[BMIM][NTf2], 80 °C, 1 h
[BMIM][BF4], 80 °C, 1 h
2a:2b:2c:2db
Yield (%)
1
2
3
4
PtCl2
PtCl2
PtCl2
PtCl2
90:10:0:0
Mixtured
90:10:0:0
Mixtured
90c
–
99
–
5
6
7
8
AuCl3
AuCl3
AuCl3
AuCl3
CH2Cl2, r.t., 1.5 h
50:38:12:0
80:20:0:0
80:20:0:0
40:10:0:50
88
92
91
80
[BMIM][PF6], r.t., 1 min
[BMIM][NTf2], r.t., 1 min
[BMIM][BF4], r.t., 1.5 h
9
AuCl
AuCl
AuCl
AuCl
CH2Cl2, r.t., 20 min
54:35:11:0
80:20:0:0
80:20:0:0
40:10:0:50
80
95
97
92
10
11
12
[BMIM][PF6], r.t., 1 min
[BMIM][NTf2], r.t., 1 min
[BMIM][BF4], r.t., 1.5 h
13
14
15
AuClPPh3/AgPF6
AuClPPh3/AgPF6
AuClPPh3/AgPF6
CH2Cl2, r.t., 20 min
[BMIM][PF6], r.t., 20 min
[BMIM][NTf2], r.t., 20 min
77:0:23:0
20:0:0:80
20:0:0:80
75
65
86
a
General procedure: dienyne 1 (0.4 mmol), PtCl2 (5 mol%) or [Au] (2 mol%), ionic liquid (0.7 mL).
b
c
Determined by 1H NMR.
See Ref. [15].
d
A complex and intractable mixture of compounds including products 2 was obtained.

X. Moreau et al. / Journal of Organometallic Chemistry 694 (2009) 561–565
563
chemistry was not particularly encouraging since several products
were formed (see above). We obtained the following results
(Table 4).
AcO
AcO
PtCl2 (5 mol%)
With platinum chloride in ionic liquids, all the attempts
exhibited very slow reactions and a majority of starting material
was recovered after many hours. We tried to force the reactions
conditions but the decompositions of the starting material was
observed. Interestingly, with AuCl and AuBr3, we obtained mix-
tures which recall the outcomes of reactions with cationic gold(I)
catalysis, and it is worthy of note that the formation of tetracyclic
derivative 8 is poorly operative.
In conclusion, we have examined well-established platinum(II),
gold(III) and gold(I)-catalyzed cycloisomerization reactions in ILs.
We could obtain similar results to the typical solution reactions
in the case of the migration of acetate based cycloisomerizations.
In some cases, much shorter reaction times were observed. Inter-
estingly, cationic gold(I) catalysis does not appear to be viable in
this type of reaction medium. With the dienyne precursor 7, it is
interesting to note that the formation of the expected tetracyclic
product 8 was not efficient in ILs. Instead, the IL reactions provided
somehow the same products we obtained with the gold(I) cationic
catalysis which suggests an important role of the reaction medium
for this transformation.
toluene, 80°C, 2 h, 98%
[BMIM][NTf2], 80°C, 1 h, 89%
neat, 80°C, 30 min, 94%
H
4
6
3
5
ref. 17
OAc
AcO
PtCl2 (5 mol%)
toluene, 80°C, 2 h, 89%
[BMIM][NTf2], 80°C, 6 h, 79%
neat, 80°C, 30 min, 74%
H
ref. 17
Scheme 1. Formation of polycyclic derivatives by platinum-catalyzed acetate
migration.
experienced with 7 to give 8 with PtCl2 or PtCl4 in solution (Table
3) [12,15]. We also checked the reactivity of this substrate with
gold catalysts and we observed shorter reaction times even at low-
er temperature. Interestingly, while AuBr3 and AuCl gave similar
results with the main formation of 8, cationic gold catalyst AuC-
lPPh3/AgSbF6 marked a significant contrast in the evolution of
the system since cyclohexadienic compound 9 was the main prod-
uct (58% yield). This type of product has been interpreted by Echa-
varren as an endo-cleavage type product [58,59,15]. Aromatic
products (10a and 10b) originating from a loss of MeOH were also
isolated as an inseparable mixture (Table 3, entry 5). We suspected
that the formation of these products could be influenced by the
amount of precatalyst mixture.
