Synthesis, characterization, and catalytic activity of cationic NHC gold(III) pyridine complexes

Article abstract of DOI:10.1021/om400338k

A series of cationic gold(I/III) pyridine complexes of the type [(L)Au(pyr)](PF₆) and [(L)AuCl₂(pyr)](PF₆), where L = IPr, (1, 5); L = IMes (2, 6); L = I^tBu, (3, 7); L = ICy (4, 8); and L = PPh₃, (9, 10), were synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The stability of the new complexes and their catalytic activity in five well-established organic transformations were assessed.

Full text of DOI:10.1021/om400338k

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Catalytic Activity of Cationic NHC
Gold(III) Pyridine Complexes
Serena Orbisaglia,†,§ Beatrice Jacques,† Pierre Braunstein,† Damien Hueber,‡ Patrick Pale,‡
́
Aurelien Blanc,*,‡ and Pierre de Fremont*,†
́
́
†Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
CS 90032, 67081 Strasbourg, France
́
de Strasbourg, 4 Rue Blaise Pascal,
de Strasbourg,
̀
degli Studi di Camerino, Via S. Agostino 1, 62032 Camerino MC, Italy
‡Laboratoire de Synthes
̀ ́ ́ ́
e, Reactivite Organique et Catalyse, Institut de Chimie (UMR 7177 CNRS), Universite
4 Rue Blaise Pascal, CS 90032, 67081 Strasbourg, France
§School of Science and Technology, Universita
S
* Supporting Information
ABSTRACT: A series of cationic gold(I/III) pyridine complexes
of the type [(L)Au(pyr)](PF6) and [(L)AuCl2(pyr)](PF6),
where L = IPr, (1, 5); L = IMes (2, 6); L = ItBu, (3, 7); L =
ICy (4, 8); and L = PPh3, (9, 10), were synthesized and
characterized by NMR spectroscopy and single-crystal X-ray
diffraction. The stability of the new complexes and their
catalytic activity in five well-established organic transformations
were assessed.
diphenylpyridine,22 or triflimidate groups23 indicate a first step
toward the elaboration of more sophisticated complexes while,
in parallel, differently functionalized carbene ligands with potentially
chelating arms (e.g., picoline,24 dialkylamine,25 pyrazole,26
pyridine27) are also being applied successfully to gold(III)
chemistry. It is important to keep in mind that the organometallic
chemistry around the gold(III) centers can be severely hampered
by reductive elimination processes.
In parallel to the development of a broad library of NHC-
gold(I/III) complexes, numerous applications in homogeneous
catalysis28 or medicinal chemistry29 have blossomed. Usually,
the NHC-Au fragment can act as a soft Lewis acid tolerant to
most oxygen-based organic functions due to its low oxophilicity.
Moreover, the gold centers stabilized by NHC ligands can
smoothly undergo a reversible two-electron redox cycleAu(I)/
(III)of catalytic relevance.30 Gold complexes have become
valuable tools in organic synthetic chemistry, promoting a broad
range of remarkable reactions including the cycloisomerization of
enynes,31 the cyclization of allenes,32 oxyarylations of alkenes,33
hydroarylations,34 hydroaminations,35 glycosilations,36 and, in
general, the activation of alkene and alkyne functions toward the
additions of nucleophiles.28 Herein, we describe the synthesis and
characterization of a new series of stable, cationic NHC gold(III)
pyridine complexes, as well as their catalytic activity in five well-
established gold-mediated transformations, including the halogen-
ation of aromatic compounds by N-halosuccinimides,37 the
INTRODUCTION
■
Since the isolation of the first stable N-heterocyclic carbene
(NHC) by Arduengo in 1991,1 these ligands became extremely
popular to design a wide variety of transition metal complexes
with enhanced stability and catalytic activities.2 Without a doubt,
organogold chemistry has taken full advantage of the easy use
of NHCs. Indeed, these ligands with their strong electron donor
ability and easy tuning of their steric properties, are able to stabi-
lize the carbophilic gold centers in various oxidation states, by
forming robust gold−carbon bonds.3 This has recently allowed
significant achievements in organogold chemistry, and various
complexes initially thought to be elusive could be isolated and
structurally characterized. Such examples include cationic NHC-
Au(I) complexes, bearing neutral and weakly coordinated groups
such as solvent molecules (MeCN, THF, DMSO),4 alkenes,5
alkynes,6 allenes,7 diimines,8 carbon monoxide,9 ammonia,10 or
silver chloride,11 and neutral complexes with uncommon anionic
groups such as fluoride,12 hydroxide,13 hydride,14 triflimidate,15
trifluoromethoxyde,16 or acetylides.17 Another interesting fea-
ture of the NHC-Au(I) cationic fragment, similar to that of the
ubiquitous (phosphine)-Au(I) cation, is its isolobal character18
with H+, which was elegantly exploited by Sadighi et al. for the
synthesis of digold hydride [Au2H core] or trigold [Au3 core]
cationic clusters.14,19 In comparison, the organometallic chem-
istry of gold(III) with NHCs appears less developed than for
gold(I) and is mainly restricted to neutral trichloride/tribromide
Au(III) complexes whose NHC ligands differ only by the steric
bulk associated with the N-alkyl/aryl groups.20 In this regard, the
recent reports of NHC-Au(III) moieties bound to fluoride,21
Received: April 19, 2013
Published: July 29, 2013
© 2013 American Chemical Society
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cycloisomerization of propargylic amides,38 the Meyer−Schuster
rearrangement of propargylic alcohols,39 the [3+3]-rearrange-
ment and Nazarov tandem reactions of enynyl acetates,40 and the
double hydroarylation of diyne diethers.41
the 31P spectra reveal the presence of PO2F2− formed by
hydrolysis of PF6−. The complexes 2−4 were crystallized by slow
evaporation of acetone/petroleum ether solutions, and their
structures were determined by single-crystal X-ray diffraction. The
complexes crystallize in the P2 /c, P2 /c, and P1 space groups,
̅
1
1
RESULTS AND DISCUSSION
respectively. The unit cell of 4 contains also four well-ordered
molecules of cocrystallized acetone in strong interaction with the
PF6− anions. All the complexes display a cationic gold(I) center in
a nearly linear coordination environment with the C−Au−N
angles comprised between 176.2(2)° and 179.1(5)°. The C−Au
bond lengths are in the range 1.979(6)−1.988(8) Å and are
similar to those found in the corresponding neutral complexes
[(NHC)AuCl].42 The N−Au bond lengths, between 2.058(5) and
2.060(6) Å, are in the range of values found for gold complexes
with pyridine donor ligands.4b,43 As expected, the PF6− anions do
not interact with the gold(I) centers, but they ensure crystal
stability through multiple H···F bonds (Figure 1).
■
Synthesis of the Au(I) Complexes. The previously reported
complex [(IPr)Au(Pyr)](PF6) (1)4b (IPr = N,N′-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) was synthesized first,
followed by the new complexes [(IMes)Au(Pyr)](PF6) (2)
(IMes = N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-yli-
dene), [(ItBu)Au(Pyr)](PF6) (3) (ItBu = N,N′-bis(tert-butyl)-
imidazol-2-ylidene), and [(ICy)Au(Pyr)](PF6) (4) (ICy = N,N′-
bis(cyclohexyl)imidazol-2-ylidene). The syntheses were carried
out using 10 equiv of pyridine and 1 equiv of silver hexa-
fluorophosphate (AgPF6) as halide abstractor in CH2Cl2, a non-
coordinating solvent. All complexes were obtained in yields
above 89% (Scheme 1).
Synthesis of the Au(III) Complexes. The complexes 1−4
were dissolved in CH2Cl2, and a slight excess of iodobenzene
dichloride (1.05 equiv) was added at room temperature to
perform their oxidation to the corresponding cationic
complexes [(IPr)AuCl2(Pyr)](PF6) (5), [(IMes)AuCl2(Pyr)]-
(PF6) (6), [(ItBu)AuCl2(Pyr)](PF6) (7), and [(ICy)-
AuCl2(Pyr)](PF6) (8), respectively. After it was stirred for 3 h,
the reaction mixture turned yellow. Upon filtration over a plug of
Celite and precipitation with pentane, the new complexes were
cleanly recovered in excellent yields (above 84%) (Scheme 2).
