Alkyne hydroarylation with Au N-heterocyclic carbene catalysts

Article abstract of DOI:10.3762/bjoc.9.29

Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC) ligands have been employed as catalysts in the intermolecular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III) complexes were able to catalyze the reaction; however, the best results were obtained with the mononuclear gold(I) complex IPrAuCl. This complex, activated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic activity and selectivity compared to more commonly employed palladium(II) catalysts. Moreover, the complex was active, albeit to a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously published complex AuCl(PPh₃). Preliminary results on intramolecular hydroarylations using this catalytic system indicate, however, that alkyne hydration by traces of water may become a serious competing reaction.

Full text of DOI:10.3762/bjoc.9.29

Alkyne hydroarylation with
Au N-heterocyclic carbene catalysts
Cristina Tubaro, Marco Baron, Andrea Biffis* and Marino Basato
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 246–253.
Dipartimento di Scienze Chimiche, Università degli Studi di Padova,
via Marzolo 1, 35131 Padova, Italy
doi:10.3762/bjoc.9.29
Received: 01 November 2012
Accepted: 14 January 2013
Published: 05 February 2013
Email:
Andrea Biffis* - andrea.biffis@unipd.it
* Corresponding author
This article is part of the Thematic Series "C–H Functionalization".
Guest Editor: H. M. L. Davies
Keywords:
alkyne; C–H functionalization; cyclisation; gold; hydroarylation;
N-heterocyclic carbenes
© 2013 Tubaro et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC) ligands have been employed as catalysts in the intermole-
cular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III) complexes were able to
catalyze the reaction; however, the best results were obtained with the mononuclear gold(I) complex IPrAuCl. This complex, acti-
vated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic
activity and selectivity compared to more commonly employed palladium(II) catalysts. Moreover, the complex was active, albeit to
a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously
published complex AuCl(PPh3). Preliminary results on intramolecular hydroarylations using this catalytic system indicate,
however, that alkyne hydration by traces of water may become a serious competing reaction.
Introduction
The hydroarylation of alkynes (Scheme 1) is arguably one of an arene adds formally trans to the triple bond of an alkyne,
the most intensively studied reactions leading to aromatic C–H generally forming the thermodynamically less favoured cis-
bond functionalization [1-7]. In this reaction, the C–H bond of arylalkene as the major product.
The study of this reaction was pioneered by the group of Fuji-
wara (hence the alternative name “Fujiwara reaction” for the
intermolecular hydroarylation of alkynes) using mainly palla-
dium(II) salts as catalyst [8-10]. Palladium complexes with
N-heterocyclic carbene (NHC) ligands have since been show-
Scheme 1: Hydroarylation of alkynes.
cased as highly efficient catalysts for this reaction [11-14].
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Beilstein J. Org. Chem. 2013, 9, 246–253.
Alternative catalytic systems based on salts or complexes of concerning the nature of the employed catalysts, simple gold
other noble metals, such as platinum [15-17], gold [18,19], or salts or phosphino complexes of gold(I) were utilized in the
rhodium [20], as well as of non-noble, electrophilic metals [21- majority of cases, although in recent years an increasing number
26] have also been successfully employed. Finally, an even of studies have been dealing with the application of NHC com-
greater number of catalysts have been proposed over the years plexes of gold for these and related reactions [48-53].
to promote alkyne hydroarylation in an intramolecular fashion
[1-7]. In such reactions, often simply termed cyclisation reac- We have an ongoing interest in the development of novel cata-
tions, the arene and the alkyne are linked through a tether, the lysts for the hydroarylation of alkynes and have extensively
nature of which can range from simple alkyl groups to ether, investigated the ability of palladium(II) complexes with
amino, ester or amido groups; depending on the nature and chelating N-heterocyclic dicarbene ligands to promote this reac-
length of the tether, different kinds of unsaturated poly(het- tion [12-14]. Recently, we have extended our interest in the
ero)cyclic compounds can be conveniently synthesized.
organometallic chemistry of such ligands to group 11 metals, in
particular gold(I) and gold(III) centers [54-56]. In the present
Recently, the unique ability of gold centres to activate C–C contribution we would like to assess the catalytic efficiency of
triple bonds towards nucleophilic attack has clearly emerged in such gold complexes with NHC ligands for the hydroarylation
the literature [27-34]. In the light of the above, it is surprising of alkynes.
