Gold phosphole complexes as efficient catalysts for alkyne activation

Article abstract of DOI:10.1021/om301244m

Gold(I) complexes bearing monophosphole ligands were synthesized, and their electronic and steric properties were compared to those of their triphenylphosphine-based counterparts. Cationic phosphole-based gold(I) complexes are active and selective in enyne cycloisomerization and in olefin cyclopropanation, with a good correlation between the ligand σ-donor ability and the catalytic activity. For the most efficient ligand, 1-phenyl-2,3,4,5-tetramethylphosphole (TMP), a highly active, selective, and stable cationic [Au(TMP)(CH₃CN)]SbF₆ complex was isolated.

Full text of DOI:10.1021/om301244m

Communication
pubs.acs.org/Organometallics
Gold Phosphole Complexes as Efficient Catalysts for Alkyne
Activation
Kevin Fourmy,†,‡ Sonia Mallet-Ladeira,†,‡ Odile Dechy-Cabaret,*,†,‡ and Maryse Gouygou*,†,‡
́
†CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
‡Universite
́
de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
S
* Supporting Information
ABSTRACT: Gold(I) complexes bearing monophosphole ligands were
synthesized, and their electronic and steric properties were compared to
those of their triphenylphosphine-based counterparts. Cationic phosp-
hole-based gold(I) complexes are active and selective in enyne
cycloisomerization and in olefin cyclopropanation, with a good correlation
between the ligand σ-donor ability and the catalytic activity. For the most
efficient ligand, 1-phenyl-2,3,4,5-tetramethylphosphole (TMP), a highly
active, selective, and stable cationic [Au(TMP)(CH3CN)]SbF6 complex
was isolated.
potential in catalysis has been underexploited in comparison to
n recent years, homogeneous gold catalysis has received
Iconsiderable attention in the field of organic synthetic
chemistry.1 Among the broad panel of gold-catalyzed trans-
formations, cycloisomerization of 1,6-enynes has emerged as a
powerful tool for the atom-economical preparation of function-
alized polycylic structures.2
Initially, gold catalysts bearing monodentate phosphine
ligands such as triphenylphosphine were used. NHC ligands3
were then widely used in gold-catalyzed cycloisomerizations,4
allowing for complementary reactivity.5 Given the major role
played by the ligand in the control of cycloisomerization
selectivity,6 more elaborate ligands were then developed,
including biaryl-like phosphines or phosphites,4c,7 as well as
various new carbenes.8
the other ligands of the series.18
The electronic properties of TMP were first determined and
compared to other phosphole ligands of the series and to
triphenylphosphine. Following the most widely used Tolman
approach,19 we estimated the electron-donating ability of
phosphole ligands by recording the IR frequency of CO in
dicarbonyl complexes [RhCl(L)(CO)2], readily prepared in
two steps from [RhCl(cod)]2, as described for N-heterocyclic
carbenes.20 The resulting stretching frequencies of the cis- and
trans-CO moieties in phosphole-based [RhCl(DBP)(CO)2],
[RhCl(TPP)(CO)2] and [RhCl(TMP)(CO)2] complexes are
summarized in Table 1 and compared with those of the
analogous [RhCl(PPh3)(CO)2] complex.21 All the νav(CO)
Phosphole ligands combine the soft donor character of
phosphines with the conformational rigidity associated with the
phosphorus heterocyclic skeleton that make them efficient in
promoting homogeneous transition-metal catalysis. The most
important examples have been reported in olefin hydro-
formylation catalyzed by Rh complexes bearing simple
monophospholes.9 Very few Au−monophosphole complexes
have been described10 and used in catalysis.11
Table 1. Determination of Electronic Parameters: IR ν(CO)
Stretching Frequencies of [RhCl(L)(CO)2] Complexes and
31P−77Se Coupling Constants for Selenides Prepared from
Selected Ligands
As part of our continuing interest in the design of phosphole-
based ligands for application in catalysis12 and in enyne
cycloisomerization,13 we report herein the synthesis of new
gold(I) phosphole complexes which are both active and
selective in various alkyne activation reactions, such as enyne
cycloisomerization and olefin cyclopropanation.