Scheme 1. Synthesis of the [(NHC)Au(Pyr)](PF6)
Complexes 1−4
1
Their H NMR spectra show a significant downfield shift
(+0.3 ppm) for the signals associated with the imidazole
backbone and the ortho-protons from the pyridine ring
compared to the starting NHC-Au(I)-pyridine complexes. The
isopropyl proton (septet) signal of 5, which is very sensitive to
the gold environment, is also shifted from 2.56 to 2.79 ppm, in
good agreement with the formation of a (IPr)Au(III) fragment.
The oxidation to gold(III) is further confirmed by the 13C NMR
spectra of 5−8, which exhibit a significant upfield shift (between
−35 and −40 ppm) for the carbenic carbons signals. Their values
range from 120.0 to 131.6 ppm and are very close to those
reported for the related [(NHC)AuCl2(NTf2)] complexes.23
The 31P NMR spectra confirm the cationic character of 5−8
with the presence of the PF6− anion. The arrangement of two
symmetrical Clcis ligands around the gold center is confirmed by
1
Their H NMR spectra clearly exhibit all the expected signals,
and the coordination of the pyridine to the gold(I) center is easily
evidenced by a downfield shift (between 0.3 and 0.5 ppm) of the
signals related to its para- and meta-proton(s). Furthermore, the
resonance of the ortho-protons is upfield shifted by 0.5 ppm upon
coordination for 1 and 2 but remains unchanged for 3 and 4. The
13C NMR spectra display the characteristic signals of the carbene
carbon of a cationic NHC-Au(I) fragment bound to a nitrogen
ligand, with chemical shifts in the range 166.2−160.2 ppm.
The 31P and 19F NMR spectra are in good agreement with the
presence of PF6−, exhibiting a septet at −144.5 ppm and a doublet
at −73.5 ppm, respectively. After two days in solution, in nondry
CH2Cl2, no change of the 1H and 13C spectra is noticeable, while
Figure 1. Ball-and-stick representation of [(IMes)Au(pyr)](PF6) (2), [(ICy)Au(pyr)](PF6) (3), and [(ItBu)Au(pyr)](PF6) (4). Hydrogen atoms
have been omitted for clarity.
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to the NHC ligand, the C−Au−N angles being comprised between
176.1(3)° and 178.7(5)°. All other interligand angles are very
close to 90° with maximum deviations of −2.8° and +3.3°. The
Au−C distances, in the range 1.997(4)−2.012(6) Å, are in good
agreement with the Au(III)−C distances found in related NHC
complexes.20a,46 The Au−N distances, in the range 2.076(3)−
2.094(5) Å, are slightly longer than in the parent NHC-Au(I)-
pyridine complexes, but in good agreement with those in other
Au(III)-pyridine complexes.45 The PF6− anions do not interact with
the gold(III) center but ensure a good crystal cohesion through
multiple H···F bonds (Figures 2, 3). In contrast to the cationic
cis-pyridine-functionalized NHC gold(III) complexes reported by
the Monkowius, Cadierno, and Helaja groups,24,27b,c the complexes
5−8 display a very selective substitution on the trans-position.
These new NHC-Au(III) complexes are air and light stable,
both in solution and in the solid state. Preliminary studies
performed in CDCl3 unveiled a partial and slow conversion of 5
Scheme 2. Synthesis of the [(NHC)AuCl2(Pyr)](PF6)
Complexes 5−8
the appearance in the infrared spectrum of an intense ν(Au−Cl)
absorption band between 372 and 385 cm−1.44 These complexes
were quantitatively crystallized by slow evaporation of their solu-
tions in acetone and petroleum ether. Their structures were
unambiguously characterized by single-crystal X-ray diffraction
studies. The complexes 5−8 crystallize in the F2dd, P21/c, P21/c,
and P1 space groups, respectively. The large unit cell of 5
̅
contains also 12 molecules of cocrystallized acetone molecules in
strong interaction with 16 PF6− anions (2 PF6− and 1.5 acetone
molecules per asymmetric unit, with Z = 8 via symmetry
operations). All the gold(III) centers exhibit the expected square-
planar coordination environment with the pyridine in trans-position
1
to [IPrAuCl3], which was detected by H NMR and also
isolated as a new crystalline polymorph.46,47 By prior filtration
of the chloroform through a plug of alumina to remove any
trace of HCl, the formation of [IPrAuCl3] could be avoided.
Figure 2. Ball-and-stick representation of [(IPr)AuCl2(pyr)](PF6) (5) and [(IMes)AuCl2(pyr)](PF6) (6). Hydrogen atoms have been omitted for
clarity.
Figure 3. Ball-and-stick representation of [(ItBu)AuCl2(pyr)](PF6) (7) and [(ICy)AuCl2(pyr)](PF6) (8). Hydrogen atoms have been omitted for
clarity.
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Clearly, the cationic NHC-gold(III) fragment exhibits a high
halide affinity.
in cis-position under very mild conditions,24,27b,c but remains
consistent with the lack of reactivity of [(IPr)AuCl2(NTf2)]
toward excess Ag(NTf2).23
The complexes 5−8 could also be obtained in excellent
yields starting from the previously reported [(NHC)AuCl3]
complexes46 and by abstracting one chloride ligand in the
presence of 10 equiv of pyridine and 1 equiv of TlPF6 or AgPF6.
Interestingly, this synthetic approach is slightly more efficient
(better yield) than the one involving direct oxidation of the
pyridine gold(I) complexes. Once again, the pyridine ligand
selectively occupies the trans-position with respect to the NHC
ligand (Scheme 3).
Phosphine Pyridine Complexes. With the phosphine
gold(I)/(III) complexes being also extremely popular and
efficient homogeneous catalysts, the synthesis of [(PPh3)-
AuCl2(Pyr)](PF6) (10) was attempted for the sake of comparison
with the NHC series. The same synthetic strategies as for 5−8
were employed. The reaction of [(PPh3)AuCl] with 1 equiv of
Ag/TlPF6 in the presence of excess pyridine led to [(PPh3)Au-
(Pyr)](PF6) (9) in high yield, following a protocol described by
Schmidbaur et al. for [(PPh3)Au(Pyr)](BF4) and [(PPh3)Au(2-
hydroxy-Pyr)](BF4).43a The 31P NMR of 10 exhibits a signal at
29.2 ppm slightly shifted compared to [(PPh3)AuCl] (33.1 ppm)
but in the range found for other cationic gold(I) phosphine
pyridine complexes. Surprisingly, the CSD does not seem to
contain any structural report for 9; thus single crystals suitable for
X-ray diffraction were grown from a solution mixture of CH2Cl2
Scheme 3. Alternative Synthesis of the
[(NHC)AuCl2(Pyr)](PF6) Complexes 5−8
and octane. Complex 9 crystallizes in the P1 space group.
̅
The unit cell contains also two molecules of cocrystallized CH2Cl2
in interaction with the PF6− anions. The gold(I) center is in a
linear coordination environment with a P−Au−N angle equal
to 178.2(2)°. The P−Au and Au−N distances of 2.232(2) and
2.066(7) Å, respectively, are in good agreement with those in
other gold(I) phosphine pyridine complexes (Figure 4).4b,43
Finally, the reaction of [(IPr)AuCl3] with 10 equiv of
pyridine and 3.5 equiv of AgPF6 was undertaken overnight
at room temperature or in refluxing CH2Cl2 with the objective
to form [(IPr)Au(Pyr)3](PF6)3. The reaction mixture was
then filtered over Celite and precipitated by addition of pentane
to yield a clear yellow powder. The 1H NMR spectrum revealed
the formation of the complex [(IPr)AuCl2(Pyr)](PF6) (5)
together with a new pyridine-containing species integrating
for three versus 5 and exhibiting signals at 8.62 and 8.01 ppm.
