Hydrophenoxylation of internal alkynes catalysed with a heterobimetallic Cu-NHC/Au-NHC system

Article abstract of DOI:10.1039/c6dt04513h

A straightforward method for the hydrophenoxylation of internal alkynes, using N-heterocyclic carbene-based copper(i) and gold(i) complexes, is described. The heterobimetallic catalytic system proceeds via dual activation of the substrates to afford the d

Full text of DOI:10.1039/c6dt04513h

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Hydrophenoxylation of internal alkynes catalysed
with a heterobimetallic Cu-NHC/Au-NHC system†
Cite this: DOI: 10.1039/c6dt04513h
Faïma Lazreg,a Stefano Guidone,a Alberto Gómez-Herrera,a Fady Nahrab and
Catherine S. J. Cazin*a,b
Received 29th November 2016,
Accepted 19th December 2016
A straightforward method for the hydrophenoxylation of internal alkynes, using N-heterocyclic carbene-
based copper(I) and gold(I) complexes, is described. The heterobimetallic catalytic system proceeds via
dual activation of the substrates to afford the desired vinylether derivatives. This methodology is shown to
be highly efficient and tolerates a wide range of substituted phenols and alkynes.
DOI: 10.1039/c6dt04513h
www.rsc.org/dalton
Introduction
The functionalisation of alkynes is a critical transformation in
synthetic chemistry leading to key building blocks.1–9 The use
of late transition-metal complexes has been widely investigated
in such transformations, allowing the formation of C–N, C–O,
C–S and C–B bonds.1–9 Recently, attention has focused on C–O
bond formation, through hydroalkoxylation and hydrophenox-
ylation reactions.10–40 The hydroalkoxylation reaction has been
widely studied and several metal complexes consisting mainly
of Cu,11 Ru,12–16 Pd,17,18 Au,19–25 Zn,26 Pt,27–29 Ir30 and Rh31
have been reported as catalysts. As for the hydrophenoxylation
of alkynes, reports remain scarce, especially in the case of non-
activated internal alkynes.32–38 Yamamoto reported the first
example in 2002, using a Pd(0) complex.32 Sahoo and co-
Scheme 1 Intermediates in the digold-catalysed hydrophenoxyl-
ation.37–40 IPr = N,N’-bis-[2,6-(di-iso-propyl)phenyl]imidazol-2-ylidene.
to generate species IV, whilst III activated the alkyne to afford
V. The coupling of both intermediates (IV and V) was followed
by protonation to afford the desired hydrophenoxylation
product.
workers achieved
a significant breakthrough when they
Based on these observations, we reasoned that a more cost-
effective method for the hydrophenoxylation of alkynes could
be developed using a different bimetallic catalytic system. To
this end, gold species II could be substituted by [Cu(OH)
(IPr)]42 (1) and gold species III could be generated from a cat-
ionic gold precursor. In this context, we now report the hydro-
phenoxylation of alkynes via a dual activation process, using a
heterobimetallic system consisting of Cu(I)- and Au(I)-NHC
(NHC = N-heterocyclic carbene) complexes, under mild and
solvent-free conditions. The use of a simple copper complex as
a gold surrogate for activation of one substrate in the addition
reaction further supports the dual activation mechanism pre-
viously described for the gold-only mediated hydrophenoxyl-
ation of alkynes.
reported on a gold(III)/phosphine system.33 More recently,
based on the discovery of a digold hydroxide species,36 Nolan
and co-workers have developed a novel methodology permit-
ting access to vinylether derivatives.37 This methodology was
later applied to access benzo[c]chromenes and benzo[b]furans
through a one-pot [Au]/[Pd]-catalysed process.38 More recently,
the mechanism of this reaction was elucidated (Scheme 1).39,40
These mechanistic studies clearly showed that gold performs a
dual role in this transformation, highlighting the need for two
gold centres acting in synergy.41 It was proposed that the
digold hydroxide Ia was in equilibrium with the gold hydroxide
II and cationic gold species III. Species II activated the phenol
aEaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST,
UK
bDepartment of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 - S3,
9000 Gent, Belgium. E-mail: Catherine.Cazin@Ugent.be
Results and discussion
Initial testing consisted of using [Cu(OH)(IPr)], 1, and two gold
complexes; [Au(OTf)(IPr)]43 (2, OTf = trifluoromethanesulfo-
nate) and [Au(NTf2)(IPr)]44 (3, NTf2 = bis(trifluoromethan-
†Electronic supplementary information (ESI) available: Detailed synthetic and
mechanistic procedures, full characterisation details for new complexes and
catalytic reactions. See DOI: 10.1039/c6dt04513h
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6aa was obtained with 97% conversion. Thus, further optimi-
sation was carried out under solvent-free conditions.