1-Phenyl-2,3,4,5-tetramethylphosphole (TMP)14 is particu-
larly attractive among the classical monophosphole ligand
series, such as 1-phenyldibenzophosphole (DBP),15 1,2,5-
triphenylphosphole (TPP),16 and 1-phenyl-3,4-dimethylphosp-
hole (DMP),17 because a good σ-donor ability along with a
moderate steric hindrance can be expected. However, its
a
tThe synthesis of the [RhCl(cod)(DMP)] intermediate complex
failed.
Received: December 21, 2012
Published: February 27, 2013
© 2013 American Chemical Society
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Communication
bands are located in the same region, accounting for similar net
donating abilities (including both σ-donor ability and π-
acceptor strength22) of the phosphole ligands and their PPh3
counterpart.
of 175−180°. Moreover, these structural parameters fall in the
range of those for previously reported Au(I) phosphole
complexes.10b,c Similar Au−Cl bond lengths and linear
character were found in NHC-based gold complexes with, as
expected, a shorter Au−C bond length within 1.94−2.01 Å.4a
Finally, to assess the steric properties of the ligands, their
buried volumes (%Vbur) were calculated from the X-ray
crystallographic data of the [Au(L)Cl] complexes using the
web application SambVca developed by Cavallo and co-
workers27 (see the Supporting Information) and were
To further characterize the electronic properties of the
ligands in the phosphole series, evaluation of the σ-donor ability
23
1
was done by measuring the magnitude of JP−Se in the 77Se
isotopomer of the corresponding phosphole selenides (Table
1).24 With the smallest value of 1JP−Se, TMP appears as the best
σ-donor ligand in our monophosphole series while PPh3
exhibits an intermediate σ-donor ability higher than that of
phenyl-substituted phospholes DBP and TPP but lower than
that of methyl-substituted phospholes DMP and TMP (Table
1).
28
compared to the buried volume of PPh3 (Table 2). As
expected, both phenyl-substituted phospholes TPP and DBP
exhibit higher %Vbur values than the methyl-substituted
phosphole TMP and all phosphole ligands were more sterically
hindered than the corresponding phosphine PPh3.
To evaluate the catalytic activity and selectivity of the gold
phosphole complexes, we first investigated the cycloisomeriza-
tion of the N-tethered 1,6-enyne 1a as a benchmark reaction.
The reaction was carried out in dichloromethane at room
temperature in the presence of a cationic catalyst generated in
situ from 5 mol % of [Au(L)Cl] and 5 mol % of AgSbF6. The
results obtained with the different phosphole complexes are
shown in Table 3.
From TMP, a new gold(I) complex was prepared under mild
conditions using [Au(SMe2)Cl] as precursor. This complex,
isolated as an air- and moisture-stable solid in good yield
(79%), was fully characterized by conventional NMR
1
techniques (31P, H, 13C), mass spectrometry, and elemental
analysis (see the Supporting Information). The structure of
[Au(TMP)Cl] has been established by X-ray diffraction studies
(Figure 1).
a
Table 3. Gold-Catalyzed Cycloisomerization of 1,6-Enynes
Figure 1. Ortep plot of [Au(TMP)Cl]. Thermal ellipsoids are drawn
at the 30% probability level, and the hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg): Au−P
= 2.224(1), Au−Cl = 2.285(1); P−Au−Cl = 179.86(5).
Other phosphole complexes of gold(I) chloride were
prepared from DBP, DMP, and TPP using the same
procedure.25 These complexes obtained in good yields (76−
87%) were fully characterized (the X-ray structure of
[Au(TPP)Cl] is described in the Supporting Information)
and were isolated as air- and moisture-stable solids, except for
[Au(DMP)Cl].
The examination of geometrical parameters within the P−
Au−Cl backbone in our chlorogold(I) phosphole complexes is
summarized in Table 2. This comparison reveals a typical Au−
P bond length in the range of 2.22−2.23 Å and a typical Au−Cl
bond length around 2.28 Å in all of the phosphole series as in
[Au(PPh3)Cl]. In all complexes, the Au center shows an almost
linear coordination with typical Cl−Au−P angles in the range
a
b
See the Supporting Information for experimental details. Deter-
mined by NMR.