This second species does not affect the symmetry of 5 by being
in close contact, ruling out any equilibrium process with the
pyridine bound to gold. The 13C NMR spectra confirmed also
the formation of 5 and of another pyridine adduct with signals
at 152.0, 140.3, and 126.3 ppm. Recently Lee et al. reported the
synthesis and characterization of [(Pyr)2Ag]2(PF6)2 by direct
addition of pyridine to AgPF6.48 Pleasingly, this complex
1
exhibits a very close H NMR signature (signals shifted by less
Figure 4. Ball-and-stick representation of [(PPh3)Au(pyr)](PF6) (9).
than 0.06 ppm) with the product found with 5, whereas both 13C
NMR spectra match perfectly. Moreover, the elemental analysis of
the powdery reaction product confirms the presence of a mix-
ture of 1 [(IPr)AuCl2(Pyr)](PF6) (5)/0.75 [(Pyr)2Ag]2(PF6)2,
in good agreement with the latter species integrating for three
pyridine versus 5. It is very likely that [(Pyr)2Ag]2(PF6)2 and 5
have a similar solubility in CH2Cl2 and pentane and were not
separated during the workup. Attempts to use more forcing
reaction conditions (boiling acetonitrile) resulted in a slow
decomposition to [(IPr)AuCl] (30%) after 2 days (Scheme 4).
Thus, replacement of one or both cis-chloride ligands from 5
by pyridine was not successful. This contrasts with the easy for-
mation of the cationic pincer-NHC-pyridine gold(III) com-
plexes for which the pyridine-arm coordinates the gold center
Hydrogen atoms have been omitted for clarity.
The oxidation of 9 with a slight excess of iodobenzene
dichloride (1.05 equiv) was attempted in dry CH2Cl2 under
protection from light and led to a yellow solution. The H
1
NMR spectrum displays a complicated pattern in the aromatic
region with indications of dynamic processes. The complete
reduction of PhICl2 proceeds cleanly, as evidenced by the for-
mation of iodobenzene. Two types of pyridine compounds
(P1/P2) (with signals different from those of the free pyridine)
are observable and display signals at 8.89 (H-ortho), 8.18 (H-para),
below 7.8 ppm, and being hidden (H-meta) for P1 and at 8.74
(H-ortho), 8.51 (H-para), and 8.01 ppm (H-meta) for P2. After
5 min reaction time, the ratio P1/P2 is equal to 9:1, after 2.5 h it
Scheme 4. Attempted Syntheses of [(IPr)Au(Pyr)3](PF6)3
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becomes 5:5, and finally after 3 days the ratio is 0:10. Interestingly,
the signals of P1 match perfectly with those of [(Pyr)AuCl3].49
The other aromatic signals account for at least two types of
triphenylphosphine-containing compounds. Importantly, there is no
trace of the starting complex 9. After 5 min, the 31P NMR spectrum
displays an intense peak at 41.0 ppm, which was not assigned, and
two minor peaks at 44.0 and 32.7 ppm. Whereas the latter can be
assigned to [(PPh3)AuCl], the former could be due to either
[(PPh3)AuCl3] or [(PPh3)2Au](PF6),50 since these complexes
have very similar resonances at 44.0 and 44.9 ppm. The situation
the formation of three minor products continues, while [(PPh3)-
AuCl3] is half consumed. Finally after 4 days, the 31P NMR
shows the complete disappearance of [(PPh3)AuCl3] and of the
species resonating at 65.9 ppm; the main product remains
[(PPh3)AuCl] with traces of PPh3O. As yellow crystals started
to appear in the NMR tube, their study by X-ray diffraction
revealed the formation of another decomposition product,
[PyrH][AuCl4].53 From the different NMR studies performed,
it appears clearly that the synthesis of 10 is highly challenging;
the cationic phosphine gold(III) pyridine complexes would likely
require more strongly donating phosphine ligands than PPh3 in
order to be thermodynamically viable (Scheme 5).54
1
being similar to the H NMR spectra, these products cannot be
unambiguously discriminated. The ratio of [(PPh3)AuCl] and
[(PPh3)AuCl3] or [(PPh3)2Au](PF6) increases with time and the
signal at 41.0 ppm fades until complete disappearance after 2.5 h.
Attempts to crystallize the reaction mixture yielded yellow and
colorless crystals, which confirmed the formation of [(PPh3)AuCl3]
and [(PPh3)AuCl].
Catalytic Properties. The complexes [(IPr)AuCl2(Pyr)]-
(PF6) (5) and [(ItBu)AuCl2(Pyr)](PF6) (7) were tested
for five, well-established gold-catalyzed transformations: the
halogenation of aromatics by N-halosuccinimides (C-1), the
cycloisomerization of propargylic amides (C-2), the Meyer−
Schuster rearrangement of propargylic alcohols (C-3), the [3+3]-
rearrangement and Nazarov tandem reactions of enynyl acetates
(C-4), and finally the double hydroarylation of diyne diethers
(C-5). Their activities were compared to those of the gold
systems usually employed for these transformations by using
similar experimental conditions (solvents, temperatures).37−41
For C-1, the naphthalene can be fully converted in 15 h to
1-bromonaphthalene (16) with N-bromosuccinimide (NBS) in
the presence of 0.1 mol % of AuCl3 at 80 °C in dichloroethane
(DCE).37 The complexes 5 and 7 do not promote any reaction
unless one equivalent of silver salt (AgSbF6) is added. Once
activated, the conversions, with 1 mol % of 5 or 7, reach a
maximum of 56% and 59%, respectively, after 20 h at 80 °C in
DCE. They exhibit a poor activity compared to AuCl3. For C-2,
the N-(prop-2-yn-1-yl)benzamide 11 can be converted to
5-methyl-2-phenyloxazole 17 in 95% yield, in the presence of
5 mol % of AuCl3 at 20 °C in CH2Cl2 for 12 h.38 Upon
activation with AgSbF6, 5 mol % of 5 and 7 convert 11 in 76%
and 75% yield, respectively, after 90 min in refluxing CH2Cl2.
They exhibit a similar activity compared to AuCl3. For C-3, the
1-(hex-1-yn-1-yl)cycloheptanol 12 can be converted in 3 h to
1-cycloheptylidenehexan-2-one in 80% yield in the presence of 2
mol % of [(PPh3)Au(NTf2)] at 20 °C in CH2Cl2.39 Reaction C-3
is unknown with any gold(III) catalyst, and 5 mol % of 5 and 7 did
not perform the desired transformation. Instead, the alcohol
elimination product (hex-1-yn-1-ylcycloheptane) (18a) was quan-
titatively formed in 1 h at room temperature in CH2Cl2. For C-4,
the 2-methylundec-1-en-3-yn-5-yl acetate 13 can be quantitatively
converted to 3-hexyl-5-methylcyclopent-2-enone 19 in the
presence of 1 mol % of [(PPh3)AuCl]/AgSbF6 at room tempera-
ture in CH2Cl2 in 30 min.40 AuCl3 can also mediate this trans-
formation but with less efficiency, yielding 48% of 19 after 30 min.
The synthesis of [(PPh3)AuCl2(pyr)](PF6) (10) was then
attempted by reacting 3.5 equiv of pyridine with [(PPh3)AuCl3]
in the presence of 1 equiv of AgPF6. After 5 min, once again the
1
aromatic region of the H NMR spectra indicates the pre-
sence of different PPh3-containing compounds. Most of the
pyridine is uncoordinated, there is only a trace of a second
pyridine species (1%) with signals at 8.89, 8.17, and 8.01 ppm,
which does not account for the formation of 9. After 2 h, the
1H NMR spectrum does not present any significant change. The
31P NMR spectrum reveals two species at 33.1 and 29.9 ppm. It
confirms also the disappearance of [(PPh3)AuCl3] within 5 min.