Decreasing the catalytic loading of both catalysts to 0.1 mol%
significantly lowered the conversion (Table 1, entry 9).
Interestingly, the use of complex 3 instead of 2 afforded only
9% conversion, indicating that the counterion plays an impor-
tant role in this reaction (Table 1, entry 10).39 When [Cu(OH)
(IPr)], 1, was used, in the absence of gold(I) complex, no cata-
Fig. 1 Catalysts used in this study.
esulfonyl)amide) (Fig. 1). The strategy makes use of the copper lytic activity was observed and only starting materials were
hydroxide synthon 1 to activate the phenolic substrate while recovered after 16 hours (Table 1, entry 11). Similarly, when
the cationic gold species concomitantly activates the alkyne.
only gold(I) complexes (2 or 3) were used, only traces of the
Diphenylacetylene and phenol were selected as model sub- desired product were observed (Table 1, entries 12 and 13).
strates for the initial investigation (Table 1). Using 1 mol% of 1 These results clearly highlight the need for a bimetallic system
and 1 mol% of 2, full conversion to the desired product was in order to effectively perform the targeted dual catalysis. Next,
achieved after 16 hours (Table 1, entry 1). When the loading of the reaction temperature was varied; the best result was
each catalyst was decreased to 0.2 mol%, significant loss of obtained at 60 °C and afforded full conversion after 16 hours,
efficiency was observed (Table 1, entries 2 and 3). Solvents using 0.2 mol% of 1 and 0.2 mol% of 2 (Table 1, entry 14). Of
such as acetonitrile, CPME (CPME = cyclopentyl methyl ether) note, lowering either the temperature or the catalyst loading
or DMSO (DMSO = dimethyl sulfoxide) led to no or low cata- resulted only in a decrease of catalytic activity (Table 1, entries
lytic activity, whereas the use of benzene afforded a 53% con- 15 and 16).
version (Table 1, entries 4–7). The most interesting result was
It should be mentioned that when the digold analogue,
obtained when no solvent was used (Table 1, entry 8). Indeed, [{Au(IPr)}2(μ-OH)][OTf] (Ib), was tested under the same con-
under solvent-free conditions, the hydrophenoxylation product ditions as entry 2 using a catalyst loading of 0.5 mol% (1 mol%
of gold), only 60% conversion was observed (Table 1, entry
19). This result strongly highlights the higher efficiency of the
heterobimetallic [Cu]/[Au] catalytic system compared to the
Table 1 Influence of temperature and solvent on the catalytic activitya
homobimetallic gold system. Furthermore, in order to
compare the present system with literature precedents, the
hydrophenoxylation reaction was performed under our opti-
mised conditions and those reported by Nolan and co-
workers,37 using the same total amount of catalysts.
Interestingly, our conditions afforded full conversion to the
desired product 6aa after 35 minutes, using 0.5 mol% of each
catalyst (1 and 2) at 60 °C; whereas digold hydroxide Ia only
afforded full conversion after 1 hour at 80 °C (Table 1, entries
17 and 18). This result emphasises a synergistic effect of the
heterobimetallic system. Although we cannot fully elimi-
nate the possibility of a homobimetallic activation route, the
latter data strongly indicate that this pathway is highly un-
likely. Finally, when the reaction was performed using gold
catalyst 2, only traces of the desired product were observed
(<3%), further demonstrating the need for a bimetallic system
(Table 1, entry 20).
Kinetic profiling of this transformation under the optimal
conditions (Table 1, entry 14) showed that, whilst the reaction
progresses slightly faster under argon, completion is also
reached within 7 hours under aerobic conditions (see the
ESI†). This result showcases the stability of the catalytic system
in air.
A substrate scope study was undertaken next to demon-
strate the versatility of the heterobimetallic system (Scheme 2).
Phenols bearing various functional groups were first screened
in the presence of diphenylacetylene as the alkyne substrate.
para-Substituted phenols bearing electron-donating groups
(EDGs), such as OMe and Me, underwent the desired trans-
formation without any loss in efficiency and both products
Loading
(mol%)
Temp.