All complexes were active in the cycloisomerization of 1a,
after addition of AgSbF6, except for [Au(DMP)Cl], which led
to the immediate formation of a black precipitate.29 The active
complexes led to high selectivity (>95%) in the cyclopropane
product 2a (Table 3, entries 1−3 and 5), and TMP appeared as
the best ligand of the series in terms of activity in the
cycloisomerization of 1a. Moreover, there is a good correlation
between the σ-donor ability of the ligand and the cyclo-
isomerization rate (see the Supporting Information), indicating
that the alkyne activation may not be the rate-determining step
in the cycloisomerization of the N-tethered enyne 1a.5b,30
In addition, the TMP-based complex proved to be very
efficient in the cycloisomerization of enynes bearing terminal
alkynes (Table 3, entries 6 and 7). 1,3-dienes 3b,c were formed
as major products from enynes 1b,c, respectively, as previously
reported for phosphine-based gold complexes.31
Table 2. Steric Parameters: Selected Bond Lengths (Å) and
Angles (deg) and %Vbur Values Calculated from X-ray
Crystallographic Data of Phosphole or Phosphine Gold(I)
Complexes [Au(L)Cl]
L
Au−P
Au−Cl
Cl−Au−P
%Vbur
DBP10a
2.221(8)
2.2241(6)
2.227(2)
2.2241(1)
2.235(3)
2.282(9)
2.282(6)
2.288(2)
2.285(4)
2.279(3)
178.8
48.7
50.8
n.d.
To extend the scope of reactions catalyzed by cationic
gold(I) phosphole complexes, we next investigated the
cyclopropanation of 4-methoxystyrene (5a) using propargyl
benzoate (6) as the gold(I) carbene precursor (Table 4).32 The
reaction was carried out in nitromethane at room temperature
TPP
176.4(2)
176.1
DMP10a
TMP
179.86(5)
179.6(1)
38.3
31.5
26
PPh3
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Organometallics
Communication
neutral complex with AgSbF6 in acetonitrile31b and fully
characterized. The X-ray diffraction structure (Figure 2)
shows the Au center in an almost linear coordination with
Au−P and Au−N bond lengths that are in the same range as in
[AuPPh3(CH3CN)]SbF6.34
Table 4. Gold-Catalyzed Cyclopropanation of Olefins
Figure 2. Ortep plot of [Au(TMP)(CH3CN)]SbF6. Thermal
ellipsoids are drawn at the 30% probability level, and the hydrogen
−
atoms and the SbF6 anion have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): Au−P = 2.220(1), Au-N =
2.042(2); P−Au−N = 178.53(6).
a
b
Determined by NMR. 4 mol equiv of olefin versus alkyne 6, 20 min.
c
4 mol equiv of olefin versus alkyne 6, 90 min.
Moreover, this new catalyst turned out to be both very stable
(several months under argon at room temperature as a white
solid) and highly efficient, since the catalytic loading could be
decreased to 0.5 mol % without any significant effect on the
conversion of the model enyne substrate 1c (see the
Supporting Information).
A 72% conversion was even achieved in 60 min when only
0.05 mol % of catalyst was used (TON = 1440, TOF = 24
min−1). Notably, the selectivity in the 1,3-dienes 3c/4c was
maintained at the 85/15 ratio when the catalytic loading was
decreased. Echavarren and co-workers reported the formation
of 3c and 4c in 100% yield and in a 50/50 ratio after 20 min of
reaction with 2% [AuPPh3(CH3CN)]SbF6 (TON = 50, TOF =
2.5 min−1).35 Thirty times less catalyst was thus needed to
accomplish the same transformation with a better selectivity in
the endodiene 3c.
In conclusion, gold(I) complexes bearing the highly donating
and not bulky tetramethylphosphole TMP are very stable
compounds and efficient catalysts in 1,6-enyne cycloisomeriza-
tions and olefin cyclopropanations, with higher activity than the
corresponding phosphine-based complexes. Further studies will
be dedicated to structural steric modulations on the phenyl
moiety of the TMP ligand to further tune the catalytic
activity.12
in the presence of a cationic catalyst generated in situ from 2
mol % of [Au(L)Cl] and 2 mol % of AgSbF6 and afforded
cyclopropane 7a as a mixture of cis and trans diastereoisomers
(Table 4, entries 1−4).