that the number of studies on the use of gold species as cata-
lysts for alkyne hydroarylation is still quite limited. A substan- Results and Discussion
tial number of reports on the intramolecular cyclisation of We recently reported on the synthesis of dinuclear gold(I) com-
arenes with tethered alkyne moieties using gold(I) or, to a lesser plexes with bridging dicarbene ligands [55], as well as on the
extent, gold(III) catalysts can be found in the literature [35-46]; preparation of the corresponding dinuclear gold(III) analogues,
however, only one additional example, beyond the two early which are obtained from the former upon oxidation with
reports by Reetz and Sommer [18] and by Shi and He [19], of bromine [54]. Complexes I–V were now tested as catalysts in
gold-catalysed intermolecular hydroarylation has been standard intermolecular hydroarylation reactions, together with
described, albeit concerning 2-substituted oxazoles as the reac- two mononuclear gold complexes previously reported in the
tion partner [47]. Investigations on the intramolecular variant literature, namely complexes VI (also termed IPrAuCl) [57] and
have focused mainly on the nature of the aromatic moiety that VII (also termed IPrAuBr3) [58] (Figure 1).
adds to the alkyne, and on the nature and/or length of the tether,
whereas concerning the alkyne moiety, only terminal, electron- The standard reaction between pentamethylbenzene and ethyl
rich alkyne groups (propargylic moieties in most instances) propiolate (Scheme 2) was initially taken as the benchmark for
were employed, with very few exceptions [36,40,44]. Finally, catalyst evaluation. Initial attempts were performed at room
Figure 1: Gold(I) and gold(III) NHC complexes employed as catalysts in this study.
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Beilstein J. Org. Chem. 2013, 9, 246–253.
Scheme 2: Hydroarylation of ethyl propiolate with pentamethylbenzene.
temperature (25 °C) with very low levels of complex II as cata- As expected, the dinuclear dicarbene gold(I) complex I was
lyst (0.005 mol %) by using trifluoroacetic acid (HTFA) or found to be inactive for the reaction, as the NHC ligands satu-
HBF4 as acidic medium plus 0.02 mol % AgTFA or AgBF4, rate the coordination sphere of the gold(I) centres. On the other
respectively, as co-catalyst to remove bromides from the coor- hand, the dinuclear dicarbene gold(III) complexes II–V, for
dination sphere of the gold centres, thereby liberating coordina- which 4 equiv of AgBF4 with respect to the complex were
tion sites at the metal and boosting its electrophilicity.
added, turned out to be active. All dinuclear complexes exhib-
ited very similar initial activity in the first few hours of reaction.
The obtained results established that the complex was inactive This observation indicates that the reactivity of the complexes is
when HTFA was employed as the acidic medium, whereas 20% not hampered by steric effects, as complexes with ligands of
yield of the desired product 3{1,1} was obtained after 18 h with widely different steric bulk, such as II and V, exhibit similar
HBF4. Consequently, a screening of the catalytic efficiency of performance. On the other hand, the complexes deactivate with
the various complexes was carried out with the latter acidic time at different rates, depending on the nature of the employed
medium; the amount of catalyst was increased tenfold (to dicarbene ligand. Complex IV turned out to be the catalyst most
0.1 mol % Au) in order to achieve faster reaction rates, whereas resistant to deactivation.
the amount of employed AgBF4 co-catalyst was always stoi-
chiometrically equivalent to the amount of bromide in the When catalysts VI, VII and KAuBr4 were employed together
employed complex. It should be mentioned that under these with the corresponding amount of AgBF4 co-catalyst, higher
reaction conditions neither AgBF4 nor HBF4 promote the reac- initial activities compared to the dinuclear dicarbene gold(III)
tion, as previously demonstrated by us in investigations on catalysts were recorded. However, whereas KAuBr4 was very
related palladium(II) catalysts for the same reaction [13]. The quickly and completely deactivated, catalysts VI and VII
conversion curves obtained with the various catalysts are retained their activity, highlighting the importance of the NHC
reported in Figure 2.
ligand in stabilizing the catalytically active species. Catalyst VI
(IPrAuCl) was particularly efficient and able to effect over 90%
yield in just 1 h with complete selectivity for the hydroaryla-
tion product. Remarkably, compared to the palladium(II) com-
plexes with chelating N-heterocyclic dicarbene ligands previ-
ously investigated by us as catalysts for the same reaction under
identical reaction conditions [13], complex VI exhibits higher
catalytic activity and complete selectivity for the insertion of
only one alkyne molecule into the aromatic C–H bond, whereas
the palladium(II) complexes predominantly yielded the product
deriving from the insertion of two alkyne molecules. The
catalytic efficiency of complexes VI and VII was evaluated
with other arene and alkyne substrates under the same reaction
conditions and the results are reported in Table 1.