(°C)
Conv.b
(%)
Entry
Catalyst
Solvent
1
2
3
4
5
6
7
8
1 + 2
1 + 2
1 + 2
1 + 2
1 + 2
1 + 2
1 + 2
1 + 2
1 + 2
1 + 3
1
1
Toluene
Toluene
Toluene
Acetonitrile
DMSO
CPME
Benzene
—
—
—
—
—
—
—
—
—
—
—
80
80
80
80
80
80
80
80
80
80
80
80
80
60
40
60
60
80
80
60
>99
83
30
0
0
6
53
97
69
9
0
2
0.5
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.2
0.2
0.2
0.2
0.2
0.1
0.5
0.5
0.5
0.2
9
10
11
12
13
14
15
16
17
18
19
20
2
3
3
1 + 2
1 + 2
1 + 2
1 + 2
Ia
Ib
2
>99 (99)c
74
64
>99d
>99e
60
Toluene
—
<3
a Reaction conditions: diphenylacetylene (0.5 mmol), phenol
(0.6 mmol), [Cu(OH)(IPr)]/[Au(OTf)(IPr)] (1 + 2) or [Cu(OH)(IPr)]/
[Au(NTf2)(IPr)] (1 + 3) (0.1–1 mol%), solvent (1 mL) or solvent-free,
16 h, under argon. b Determined by GC, relative to diphenylacetylene,
average of two reactions. c Isolated yield in parentheses. d Full conver-
sion reached after 35 minutes. e Full conversion reached after 1 hour.
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0.4 mol%; under these conditions, the desired products 6an
and 6ar were obtained in good isolated yields (96% and 89%,
respectively). ortho-Substituted phenyl- and allylphenols (5p
and 5q) also needed 0.4 mol% catalyst loading to afford good
isolated yields of the desired products (6ap, 79%; 6aq, 75%),
presumably for steric reasons. meta-Disubstituted 3,5-xylenol
(5s) also performed well under our conditions, affording the
desired product 6as in excellent isolated yield (96%). However,
when ortho-disubstituted 2,6-xylenol (5t) was used, a significant
decrease in reaction efficiency was observed and the desired
product 6at was only obtained in moderate yield (46%), using
0.4 mol% catalyst loading. Considering the high steric hin-
drance induced by phenol 5t, this last example still constitutes
an impressive result for this heterobimetallic catalytic system.
Finally, 2-naphthol was subjected to the optimal reaction con-
ditions, affording the desired product 6au in 81% isolated yield.
Next the alkyne substrate was varied (Scheme 3). A number
of symmetrical and unsymmetrical alkyne derivatives, bearing
alkyl, aryl and heteroaryl groups, successfully underwent con-
version under our reaction conditions to afford moderate to
very good isolated yields of the products. When 3-hexyne (4b)
was used, the desired product 6ba was obtained in very good
yield (90%), thus further underlining the versatility of such a
methodology. In all cases where unsymmetrical alkynes were
used, moderate to good yields were obtained; however, a
mixture of Z/Z′ (6ca–ja/6′ca–ja) was always observed with a
ratio varying from 7 : 3 to 2 : 8.
Scheme 2 Reaction scope of the hydrophenoxylation reaction cata-
lysed by a [Cu]/[Au] bimetallic system; variation of the phenol. Reaction
conditions: alkyne (0.5 mmol), Ar–OH (0.6 mmol), [Cu(OH)(IPr)]/
[Au(OTf)(IPr)] (1 : 1) (0.2/0.2 mol%), solvent-free, 16 h, 60 °C, under Ar.
Conversion determined by GC based on the alkyne; minimum average
of 2 reactions. Isolated yield in parentheses. a 0.4 mol% of both catalysts.
A mechanism operating via a dual activation process is pro-
posed and illustrated in Scheme 4, based on recent reports by
Nolan and co-workers (Scheme 1)37–40 and our own mecha-
nistic investigations (Schemes 5–7).
(6ab and 6ac) were isolated in 99% yield. Interestingly,
phenols bearing para-substituted B(pin) (pin = pinacolato)
and fluoro-groups were also successfully transformed under
our conditions, affording the desired products 6ad and 6ae in
good isolated yields (78% and 96%, respectively). The case of
B(pin)-substituted phenol represents an attractive feature as it
could allow further functionalisation in possible tandem reac-
tions.45 In contrast, phenols bearing electron-withdrawing
groups (EWGs), such as the weakly deactivating Cl group and
moderate to strong deactivating groups (C(O)H, C(O)Me, CN
and NO2), afforded low conversion to the desired products. To
overcome this problem, the catalyst loading was increased to
0.4 mol%; under these conditions, the desired products 6af,
6ag, 6ah, 6aj and 6ak were obtained in good to excellent
yields. Interestingly, 6ai, bearing a moderately deactivating
group (C(O)OMe), was obtained in good yield (68%), using
only 0.2 mol% catalyst loading. meta- and ortho-Substituted
phenols were also screened under these conditions. Following
the same trend as before, meta-substituted methyl- and fluoro-
phenols (5l and 5m) as well as ortho-substituted methylphenol
(5o) performed very well under our optimal conditions,
affording the desired products 6al, 6am and 6ao in 95%, 85%
and 81% isolated yields, respectively. Similarly, meta- and
ortho-substituted chlorophenols (5n and 5r) only afforded high
Scheme 3 Reaction scope of the hydrophenoxylation reaction cata-
lysed by a [Cu]/[Au] bimetallic system; variation of the alkyne. Reaction
conditions: alkyne (0.5 mmol), phenol (0.6 mmol), [Cu(OH)(IPr)]/
[Au(OTf)(IPr)] (1 : 1) (0.2/0.2 mol%), solvent-free, 16 h, 60 °C, under Ar.