TMP appeared here also as the best ligand of the series in
terms of activity in the cyclopropanation of 5a, with the same
kind of correlation between the σ-donor ability of the ligand
and the alkyne conversion rate as previously observed for the
cycloisomerization. Surprisingly, [Au(DMP)Cl] did not
decompose upon addition of AgSbF6 in nitromethane and
was able to catalyze the cyclopropanation (Table 4, entry 3).
The diastereoselectivity of the cyclopropanation of 5a did not
show any significant dependence on the phosphole ligand, in
spite of their different steric properties. Notably 6 was fully
converted into 7a with [Au(TMP)Cl] as catalytic precursor
(Table 4, entry 4) in 20 min with a 57% isolated yield in
cyclopropane, whereas [Au(PPh3)Cl] led to undesired
polymeric products under the same conditions (Table 4,
entry 5), indicating that TMP is more selective than its
phosphine counterpart. In addition, the TMP-based complex
proved to be efficient and selective in the cyclopropanation of
various olefins, including styrene (5b; Table 4, entry 6),
sterically hindered 4-tert-butoxystyrene (5c; Table 4 entry 7),
and even strained bicyclic norbornylene (5d; Table 4 entry 8),
which usually requires drastic conditions.33
From a practical point of view, [Au(TMP)(NTf2)] was then
prepared in good yield (66%), analogously to the air-stable
[Au(PPh3)(NTf2)] complex developed by Gagosz et al.,11 to
avoid the use of a silver salt as cocatalyst. This new complex
was evaluated in the cycloisomerization of enyne 1c. [Au-
(TMP)(NTf2)] is slightly less active than the [Au(TMP)Cl]/
AgSbF6 system, as 98% conversion was achieved in 20 min with
a 5 mol % catalytic load, leading to 1,3-dienes 3c and 4c in a
85/15 ratio. However, [Au(TMP)(NTf2)] turned out to be
much less stable than [Au(PPh3)(NTf2)] and [Au(TMP)Cl],
since the solid stored under argon turned black within several
hours.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files detailing the preparation of
complexes and starting enyne substrates, catalysis protocol,
characterization of products formed, and X-ray crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +33 534 32 35 74. E-mail: odile.dechycabaret@ensiacet.fr
(O.D.-C.); maryse.gouygou@lcc-toulouse.fr (M.G.).
Finally, a cationic TMP-Au(I) complex could be isolated and
tested in enyne cycloisomerization. [Au(TMP)(CH3CN)]SbF6
was obtained by chloride abstraction from the corresponding
Notes
The authors declare no competing financial interest.
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Communication
̈
(13) (a) Costes, P.; Weckesser, J.; Dechy-Cabaret, O.; Urrutigoıty,
ACKNOWLEDGMENTS
■
M.; Kalck, P. Appl. Organomet. Chem. 2008, 22, 211. (b) Fuente-
Thanks are due to the CNRS, the French “Minister
l′Education Nationale et de la Recherche”, and Dr. I. Dixon for
helpful discussions during the preparation of this paper.
̀
e de
Hernandez, A.; Costes, P.; Kalck, P.; Ruiz-García, J. A.; Jau
U.; Urrutigoïty, M.; Dechy-Cabaret, O. Cat. Comm. 2010, 12, 142.
(c) Fuente-Hernandez, A.; Costes, P.; Kalck, P.; Jauregui-Haza, U.;
́
regui-Haza,
́
Dechy-Cabaret, O.; Urrutigoïty, M. Appl. Organomet. Chem. 2011, 25,
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́ ́
ez, E.; Nieto-Oberhuber, C.; Lopez, S.; Benet-
́
́
́
̃
̃
̈
́
S.; Kalck, P.; Daran, J.-C.; Urrutigoıty, M.; Gouygou, M.
Juge,
Organometallics 2012, 31, 857.
1574
dx.doi.org/10.1021/om301244m | Organometallics 2013, 32, 1571−1574