Figure 2: Yield in 3{1,1} versus time diagram for the reaction of
pentamethylbenzene and ethyl propiolate catalysed by complexes
II–VII and KAuBr4 at room temperature in HBF4 and with added
AgBF4: complex II (black squares); complex III (circles); complex IV
(squares); complex V (black circles); complex VI (black triangles);
complex VII (triangles); KAuBr4 (asterisks). Reaction conditions:
1 equiv arene, 1 equiv alkyne, 1 equiv tetrafluoroboric acid,
0.1 mol % Au, 0.1–0.4 mol % AgBF4, 1,2-dichloroethane, 25 °C.
The catalytic activity of the complexes remained high also with
less substituted substrates, complex VI being systematically
superior to complex VII. The selectivity of the reaction was,
however, hampered by the formation of significant amounts of
products deriving from the addition of two molecules of alkyne
to the arene (product type 4), which was invariably recorded
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Beilstein J. Org. Chem. 2013, 9, 246–253.
when more than one C–H group was available for reaction. from insertion of more than one alkyne molecule into the same
Other by-products that were observed on using Pd catalysis, arene C–H bond, were however never detected with Au cata-
such as, e.g., products of double-bond isomerisation or deriving lysts. Only in the case of p-xylene was the reaction again fully
Table 1: Hydroarylation of alkynes using gold NHC catalysts: screening of different arenes and alkynes.a
Catalyst
Arene
Alkyne
Time (h)
Arene conversion, %
(alkyne conversion)
Yield (%)b
VI
2{1}
1
3
90 (90)
>99 (>99)
3{1,1}
3{2,1}
90
>99
1{1}
VI
2{1}
1
5
68 (94)
72 (100)
42
44
4{2,1}
26
28
1{2}
1{3}
VI
VI
2{1}
2{1}
1
5
72 (95)
74 (98)
3{3,1}
3{4,1}
49
50
4{3,1}
4{4,1}
23
24
5
35 (38)
32
3
1{4}
1{5}
1{1}
1{2}
VI
2{1}
2{1}
2{1}
1
5
20 (20)
45 (45)
3{5,1}
3{1,1}
3{2,1}
20
45
4{5,1}
0
0
VII
VII
1
5
43 (43)
91 (91)
43
91
1
5
18 (19)
58 (72)
17
45
4{2,1}
1
13
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Beilstein J. Org. Chem. 2013, 9, 246–253.
Table 1: Hydroarylation of alkynes using gold NHC catalysts: screening of different arenes and alkynes.a (continued)
VII
2{1}
1
5
66 (86)
71 (93)
3{3,1}
46
50
4{3,1}
20
22
1{3}
1{1}
1{1}
1{1}
1{1}
VI
2{2}
1
5
23 (23)
58 (58)
3{1,2}
23
58
VI
VI
VI
2{3}
2{1}
2{1}
5
0
3{1,3}
3{1,1}
3{1,1}
0
51
15
5
51 (51)c
15 (15)d
48
aReaction conditions: 1 equiv arene, 1 equiv alkyne, 1 equiv tetrafluoroboric acid, 0.1 mol % Au, 0.1 or 0.3 mol % AgBF4, 1,2-dichloroethane, 25 °C.
bThe yield was determined by 1H NMR spectroscopy. cReaction performed with trifluoroacetic acid. dReaction performed without acid.
selective, albeit sluggish. Variations of the alkyne substrate a closer comparison between the catalytic efficiency of VI and
made it apparent that electron-rich alkynes, such as 1-hexyne, that of the previously employed gold(I) catalysts, such as
are not viable substrates for this reaction, and that electron- AuCl(PPh3), we subjected complex VI to the same catalytic test
poor, internal alkynes react only scarcely under these condi- performed by Reetz and Sommer with the phosphino complex
tions.