Isolated yields are given; Z/Z’ ratio in parentheses is determined by 1H
conversions when the catalyst loading was increased to NMR. a Isolated yield of each isomer.
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Scheme 7 Stoichiometric tests to elucidate the reaction mechanism;
confirming the involvement of intermediate species F and G in the cata-
lytic cycle.
Similarly to the reports on the digold hydroxide species Ia
(Scheme 1),37–40 we suspected an equilibrium between
species (1 + 2) and (A + B) via intermediate C. Indeed, upon
mixing complexes 1 and 2, a mixture was obtained (Mixture
A, Scheme 6); the same mixture was also obtained upon
mixing complexes A and B, highlighting the existence of an
equilibrium between these species as proposed in Scheme 4.
Scheme 4 Proposed mechanism for the hydrophenoxylation of alkynes.
In
a
similar manner, we observed an equilibrium
between species (F + 2) and (B + E), most likely via inter-
mediate D;38 upon mixing (F + 2) and (B + E), the same
mixture was obtained (Mixture B, Scheme 6). Interestingly,
when phenol 5a was added to Mixture A, we observed
the formation of Mixture B after 1 hour at 45 °C. The
addition of diphenylacetylene 4a to Mixture B delivers a
complex reaction blend; however, the formation of the final
product 6aa was not observed. Nonetheless, when phenol 5a
was added to the latter mixture, product 6aa was finally
obtained along with the concurrent formation of Mixture B
(Scheme 6).
Scheme 5 Stoichiometric tests to elucidate the reaction mechanism;
formation of F and G.
Furthermore, upon reacting copper complex F with inter-
mediate G, the same complex reaction mixture as above was
obtained; and in a similar fashion, addition of phenol 5a
allowed us to observe the formation of product 6aa and
Mixture B (Scheme 7). The same result can be obtained by
reacting F with G, in the presence of phenol under our
optimal conditions (solvent-free, at 60 °C for 16 h). Finally,
the catalytic involvement of the isolated copper phenoxide F
was further ascertained by carrying out a catalytic test
using 0.2 mol% of F and 0.2 mol% of 2 (Scheme 7). Indeed,
under the standard conditions, full conversion to 6aa was
obtained.
These observations suggest that the mixture of 1 and 2
(Mixture A) first reacts with phenol 5a to afford Mixture B.
The latter enters the catalytic cycle by further reaction
with diphenylacetylene 4a to give F and G. Reaction of F and
G affords species H, which is in equilibrium with I and
(J + B). The addition of phenol (5a) drives the equilibrium
Scheme 6 Stoichiometric tests to elucidate the reaction mechanism;
establishing the equilibrium in Mixture A and Mixture B.
As shown in Scheme 5, phenol 5a only reacts with copper towards the formation of the final product 6aa, thus
hydroxide 1 to afford copper phenoxide F, whilst diphenylacetyl- closing the catalytic cycle via the concurrent regeneration of
ene 4a only reacts with gold species 2 to afford intermediate G.
Mixture B.
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13 D. B. Grotjahn, C. D. Incarvito and A. L. Rheingold, Angew.
Chem., Int. Ed., 2001, 40, 3884.
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A new dual catalytic process based on a heterobimetallic 14 D. B. Grotjahn and D. A. Lev, J. Am. Chem. Soc., 2004, 126,
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reported. The methodology was shown to be highly efficient at 15 A. Varela-Fernández, C. González-Rodríguez, J. A. Varela,
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ethers. Various alkynes were also tested and a mixture of Z/Z′ 18 I. Kadota, L. M. Lutete, A. Shibuya and Y. Yamamoto,
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Acknowledgements
The authors gratefully acknowledge the Royal Society
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