All these findings are of interest and they confirm that ILs can be
advantageous since high-throughput procedures can be at hand
after optimization, and as well as opportunities of new reactivity
with already known precursors. Such particular reactions
conditions could be of a great help for mechanistic investigations
in using the property of ionic liquid to promote charges formations.
3. Experimental
Indeed, with 20 mol% of AuClPPh3/AgSbF6, a mixture of 10a and
10b was obtained in 19% yield accompanied with 42% of a new
product, the cyclohexenone 11 (Scheme 2). The latter presumably
originates from a proton or metal-catalyzed [3,3]-sigmatropic
rearrangement of 9 followed by an hydrolysis of the enol-ether
intermediate. A loss of methanol would lead to the formation of
the conjugated triene 12. cationic [1,2] methyl shifts and further
aromatization deliver the corresponding allyl-pseudocumene
derivatives 10a and 10b [60,61].
3.1. General remarks
All reactions were run under an argon or a nitrogen atmosphere
in anhydrous solvents and flame-dried flask. Toluene and dichloro-
methane were distilled from CaH2. Thin-layer chromatography
(TLC) was performed on Merck 60 F254 silica gel. Merck Gerudan
SI 60 Å silica gel (35–70 l) was used for column chromatography.
NMR spectra were recorded at room temperature on a 400 MHz
Bruker ARX400 spectrometer. Chemical shifts are given in parts
per million, referenced to the residual proton resonance of the sol-
vents (=7.26 ppm for CDCl3) or to the residual carbon resonance of
the solvent (=77.16 ppm for CDCl3). High-resolution mass spectra
(HRMS) were measured by the Service de Spectrométrie de Masse
de l’Université Pierre et Marie Curie-Paris 6. Infrared (IR) spectra
were recorded on a Bruker Tensor 27 spectrometer. Gold and
platinum catalysts were purchased from Strem. Cycloisomeriza-
tions were performed in freshly degassed (2–3 freeze–pump–thaw
cycles or 30 min argon bubbling) solvents. [BMIM][PF6],
[BMIM][NTf2] and [BMIM][BF4] were purchased from Solvionic
and kept 4 h under vacuum (10À2 bar) before use. The preparation
of the dienyne substrates 1 and 5 have been reported [15], and the
corresponding cycloisomerization products 2 have been character-
ized [31]. The enynes 3 and 5 have been prepared and the cycloiso-
merization products have been characterized [17].
We then studied the metal-catalyzed transformations of 7 in
ILs. We focus on PtCl2, AuBr3 and AuCl since cationic gold(I) proved
to be touchy in our previous study and here also the solution
Table 3
Cycloisomerization of dienyne 7 in usual solvents.
MeO
MeO
OMe
[M]
+
conditions
7
8
9
Entry
[M]
Conditions
8c (%)
9c (%)
1
2
3
4
5
PtCl2
PtCl4
Toluene, 80 °C, 2 h
Toluene, 80 °C, 2 h
CH2Cl2, r.t., 20 min
CH2Cl2, r.t., 20 min
CH2Cl2, r.t., 20 min
73e
77e
69
72
22
–
–
–
a
a
3.2. General procedure for the cycloisomerization in usual solvent
b
AuBr3
AuClb
–
PtCl2 (5 mol%), AuCl (2 mol%), AuCl3 (2 mol%) or AuClPPh3
(2 mol%) and AgSbF6 (2 mol%) were introduced in a round bottom
flask under an argon atmosphere. Anhydrous toluene or CH2Cl2
(0.025 M) was then added. Dienyne (0.4 mmol) was introduced
and the reaction mixture was stirred at the appropriate tempera-
ture (for 1–2 h at 80 °C with PtCl2, 1–90 min at rt with gold
a
AuClPPh3/AgSbF6
58d
a
5 mol% of catalyst.