Once again, the species with the resonance at 33.1 ppm is likely
to be [(PPh3)AuCl], whereas the species at 29.9 ppm might be
either [(PPh3)2Au]Cl or PPh3O owing to some trace of water
appearing with time.51 It is also important to note that all the
syntheses based on the gold(I/III) phosphine systems led
ultimately to the appearance of colloids or of a gold mirror. As
it is known that [(PPh3)AuCl3] is prompt to decompose,52
a
solution of [(PPh3)AuCl3] containing 3.5 equiv of pyridine was
prepared in CD2Cl2 and kept cold (with dry ice) in the dark,
to ensure its stability prior to addition of the silver salt. After
15 min, its 31P NMR spectrum showed already the formation in
very small quantities of two new products with signals at 33.1
and 27.3 ppm, assigned to [(PPh3)AuCl] and likely PPh3O,
respectively. After 3 h at low temperature, the concentration of
both products increases slowly, the formation of [(PPh3)AuCl]
1
being the fastest. The H NMR spectrum reveals that most of
the pyridine is free, while a minor species (2% vs free pyridine)
has signals at 8.89, 8.17, and 8.01 ppm. Then, warming up
the solution to room temperature accelerates the formation of
[(PPh3)AuCl] and PPh3O. After 1.75 h, formation of a third
species with a 31P chemical shift at 65.9 ppm occurs. After 4 h,
Scheme 5. Decomposition Pathways upon the Attempted Syntheses of 10
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Table 1. Summary of the Catalytic Results with [(IPr)AuCl2(Pyr)](PF6) (5) and [(ItBu)AuCl2(Pyr)](PF6) (7)
a
b
Activated with AgSbF6. Mixture of 5-methyl-2-phenyloxazole 17 and 5-methylene-2-phenyl-4,5-dihydrooxazole 17a (ratio 7:93 and 37:63,
c
respectively). Only the elimination product 18a was formed (see Experimental Section).
Table 2. Summary of the Catalytic Tests with 1
a
1
No trace of isomerized product 17 was detected by H NMR analysis.
Upon activation with AgSbF6, 1 mol % of 5 and 7 convert 13 in
75% and 81% yield after 1 and 3 h, respectively, in refluxing
CH2Cl2. Once again 5 and 7 are more sluggish than AuCl3,
but better yields are obtained. Finally, for C-5, the 1,6-
diphenoxyhexa-2,4-diyne 15 was converted to 2H,2′H-4,4′-
bichromene (20) in 95% yield in the presence of 5 mol % of
[(PPh3)AuCl]/AgSbF6 after 20 min at 30 °C in CH2Cl2.41
The reaction does not proceed with AuCl3/AgOTf. Upon
activation with AgSbF6, 5 mol % of 5 and 7 convert 15 in 70%
and 69% yield, respectively, after 4 h in refluxing CH2Cl2.
Their activities are poorer compared to [(PPh3)AuCl]/AgSbF6
but better than with the AuCl3/AgOTf system (Table 1).
The complexes 5 and 7 display poor to moderate activities
compared to the reference systems for the five transformations
studies. Surprisingly, they do not show any sign of activity in
the absence of a silver salt. This may be related to recent
mechanistic investigations by Shi et al., who have suggested an
involvement of Au−Ag bimetallic catalysts.55 It also emphasizes
the difficulties in gaining a clear picture of the active species
involved. A closer look at the 1H NMR spectra of the crude reac-
tions performed with 5% mol catalysts allows the detection of the
catalyst fragments. C-1, C-3, and C-4 seem to proceed exclusively
via some NHC-gold(III) fragments (including 5 and 7), while C-2
and C-5 seem to involve exclusively some NHC-gold(I) fragments
(including 1 and 3). These results contrast with the inertness of
1 to promote the allylic acetate rearrangement4a and suggest a
reaction pattern even more complicated than anticipated owing to
the easy reduction of the gold(III) halide complexes.
17a and 19 in 77% and 72% yield, respectively. More
interestingly, 1 was revealed to be equally active without addition
of AgSbF6 (Table 2). The H NMR spectra of the crude reac-
1
tion show the presence of only 1 for the catalysis made without
silver and of 1 plus a second unidentified NHC-Au(I) species
in the presence of AgSbF6. These results hint at the possibility
of using more often NHC-Au-pyridine systems in catalysis,
in particular the gold(I) complexes, which are extremely stable.
They also reveal the need for a better understanding of the role of
the silver salts and of the changes in the gold coordination sphere
to predict catalytic activity.
CONCLUSION
■
A series of new cationic gold(III) monopyridine NHC complexes
was readily prepared in excellent yields. They were tested in
five gold-mediated organic transformations and display poor to
moderate catalytic activity compared to the reference systems.
The gold(III) complexes strongly tend to be reduced under the
catalytic conditions, thus complicating the reaction pattern.
Moreover, the gold(III) complexes were inactive without addi-
tion of silver salt, while the parent gold(I) pyridine complexes did
not require the use of silver salts. The synthesis of a bis- or tris-
pyridine gold(III) NHC complexes failed, the reaction stopping
after the formation of the monopyridine complex. Finally the
related cationic gold(III) monopyridine PPh3 complex was not
accessible using the synthetic strategy used with the NHC ligands,
and extensive decomposition or ligand redistribution products
were observed.
To further confirm the catalytic potential of 1, tests were
made for C-2 and C-4. Complex 1 promotes C-2 and C-4 with
a moderate activity, comparable to 5, thus yielding exclusively
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C16H25AuF6N3P: C, 31.96; H, 4.19; N, 6.99. Found: C, 32.25; H,
4.24; N, 7.02.
Synthesis of [(ICy)Au(Pyr)](PF6) (4). A procedure similar to that
EXPERIMENTAL SECTION
■
General Considerations. Proton (1H NMR), carbon (13C NMR),
phosphorus (31P NMR), and fluorine (19F NMR) nuclear magnetic
resonance spectra were recorded on the following instruments: Bruker
AVANCE I, 300 MHz spectrometer; Bruker AVANCE III, 400 MHz
spectrometer; and Bruker AVANCE I, 500 MHz spectrometer,
respectively. The chemical shifts are given in parts per million
(ppm). Data are presented as follows: chemical shift, multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sept. =
septet, m = multiplet, b = broad), coupling constants (J/Hz), and
integration. Assignments were determined on the basis of either
unambiguous chemical shifts or coupling patterns. The residual solvent
proton (1H) or carbon (13C) resonance, or the PF6 anion signals (31P,
19F) were used as reference values. For 1H NMR: CDCl3 = 7.26 ppm,
CD2Cl2 = 5.32 ppm. For 13C{1H} NMR: CDCl3 = 77.1 ppm, CD2Cl2
used for compound 2 gave 4 as a white solid (177 mg, 0.27 mmol).
1
Yield: 95%. H NMR (CD2Cl2, 500 MHz): 8.61 (d, J = 7 Hz, 2H,
CHo‑Py), 8.16 (t, J = 7 Hz, 1H, CHp‑Py), 7.76 (t, J = 7 Hz, 2H, CHm‑Py),
7.16 (s, 2H, CHimidazole), 4.45−4.52 (m, 2H, CHCy), 2.12−2.17 (m,
4H, CH2Cy), 1.91−1.97 (m, 4H, CH2Cy), 1.69−1.80 (m, 6H, CH2Cy),
1.44−1.53 (m, 4H, CH2Cy), 1.22−1.31 (m, 2H, CH2Cy). 13C{1H}
NMR (CD2Cl2, 125.7 MHz): 161.0 (Ccarbene), 151.9 (CHo‑Py), 141.9
(CHp‑Py), 127.5 (CHm‑Py), 119.1 (CHimidazole), 62.4 (CHCy), 34.7
(CH2Cy), 25.9 (CH2Cy), 25.4 (CH2Cy). 31P{1H} NMR (CD2Cl2, 121.4
1
−
MHz): −144.5 (sept., J(31P−19F) = 710 Hz, PF6 ). 19F{1H} NMR
1
−
(CD2Cl2, 282.2 MHz): −73.3 (d, J(19F−31P) = 710 Hz, PF6 ). Anal.
Calcd for C20H29AuF6N3P: C, 36.76; H, 4.47; N, 6.43. Found: C,
37.14; H, 4.60; N, 6.38.