(Scheme 3) [18]. The catalytic test performed with catalyst VI
resulted in the exclusive formation of the hydroarylation pro-
The high catalytic activity of complex VI prompted us to eval- duct in 45% yield. Catalyst AuCl(PPh3) was reported instead to
uate its efficiency also under less acidic conditions. Hydroaryla- produce, under the same reaction conditions, the hydroarylation
tion of ethyl propiolate with pentamethylbenzene run with 0.1% product in 56% yield, together with 28% yield of the product
VI and 0.1% AgTFA in HTFA yielded 51% pure monohy- deriving from insertion of two alkyne molecules into two C–H
droarylated product after 5 h. On the other hand, the reaction bonds of mesitylene [18]. Thus, it can be stated that catalyst VI
run with 0.1% VI and 0.1% AgBF4 under neutral conditions is apparently less active but more selective under these neutral
yielded only 15% product after 48 h. Thus, the nature and reaction conditions.
amount of acid have a very strong influence on catalytic effi-
ciency, as in the case of catalysis by palladium(II) NHC com- Finally, we preliminarily investigated the capability of com-
plexes [13]; in contrast to Pd, though the catalyst remains plexes VI and VII to act as catalysts for intramolecular alkyne
slightly active even under neutral conditions. This result was hydroarylation reactions. As mentioned in the Introduction,
expected, as in early examples of the use of gold catalysts for gold salts and complexes have been extensively employed for
intermolecular alkyne hydroarylations the reaction was invari- intramolecular cyclisations of this kind [35-46], but in most
ably done without acid addition, although much more forceful instances only terminal pendant alkynes have been employed as
conditions (larger amount of catalyst, higher temperature, reacting groups. Furthermore, to the best of our knowledge
longer reaction times) were applied [18,19,47]. In order to have substrates with amido tethers between the aryl and the alkyne,
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Beilstein J. Org. Chem. 2013, 9, 246–253.
Scheme 3: Hydroarylation experiment with catalyst VI under neutral conditions.
such as substrates 5 (Scheme 4), have never been reported to
undergo cyclisation with gold catalysts, whereas there are
examples of the use of palladium catalysts with these substrates
leading to different products in dependence on the reaction
conditions: reaction under neutral conditions produces the
5-exo-dig cyclisation product 6 [59], whereas in the presence of
an acid the 6-endo-dig product 7 is formed [60]. Thus, we set
out to evaluate the reactivity of substrates 5 with catalysts VI
and VII.
Table 2: Intramolecular alkyne hydroarylation under acidic conditions.a
Substrate Catalyst Conversion (%)b
Yield (%)b
5{1}
5{1}
5{2}
5{2}
VI
VII
VI
89
31
54
0
7{1} 12 8{1} 78
7{1} 10 8{1}
-
7{2}
7{2}
7
0
8{2} 47
8{2}
VII
0
aReaction conditions: 1 equiv substrate, 20 equiv trifluoroacetic acid,
1 mol % Au, 1 mol % AgBF4, 1,2-dichloroethane, room temperature,
24 h. bThe conversion and the yields were determined by 1H NMR
spectroscopy.
Under neutral conditions the reaction gave no yield in the
desired cyclised product and the substrate was recovered
unchanged in all cases. Therefore, we moved to investigate the
reaction in the presence of trifluoroacetic acid, using a reaction
protocol previously employed by Fujiwara for running analo-
gous reactions with palladium(II) catalysts [60]. The results are
reported in Table 2.
can be expected that alkyne hydration by traces of water may
become a serious competitive reaction despite the low concen-
tration of water in the reaction mixture. On the basis of the
above, it can be hypothesised that in order to steer the reaction
towards the hydroarylation product, more activating, hence
electron-donating substituents should be installed on the aryl
ring. Experiments towards this goal are currently underway.
Catalyst VII was completely inactive even under acidic condi-
tions with substrate 5{2}, whereas with substrate 5{1} it reacted
sluggishly forming complex product mixtures containing also Conclusion
the 6-endo-dig cyclisation product 7{2}. On the other hand, with In conclusion, we have demonstrated that gold complexes with
complex VI moderate to very good conversions of the N-heterocyclic carbenes are active catalysts for alkyne hydro-
substrates were obtained, but the main reaction product was arylations under acidic conditions. Mononuclear complexes
invariably the product of hydration of the triple bond 8, whereas appear more active than dinuclear ones, and gold(I) complexes
the 6-endo-dig cyclisation product 7 was present in minor are more active and selective than analogous gold(III) com-
amounts. Gold(I) NHC complexes such as VI are known to be plexes. Under neutral reaction conditions, mononuclear gold(I)
extremely efficient catalysts for alkyne hydration [61], hence it NHC complexes appear less active but more selective than the
Scheme 4: Intramolecular cyclisation through hydroarylation investigated in this work.
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Beilstein J. Org. Chem. 2013, 9, 246–253.
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