2 mol% of catalyst.
Isolated yields.
5% of 10a and 10b have been isolated.
See Ref. [15].
b
c
d
e

564
X. Moreau et al. / Journal of Organometallic Chemistry 694 (2009) 561–565
OMe
AuClPPh3/AgSbF6
20 mol%
O
+
+
CH2Cl2, r.t., 20 min
(19%)
7
10b
10a
11, 42%
[M]
[M]
MeO
MeO
via:
[3,3]
11
[M] = H or [Au]
- MeOH
10a
10b
[M]
[M]
12
Scheme 2. Behavior of dienyne 7 with gold(I) catalyst.
2962, 2933, 2874, 1740, 1720, 1236 cmÀ1
C14H22O3Na [M+Na]: 261.14612; Found: 261.14603.
. HRMS Calc. for
Table 4
Behavior of dienyne 7 with gold(I) catalyst in ionic liquids.
Cyclohexene 9: Colorless oil. 1H NMR (CDCl3, 400 MHz) d 5.94 (d,
J = 9.8 Hz, 1H), 5.84 (m, 1H), 5.51 (d, J = 9.8 Hz, 1H), 5.11–5.05
(overlap, 4H), 3.09 (s, 3H), 2.58 (dd, J = 14.6, 6.4 Hz, 1H), 2.36 (dd,
J = 14.6, 6.8 Hz, 1H), 1.85 (d, J = 14.0, 1H), 1.51 (d, J = 14.0 Hz, 1H),
1.14 (s, 3H), 1.02 (s, 3H). 13C NMR (CDCl3, 100 MHz) d 142.3 (C),
139.6 (CH), 134.6 (CH), 125.5 (CH), 117.6 (CH2), 113.0 (CH2), 76.5
(C), 49.5 (CH3), 44.3 (CH2), 40.4 (CH2), 33.4 (C), 31.5 (CH3), 30.6
(CH3). IR (neat) 2933, 1460, 1079 cmÀ1. HRMS Calc. for C13H20ONa
[M+Na]: 215.14064; Found: 215.14032.
MeO
MeO
OMe
[Au] (5 mol%)
+ 10
+
r.t., conditions
7
8
9
Entry
[Au]
Conditions
7:8:9:10
Yield (%)
1
2
3
4
AuBr3
AuBr3
AuCl
[BMIM][PF6], 2.5 h
[BMIM][NTf2], 2.5 h
[BMIM][PF6], 2.5 h
[BMIM][NTf2], 2.5 h
32:10:58:0
48:0:52:0
50:19:31:0
31:10:24:35
69
69
73
81
Cyclohexenone 11: Colorless oil. 1H NMR (CDCl3, 400 MHz) d
6.56 (t, J = 4.4 Hz, 1H), 5.77 (ddt, J = 20.2, 16.8, 6.4 Hz, 1H), 4.97
(m, 2H), 2.31–2.15 (overlap, 8H), 1.02 (s, 6H). 13C NMR (CDCl3,
100 MHz) d 199.7 (C), 143.4 (CH), 138.4 (CH), 138.2 (C), 115.1
(CH2), 52.3 (CH2), 40.3 (CH2), 34.2 (C), 32.9 (CH2), 28.9 (CH2),
28.4 (2 CH3). IR (neat) 2923, 1675, 1464 cmÀ1. HRMS Calc. for
C12H19O [M+H]: 179.14304; Found: 179.14297.