−
1
= 53.8. For 31P{1H} NMR: PF6 = −141.3 ppm (sept., J(31P−19F) =
Synthesis of [(IPr)AuCl2(Pyr)](PF6) (5). PhICl2 (71 mg, 0.26 mmol,
1.05 equiv) was added to a solution of [(IPr)Au(Pyr)](PF6) (200 mg,
0.25 mmol, 1.0 equiv) in CH2Cl2 (10 mL). The reaction mixture was
stirred at room temperature for 3 h and then filtered through Celite.
The filtrate was added to pentane (30 mL), and the resulting yellow
precipitate was filtered, washed with pentane (3 × 10 mL), and dried
under vacuum to afford 5 (188 mg, 0.21 mmol) as a yellow solid.
−
1
712.0 Hz). For 19F{1H} NMR: PF6 = −74.0 ppm (d, J(31P−19F) =
712.0 Hz). IR spectra were recorded in the region 4000−200 cm−1 on
a Nicolet 6700 FT-IR spectrometer (ATR mode, diamond crystal).
Elemental analyses were performed by the “Service de Microanalyses”,
́
Universite de Strasbourg. For the X-ray diffraction studies, the
intensity data were collected at 173(2) K on a Kappa CCD
diffractometer 88 (graphite-monochromated Mo Kα radiation, λ =
0.71073 Å). Crystallographic and experimental details for all the
structures are summarized in the Supporting Information (p S13). The
structures were solved by direct methods (SHELXS-97) and refined by
full-matrix least-squares procedures (based on F2, SHELXL-97) with
anisotropic thermal parameters for all the non-hydrogen atoms.56 The
hydrogen atoms were introduced into the geometrically calculated
positions (SHELXL-97 procedures) and refined riding on the
corresponding parent atoms.
1
Yield: 84%. H NMR (CD2Cl2, 400 MHz): 8.35 (d, J = 7 Hz, 2H,
CHo‑Py), 8.12 (t, J = 7 Hz, 1H, CHp‑Py), 7.62−7.68 (m, 6H, CHm‑Py and
CHimidazole and CHp‑Ar), 7.45 (d, J = 8 Hz, 4H, CHm‑Ar), 2.79 (sept., J =
7 Hz, 4H, CHiPr), 1.41 (d, J = 7 Hz, 12H, CH3iPr), 1.20 (d, J = 7 Hz,
12H, CH3iPr). 13C{1H} NMR (CD2Cl2, 100.6 MHz): 149.2 (CHo‑Py),
146.5 (Co‑Ar), 143.5 (CHp‑Py), 132.8 (CHp‑Ar), 132.0 (Ci‑Ar), 131.6
(Ccarbene), 128.6 (CHimidazole), 128.0 (CHm‑Py), 125.4 (CHm‑Ar), 29.6
(CHiPr), 26.7 (CH3iPr), 22.9 (CH3iPr). 31P{1H} NMR (CD2Cl2, 162.0
1
−
MHz): −144.5 (sept., J(31P−19F) = 710 Hz, PF6 ). 19F{1H} NMR
(CD2Cl2, 282.2 MHz): −73.4 (d, J(19F−31P) = 710 Hz, PF6 ). Anal.
Calcd for 2(C32H41AuCl2F6N3P)·C3H6O: C, 44.24; H, 4.88; N, 4.62.
Found: C, 44.28; H, 4.92; N, 4.60. IR: ν(cm−1): 385 (Au−Clcis).
Synthesis of [(IMes)AuCl2(Pyr)](PF6) (6). A procedure similar to that
used for compound 5 gave 6 as a yellow solid (183 mg, 0.23 mmol).
1
−
Synthesis of the Gold Complexes. All reactions were carried out
in air with nondried solvents, unless stated otherwise. CD2Cl2 was
distilled from CaH2, degassed, and stored over 4 Å molecular sieves.
PhICl2 was prepared following a procedure described by Kalyani and
Sanford57 and stored at −30 °C. The silver hexafluorophosphate was
stored away from light under argon prior to use. All other reagents
(including the pyridine) were used as received from commercial
suppliers. All gold(III) complexes were made and kept in the dark.
Yields of the new complexes are based on the precursor gold
complexes.
́
1
Yield: 82%. H NMR (CD2Cl2, 300 MHz): 8.39 (d, J = 7 Hz, 2H,
CHo‑Py), 8.12 (t, J = 7 Hz, 1H, CHp‑Py), 7.65 (t, J = 7 Hz, 2H, CHm‑Py),
7.58 (s, 2H, CHimidazole), 7.13 (s, 4H, CHAr), 2.40 (s, 6H, p-CH3), 2.28
(s, 12H, o-CH3). 13C{1H} NMR (CD2Cl2, 125.7 MHz): 149.4
(CHo‑Py), 143.4 (CHp‑Py), 142.1 (Cp‑Ar), 135.8 (Co‑Ar), 132.4 (Ci‑Ar),
130.7 (Ccarbene), 130.5 (CHm‑Ar), 127.9 (CHimidazole or CHm‑Py), 127.8
(CHimidazole or CHm‑Py), 21.4 (p-CH3), 18.7 (o-CH3). 31P{1H} NMR
Synthesis of [(IMes)Au(Pyr)](PF6) (2). AgPF6 (96 mg, 0.37 mmol,
1.0 equiv) was added to a solution of [(IMes)AuCl] (200 mg, 0.37
mmol, 1.0 equiv) and pyridine (0.30 mL, 3.7 mmol, 10 equiv) in
CH2Cl2 (5 mL). The reaction mixture was stirred at room temperature
overnight and then filtered over Celite. The filtrate was added to
pentane (30 mL), and the resulting precipitate was filtered, washed
with pentane (3 × 10 mL), and dried under vacuum to afford 2 (240
1
−
(CD2Cl2, 121.4 MHz): −144.5 (sept., J(31P−19F) = 710 Hz, PF6 ).
19F{1H} NMR (CD2Cl2, 282.2 MHz): −73.3 (d, J(19F−31P) = 710
1
−
Hz, PF6 ). Anal. Calcd for C26H29AuCl2F6N3P: C, 39.21; H, 3.67; N,
5.28. Found: C, 39.06; H, 3.72; N, 5.24. IR: ν(cm−1): 381 (Au−Clcis).
Synthesis of [(ItBu)AuCl2(Pyr)](PF6) (7). A procedure similar to that
used for compound 5 gave 7 as a yellow solid (531 mg, 0.79 mmol).
1
mg, 0.33 mmol) as a white solid. Yield: 89%. H NMR (CD2Cl2, 500
MHz): 8.06 (d, J = 7 Hz, 2H, CHo‑Py), 8.00 (t, J = 7 Hz, 1H, CHp‑Py),
7.55 (t, J = 7 Hz, 2H, CHm‑Py), 7.33 (s, 2H, CHimidazole), 7.10 (s, 4H,
CHAr), 2.38 (s, 6H, p-CH3), 2.17 (s, 12H, o-CH3). 13C{1H} NMR
(CD2Cl2, 125.7 MHz): 166.2 (Ccarbene), 151.4 (CHo‑Py), 141.9
(CHp‑Py), 140.9 (Cp‑Ar), 135.2 (Co‑Ar), 134.6 (Ci‑Ar), 130.0 (CHm‑Ar),
127.2 (CHm‑Py), 124.2 (CHimidazole), 21.3 (p-CH3), 18.0 (o-CH3).
1
Yield: 95%. H NMR (CD2Cl2, 500 MHz): 8.90 (d, J = 7 Hz, 2H,
CHo‑Py), 8.28 (t, J = 7 Hz, 1H, CHp‑Py), 7.86 (t, J = 7 Hz, 2H, CHm‑Py),
7.70 (s, 2H, CHimidazole), 2.00 (s, 18H, C(CH3)3). 13C{1H} NMR
(CD2Cl2, 125.7 MHz): 150.0 (CHo‑Py), 143.4 (CHp‑Py), 128.3
(CHm‑Py), 124.4 (CHimidazole), 120.0 (Ccarbene), 63.2 (C(CH3)3), 32.1
(C(CH3)3). 31P{1H} NMR (CD2Cl2, 121.4 MHz): −144.5 (sept.,
31P{1H} NMR (CD2Cl2, 121.4 MHz): −144.5 (sept., J(31P−19F) =
1
−
1J(31P−19F) = 710 Hz, PF6 ). 19F{1H} NMR (CD2Cl2, 282.2 MHz):
−
710 Hz, PF6 ). 19F{1H} NMR (CD2Cl2, 282.2 MHz): −73.5 (d,
−73.3 (d, 1J(19F−31P) = 710 Hz, PF6−). Anal. Calcd for
C16H25AuCl2F6N3P: C, 28.59; H, 3.75; N, 6.25. Found: C, 28.74; H,
3.77; N, 6,25. IR: ν(cm−1): 374 (Au−Clcis).
−
1J(19F−31P) = 710 Hz, PF6 ). Anal. Calcd for C26H29AuF6N3P: C,
43.05; H, 4.03; N, 5.79. Found: C, 43.22; H, 4.13; N, 5.80.
Synthesis of [(ItBu)Au(Pyr)](PF6) (3). A procedure similar to that
used for compounds 2 gave 3 as a white solid (261 mg, 0.43 mmol).
Synthesis of [(ICy)AuCl2(Pyr)](PF6) (8). A procedure similar to that
used for compound 5 gave 8 as a yellow solid (238 mg, 0.33 mmol).
Yield: 87%. H NMR (CD2Cl2, 400 MHz): 9.04 (d, J = 8 Hz, 2H,
1
1
Yield: 89%. H NMR (CD2Cl2, 300 MHz): 8.59 (d, J = 7 Hz, 2H,
CHo‑Py), 8.16 (t, J = 7 Hz, 1H, CHp‑Py), 7.77 (t, J = 7 Hz, 2H, CHm‑Py),
7.29 (s, 2H, CHimidazole), 1.92 (s, 18H, C(CH3)3). 13C{1H} NMR
(CD2Cl2, 125.7 MHz): 160.2 (Ccarbene), 152.2 (CHo‑Py), 141.9
(CHp‑Py), 127.6 (CHm‑Py), 118.3 (CHimidazole), 59.6 (C(CH3)3), 32.3
(C(CH3)3). 31P{1H} NMR (CD2Cl2, 121.4 MHz): −144.5 (sept.,
CHo‑Py), 8.29 (t, J = 8 Hz, 1H, CHp‑Py), 7.86 (t, J = 8 Hz, 2H, CHm‑Py),
7.44 (s, 2H, CHimidazole), 4.42−4.48 (m, 2H, CHCy), 2.20−2.27 (m,
4H, CH2Cy), 1.94−2.01 (m, 4H, CH2Cy), 1.66−1.83 (m, 6H, CH2Cy),
1.44−1.57 (m, 4H, CH2Cy), 1.24−1.35 (m, 2H, CH2Cy). 13C{1H}
NMR (CD2Cl2, 125.7 MHz): 149.7 (CHo‑Py), 143.6 (CHp‑Py), 128.0
(CHm‑Py), 124.0 (Ccarbene), 122.6 (CHimidazole), 62.6 (CHCy), 33.9
(CH2Cy), 25.6 (CH2Cy), 25.1 (CH2Cy). 31P{1H} NMR (CD2Cl2, 121.4
−
1J(31P−19F) = 710 Hz, PF6 ). 19F{1H} NMR (CD2Cl2, 282.2 MHz):
−73.4 (d, 1J(19F−31P) = 710 Hz, PF6−). Anal. Calcd for
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1
−
MHz): −144.4 (sept., J(31P−19F) = 710 Hz, PF6 ). 19F{1H} NMR
11 (1.828 g, 90%) as a white solid. TLC: Rf 0.63 (EA/cyclohexane
40%). H NMR (300 MHz, CDCl3): 2.29 (t, J = 2.6 Hz, 1 H), 4.26
(dd, J = 5.2, 2.6 Hz, 2 H), 6.29 (br, 1 H), 7.41−7.55 (m, 3 H), 7.75−
7.82 (m, 2 H). 13C{1H} NMR (125 MHz, CDCl3): 29.9, 72.0, 79.5,
127.0, 128.7, 131.9, 133.7, 167.1.
1
−
1
(CD2Cl2, 282.2 MHz): −73.0 (d, J(19F−31P) = 710 Hz, PF6 ). Anal.
Calcd for C20H29AuCl2F6N3P: C, 33.16; H, 4.04; N, 5.80. Found: C,
33.26; H, 4.17; N, 5.67. IR: ν(cm−1): 372 (Au−Clcis).
The complexes 5−8 were also obtained by following the second
procedure described below for 5.
Synthesis of [(IPr)AuCl2(Pyr)](PF6) (5). Solid AgPF6 (35 mg,
0.14 mmol, 1 equiv) was added to a solution of [(IPr)AuCl3] (100 mg,
0.14 mmol, 1 equiv) and pyridine (0.11 mL, 1.4 mmol, 10 equiv) in
CH2Cl2 (5 mL). The reaction mixture was stirred at room temperature
overnight and then filtered over Celite. The filtrate was added in
pentane (30 mL). The resulting yellow precipitate was filtered,
washed with pentane (3 × 10 mL), and dried under vacuum to afford
5 (120 mg, 0.13 mmol) as a yellow solid. Yield: 93%.
1-(Hex-1-yn-1-yl)cycloheptanol (12) (ref 39). To a solution of
nBuLi (1.6 M in hexanes, 6.1 mmol) in THF (10 mL) was added
1-hexyne (6.1 mmol) at −78 °C under argon. The resulting mixture
was stirred at this temperature for 15 min and was then allowed
to warm to −20 °C over 5 min. Cycloheptanone (6.1 mmol) was then
added at −78 °C. The reaction mixture was stirred for 30 min at −78 °C,
then allowed to reach room temperature, and further stirred for 30 min
(monitored by TLC). The mixture was quenched with a saturated
NH4Cl solution and extracted with Et2O (2×). The combined organic
extracts were dried over MgSO4 and evaporated. Flash column
chromatography over silica gel (EA/cyclohexane 10%) gave the desired
product 12 (1.041 g, 88%) as a colorless oil. TLC: Rf 0.40 (EA/
Synthesis of [(PPh3)Au(Pyr)](PF6) (9). A procedure similar to that
used for compounds 2−4 gave 9 as a white solid (257 mg, 0.38 mmol).
Yield: 95%. H NMR (CD2Cl2, 300 MHz): 8.64 (d, J = 7 Hz, 2H,
1
CHo‑Py), 8.15 (t, J = 7 Hz, 1H, CHp‑Py), 7.78 (t, J = 7 Hz, 2H, CHm‑Py),
7.68−7.56 (m, 15H, PPh3). 13C{1H} NMR (CD2Cl2, 75.5 MHz): 151.4
(CHo‑Py), 142.2 (CHp‑Py), 134.6 (d, 2J(13C−31P) = 13 Hz, CHo‑Ph), 133.2
(d, 4J(13C−31P) = 3 Hz, CHp‑Ph), 130.1 (d, 3J(13C−31P) = 12 Hz,
CHm‑Ph), 127.3 (CHm‑Py), 127.2 (d, 1J(13C−31P) = 66 Hz, Ci‑Ph). 31P{1H}
NMR (CD2Cl2, 121.5 MHz): 29.2 (s, PPh3), −144.5 (septet, 1J(31P−19F) =
710 Hz, PF6−). 19F{1H} NMR (CD2Cl2, 282.4 MHz): −73.2 (d,
1J(19F−31P) = 710 Hz, PF6−). Anal. Calcd for C23H20AuF6NP2: C,
40.43; H, 2.95; N, 2.05. Found: C, 40.35; H, 3.00; N, 2.16.
1
cyclohexane 20%). H NMR (300 MHz, CDCl3): 0.90 (t, J = 7.2 Hz,
3 H), 1.34−1.67 (m, 12 H), 1.72−1.84 (m, 3 H), 1.86−2.00 (m, 2 H),
2.20 (t, J = 7.0 Hz, 2 H). 13C{1H} NMR (125 MHz, CDCl3): 13.6, 18.3,
21.9, 22.3, 27.9, 30.9, 43.4, 71.9, 84.0, 82.9.