Allyl-trimethylbenzene derivatives 10a and 10b: Inseparable mix-
ture 1.0/1.7 (m)/(M). Colorless oil. 1H NMR (CDCl3, 400 MHz) d
6.96–6.93 (overlap, 4H, M and m), 6.01–5.88 (overlap, 2H, M and
m), 5.05–5.02 (overlap, 3H, M and m), 4.88 (ddd, J = 17.1, 3.8,
1.8 Hz, 1H, m), 3.44 (dt, J = 5.7, 1.8 Hz, 2H, m), 3.33 (br. d,
J = 6.7 Hz, 2H, M), 2.29 (s, 3H, m), 2.28 (s, 3H, m), 2.25–2.21(overlap,
12H, M and m). 13C NMR (CDCl3, 100 MHz) d 137.2 (CH, M), 136.1
(Carom, m), 135.8 (CH, m), 135.5 (Carom, M), 135.3 (Carom, m),
134.5 (Carom, m), 134.4 (Carom, M), 134.3 (Carom, m), 134.1 (Car-
om, M), 133.6 (Carom, M), 131.7 (CH, M), 130.7 (CH, m), 127.9 (CH,
M), 127.5 (CH, m), 115.4 (CH2, M), 115.0 (CH2, m), 37.5 (CH2, M),
34.1 (CH2, m), 20.9 (CH3, m), 20.1(CH3, m), 19.3 (2 CH3, M), 18.9
(CH3, M), 15.6 (CH3, m). IR (neat) 2923, 2853, 1461 cmÀ1. HRMS
Calc. for C12H17 [M+H]: 161.13248; Found: 161.13224.
AuCl
catalysts) and was monitored by TLC. After completion, the solu-
tion was filtered through a pad of silica and the pad was washed
with diethylether. The solvent was removed under reduced pres-
sure and the crude mixture was purified by flash chromatography.
3.3. General procedure for the cycloisomerization in ionic liquid
PtCl2 (5 mol%) or gold salts (2 mol%) was added to ionic liquid
(0.7 mL). The mixture was stirred for 5 min and warmed to the de-
sired temperature (80 °C for PtCl2, r.t. for gold salts) and dienyne
(0.4 mmol) was added. The mixture was stirred at this temperature
for the indicated time and extracted with Et2O (3 Â 1 mL). The sol-
vent was evaporated under reduced pressure and the crude mix-
ture was purified by flash chromatography.
3.4. Catalyst recycling
Same procedure with dry cyclohexane (5 Â 1 mL) as extracting
solvent. The ionic liquid media was taken up in benzene, stirred
5 min and the solvent was evaporated. The dienyne 1 was then
added to this mixture for the next run.
Acknowledgments
The authors thank the MRES, the CNRS, the IUF (MM) and the
ANR BLAN 06-2_159258 ‘‘Allènes” for financial support.
Ketone 2d: Colorless oil. 1H NMR (CDCl3, 400 MHz) d 5.72 (dd,
J = 17.6, 10.8 Hz, 1H), 5.53 (ddt, J = 17.6, 10.4, 7.6 Hz, 1H), 5.04
(m, 2H), 4.84 (m, 2H), 2.96 (dd, J = 14.4, 7.6 Hz, 1H), 2.62 (dd,
J = 14.4, 7.6 Hz, 1H), 2.29 (d, J = 15.2 Hz, 1H), 2.15 (s, 3H), 2.09 (d,
J = 15.2 Hz, 1H), 2.04 (s, 3H), 1.02 (s, 3H), 0.96 (s, 3H). 13C NMR
(CDCl3, 100 MHz) d 209.3 (C), 170.0 (C), 147.9 (CH), 131.2 (CH),
119.7 (CH2), 110.5 (CH2), 90.5 (C), 45.7 (CH2), 41.2 (CH2), 36.6
(C), 28.3 (CH3), 28.1 (CH3), 28.0 (CH3), 22.2 (CH3). IR (neat) 3081,
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