2-Methylundec-1-en-3-yn-5-yl Acetate (13).58 To a solution of
nBuLi (1.6 M in hexanes, 7.6 mmol) in THF (10 mL) was added
Synthesis of [(PPh3)AuCl2(Pyr)](PF6) (10). PhICl2 (63 mg, 0.23 mmol,
1.05 equiv) was added to a solution of [(PPh3)Au(Pyr)](PF6) (9)
(150 mg, 0.22 mmol, 1.0 equiv) in CH2Cl2 (10 mL). The reaction
mixture was stirred at room temperature for 3 h and then filtered
through Celite. The filtrate was added to pentane (30 mL), and the
resulting yellow precipitate was filtered, washed with pentane (3 × 10 mL),
and dried under vacuum to afford a mixture of compounds.
Synthesis of [(PPh3)AuCl2(Pyr)](PF6) (10): NMR Studies. PhICl2
(4.0 mg, 0.16 mmol, 1.05 equiv) was added to a solution of
[(PPh3)Au(Pyr)](PF6) (10 mg, 0.15 mmol, 1.0 equiv) in CD2Cl2
(1 mL). The reaction mixture was filtered through Celite and
monitored over time.
Synthesis of [(PPh3)AuCl2(Pyr)](PF6) (10): NMR Studies. AgPF6
(5.0 mg, 0.02 mmol, 1.0 equiv) was added to a solution of [(PPh3)AuCl3]
(10 mg, 0.02 mmol, 1.0 equiv) and pyridine (0.02 mL, 0.2 mmol, 10 equiv)
in CD2Cl2 (3 mL). The reaction mixture was filtered through Celite and
monitored over time.
Homogenous Catalysis. Reagents and solvents were purified
using standard methods. Dichloromethane and dichloroethane were
distilled from CaH2 under an argon atmosphere; pyrimidine was distilled
from KOH; tetrahydrofuran (THF) was distilled from sodium metal/
benzophenone and stored under an argon atmosphere. Anhydrous
reactions were carried out in flame-dried glassware and under an argon
atmosphere. All extractive procedures were performed using nondistilled
solvents. Analytical thin layer chromatography (TLC) was carried out on
silica gel 60 F254 plates with visualization by ultraviolet light, cerium
ammonium molybdate, or potassium permanganate dip. Flash column
chromatography was carried out using silica gel 60 (40−63 μm), and the
procedure included the subsequent evaporation of solvents in vacuo.
N-(Prop-2-yn-1-yl)benzamide (11).38 To a solution of
propargylamine (12.7 mmol) in CH2Cl2 (28 mL) were added Et3N
2-methyl-1-buten-3-yne (7.6 mmol) at −78 °C under argon. The
resulting mixture was stirred at this temperature for 15 min and was
then allowed to warm to −20 °C over 5 min. Heptanal (7.6 mmol) was
then added at −78 °C. The reaction mixture was stirred for 30 min at
−78 °C, then allowed to reach room temperature, and further stirred
for 1.5 h (monitored by TLC). The mixture was quenched with Ac2O
(25 mmol) and stirred at 0 °C for 2 h. A saturated NH4Cl solution was
then added, and the mixture was extracted with Et2O (2×). The combined
organic extracts were dried over MgSO4 and evaporated. Flash column
chromatography over silica gel (EA/cyclohexane 5%) gave the desired
product 13 (0.953 g, 56%) as a yellowish oil. TLC: Rf 0.58 (EA/
1
cyclohexane 20%). H NMR (300 MHz, CDCl3): 0.87 (t, J = 6.7 Hz,
3 H), 1.28−1.42 (m, 8 H), 1.72−1.80 (m, 2 H), 1.87 (s, 3 H), 2.07 (s,
3 H), 5.23 (m, 1 H), 5.30 (m, 1 H), 5.48 (t, J = 6.6 Hz, 1 H). 13C{1H}
NMR (125 MHz, CDCl3): 14.1, 21.2, 22.6, 23.4, 25.0, 28.8, 31.7, 34.9,
64.5, 85.6, 86.4, 122.7, 126.1, 170.1.
(14.0 mmol) at 0 °C and benzoyl chloride (19.1 mmol). The resulting
mixture was stirred at room temperature for 2 h. The mixture was
quenched with MeOH (19 mL), stirred for 1 h, and then evaporated.
The crude was then taken in CH2Cl2 and 1 N HCl(aq) and extracted
with CH2Cl2 (2×). The combined organic extracts were washed with
brine, dried over MgSO4, and evaporated. Flash column chromatog-
raphy over silica gel (EA/cyclohexane 30%) gave the desired product
(Prop-2-yn-1-yloxy)benzene (14).41 To a solution of phenol
(63.8 mmol) in DMF (60 mL) were added K2CO3 (127.5 mmol)
and propargyl bromide (80 wt % in toluene, 95.6 mmol). The resulting
mixture was stirred at room temperature overnight. Water was then
added, and the mixture was extracted with Et2O (3×). The combined
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AgSbF6 (5 mol %) in CH2Cl2 (5 mL). The resulting mixture was
stirred at reflux (2 h for IPr and overnight for ItBu). The mixture was
evaporated without any workup. Flash column chromatography over silica
gel (EA/cyclohexane 30%) gave a mixture of desired methyl-oxazole 17
and methylene-dihydrooxazole 17a as a colorless oil. [(IPr)AuCl2(pyr)]-
(PF6): 38.0 mg, 76% (17:17a/7:93). [(ItBu)AuCl2(pyr)](PF6): 37.7 mg,
75% (17:17a/37:63).
organic extracts were washed with water and brine, dried over MgSO4,
and evaporated. Flash column chromatography over silica gel (EA/
cyclohexane 10%) gave the desired product 14 (7.426 g, 88%) as a
yellow oil. TLC: Rf 0.68 (EA/cyclohexane 20%). 1H NMR (400 MHz,
CDCl3): 2.52 (t, J = 2.4 Hz, 1 H), 4.70 (d, J = 2.4 Hz, 2 H), 6.95−7.04
(m, 3 H), 7.27−7.35 (m, 2 H). 13C{1H} NMR (125 MHz, CDCl3):
58.8, 75.4, 78.7, 114.9, 121.6, 129.5, 157.6.
5-Methyl-2-phenyloxazole (17). 1H NMR (300 MHz, CDCl3):
2.40 (d, J = 1.1 Hz, 3 H), 6.84 (q, J = 1.1 Hz, 1 H), 7.39−7.56 (m, 3 H),
7.97−8.02 (m, 2 H). 13C{1H} NMR (125 MHz, CDCl3): 11.1, 124.2,
126.0, 127.8, 128.7, 129.9, 148.9, 160.7.
5-Methylene-2-phenyl-4,5-dihydrooxazole (17a).59 1H NMR
(300 MHz, CDCl3): δ 4.37 (q, J = 2.7 Hz, 1 H), 4.66 (t, J = 2.8 Hz,
2 H), 4.82 (q, J = 3.0 Hz, 1 H), 7.39−7.55 (m, 3 H), 7.95−8.01 (m,
2 H). 13C{1H} NMR (125 MHz, CDCl3): 57.8, 83.8, 126.8, 128.0, 128.5,
131.8, 158.9, 163.8.
1,6-Diphenoxyhexa-2,4-diyne (15) (ref 41). To a solution of
(prop-2-yn-1-yloxy)benzene (14) (28.0 mmol) in CH2Cl2 (55 mL)
1-Cycloheptylidenehexan-2-one (18),39 C-3. A solution of 1-(hex-
1-yn-1-yl)cycloheptanol (12) (0.309 mmol) in CH2Cl2 (2 mL) was
was added pyrimidine (28.0 mmol) and Cu(OAc)2·H2O (2.8 mmol).
The resulting mixture was stirred at room temperature overnight
under air (open flask). The mixture was then filtered over a pad of
Celite, washed with water and saturated NH4Cl solution, dried over
MgSO4, and evaporated. Flash column chromatography over silica gel
(EA/cyclohexane 5%) gave the desired product 15 (2.164 g, 55%) as
a yellowish solid. TLC: Rf 0.34 (EA/cyclohexane 5%). 1H NMR
(300 MHz, CDCl3): 4.75 (s, 4 H), 6.91−7.05 (m, 6 H), 7.27−7.35 (m,
4 H). 13C{1H} NMR (125 MHz, CDCl3): 56.2, 71.1, 74.7, 114.9,
121.8, 129.6, 157.4.
added to a solution of catalyst (5 mol %) in CH2Cl2 (3 mL). The
resulting mixture was stirred for 20 h at room temperature, then 4 h at
reflux. No conversion was observed using either the IPr or ItBu catalyst
(1H NMR analysis showed only the starting material).
Activated Conditions. A solution of 1-(hex-1-yn-1-yl)cycloheptanol
(12) (0.309 mmol) in CH2Cl2 (2 mL) was added to a solution of
catalyst (5 mol %) activated with AgSbF6 (5 mol %) in CH2Cl2 (3 mL).
The resulting mixture was stirred for 1 h at reflux or 20 h at room
temperature. No conversion was observed using either the IPr or ItBu
catalyst even under thermal conditions (1H NMR analysis showed only
elimination product 18a: 1-(hex-1-yn-1-yl)cyclohept-1-ene). 1H NMR
(400 MHz, CDCl3): δ 0.91 (t, J = 7.3 Hz, 3 H), 1.35−2.95 (m, 11 H),
2.11−2.19 (m, 2 H), 2.26−2.34 (m, 3 H), (t, J = 6.7 Hz, 1 H).
3-Hexyl-5-methylcyclopent-2-enone (19),40 C-4. A solution of
2-methylundec-1-en-3-yn-5-yl acetate (13) (0.270 mmol) in wet CH2Cl2
Gold(III)-Catalyzed Standard Organic Reactions (Catalysts: 5 and
7). 1-Bromonaphthalene (16),37 C-1. Naphthalene (0.780 mmol) and
then NBS (0.780 mmol) were added to a solution of catalyst (1 mol %)
in CH2Cl2 (5 mL). The resulting mixture was stirred for 20 h at 80 °C.
No conversion was observed using either IPr or ItBu catalyst (1H NMR
analysis showed only the starting material).
Activated Conditions. Naphthalene (0.780 mmol) and then NBS
(0.780 mmol) were added to a solution of catalyst (1 mol %) activated
with AgSbF6 (1 mol %) in CH2Cl2 (5 mL). The resulting mixture was
stirred for 20 h at 80 °C. The mixture was evaporated (no more trace of
NBS by 1H NMR) without any workup. Flash column chromatography
over silica gel (cyclohexane) gave a mixture of remaining naphthalene
and desired product 16. Yield calculated with internal standard
(hexamethylbenzene): [(IPr)AuCl2(pyr)](PF6), 56%; [(ItBu)AuCl2-
(pyr)](PF6), 59%.
(2 mL) was added to a solution of catalyst (5 mol %) in wet CH2Cl2
(3 mL). The resulting mixture was stirred 20 h at room temperature, then
4 h at reflux. No conversion was observed using either IPr or ItBu catalyst
(1H NMR analysis showed only the starting material).
Activated Conditions. A solution of 2-methylundec-1-en-3-yn-5-yl
acetate (13) (0.270 mmol) in wet CH2Cl2 (2 mL) was added to
a solution of catalyst (5 mol %) activated with AgSbF6 (5 mol %) in
wet CH2Cl2 (3 mL). The resulting mixture was stirred for 1.5 h at
reflux. The mixture was evaporated without any workup. Flash column
chromatography over silica gel (EA/cyclohexane 5%) gave the desired
product 19 as a yellowish oil. [(IPr)AuCl2(pyr)](PF6): 39.6 mg, 81%.
[(ItBu)AuCl2(pyr)](PF6): 33.8 mg, 70%.
1H NMR (300 MHz, CDCl3): 7.33 (t, J = 7.8 Hz, 1 H), 7.62−7.44
(m, 2 H), 7.86−7.75 (m, 3 H), 8.23 (d, J = 8.5 Hz, 1 H). 13C{1H}
NMR (125 MHz, CDCl3): 122.8, 126.1, 126.6, 127.0, 127.2, 127.8,
128.2, 129.8, 131.9, 134.6.
1H NMR (300 MHz, CDCl3): 0.89 (t, J = 6.9 Hz, 3 H), 1.15 (d, J = 7.5
Hz, 3 H), 1.26−1.38 (m, 6 H), 1.52−1.62 (m, 2 H), 2.16 (d, JAB = 18.3 Hz,
1 H), 2.83 (t, J = 7.7 Hz, 2 H), 2.36−2.45 (m, 1 H), 2.81 (dd, JAB = 18.3
Hz, J = 6.7 Hz, 1 H), 5.90 (s, 1 H). 13C{1H} NMR (75 MHz, CDCl3):
14.0, 16.5, 22.5, 27.0, 29.0, 31.6, 33.5, 40.4, 40.7, 128.2, 181.4, 212.7.
2H,2′H-4,4′-Bichromene (20),41 C-5. 1,6-Diphenoxyhexa-2,4-diyne
(15) (0.381 mmol) was added to a solution of catalyst (5 mol %) in
5-Methyl-2-phenyloxazole (17),38 C-2. N-(Prop-2-yn-1-yl)-
benzamide (11) (0.314 mmol) was added to a solution of catalyst
(5 mol %) in CH2Cl2 (5 mL). The resulting mixture was stirred at
reflux for 20 h. No conversion was observed using either IPr or ItBu
catalyst (1H NMR analysis showed only the starting material).
Activated Conditions. N-(Prop-2-yn-1-yl)benzamide (11) (0.314
mmol) was added to a solution of catalyst (5 mol %) activated with
CH2Cl2 (5 mL). The resulting mixture was stirred for 20 h at room
temperature, then 4 h at reflux. No conversion was observed using
4161
dx.doi.org/10.1021/om400338k | Organometallics 2013, 32, 4153−4164

Organometallics
Article
either IPr or ItBu catalyst (1H NMR analysis showed only the starting
material).
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
Activated Conditions. 1,6-Diphenoxyhexa-2,4-diyne (15) (0.381
mmol) was added to a solution of catalyst (5 mol %) activated
with AgSbF6 (5 mol %) in CH2Cl2 (5 mL). The resulting mixture
was stirred for 4 h at reflux. The mixture was evaporated without
any workup. Flash column chromatography over silica gel (EA/
cyclohexane 5%) gave the desired product (20) as a yellow solid.
[(IPr)AuCl2(pyr)](PF6): 69.7 mg, 70%. [(ItBu)AuCl2(pyr)](PF6):
68.7 mg, 69%.
ACKNOWLEDGMENTS
The Centre National de la Recherche Scientifique (CNRS) and
■
the Minister
̀
e de la Recherche are gratefully acknowledged for
financial support of this work. Johnson Matthey PLC is also
gratefully acknowledged for a generous loan of gold metal. A.B.
and D.H. thank the Agence Nationale de la Recherche (ANR)
for financial support and a Ph.D. fellowship (Grant ANR-11-
JS07-001-01 Synt-Het-Au). S.O. thanks Prof. Claudio Pettinari
and Prof. Fabio Marchetti for encouraging and financially
supporting her Ph.D. mobility between the University of Camerino
and the University of Strasbourg.
1H NMR (300 MHz, CDCl3): 4.89 (d, J = 3.8 Hz, 2 H), 5.83 (t, J =
3.8 Hz, 1 H), 6.77 (td, J = 7.5, 1.2 Hz, 1 H), 6.86 (dd, J = 8.1, 1.2 Hz,
1 H), 6.89 (dd, J = 7.5, 1.7 Hz, 1 H), 7.11 (ddd, J = 8.1, 7.5, 1.7 Hz,
1 H). 13C{1H} NMR (125 MHz, CDCl3): 65.4, 116.2, 121.5, 122.9,
125.9, 129.5, 134.0, 154.1.
Gold(I)-Catalyzed Standard Organic Reactions (catalyst: 1).
5-Methyl-2-phenyloxazole (17),38 C-2. Activated Conditions.
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ASSOCIATED CONTENT
* Supporting Information
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