Commercial Supported Gold Nanoparticles Catalyzed Alkyne Hydroamination and Indole Synthesis

Article abstract of DOI:10.1002/adsc.201600804

Commercial gold nanoparticles supported on titanium dioxide (TiO₂) were found to be a highly efficient catalyst for alkyne hydroamination. Terminal alkynes could easily undergo intermolecular hydroamination with low catalyst loadings (0.2 mol% Au) under solvent-free conditions. Indoles were efficiently synthesized using microwave heating through intramolecular hydroamination. (Figure presented.).

Full text of DOI:10.1002/adsc.201600804

UPDATES
DOI: 10.1002/adsc.201600804
Commercial Supported Gold Nanoparticles Catalyzed Alkyne Hy-
droamination and Indole Synthesis
Shengzong Liang,a Luisa Hammond,b Bo Xu,c,* and Gerald B. Hammonda,*
a
Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA
E-mail: gb.hammond@louisville.edu
Williams College, Williamstown MA, 01267, USA
College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai
b
c
201620, Peopleꢀs Republic of China
E-mail: bo.xu@dhu.edu.cn
Received: July 22, 2016; Revised: August 4, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600804.
Abstract: Commercial gold nanoparticles supported
on titanium dioxide (TiO2) were found to be
a highly efficient catalyst for alkyne hydroamina-
tion. Terminal alkynes could easily undergo inter-
molecular hydroamination with low catalyst load-
ings (0.2 mol% Au) under solvent-free conditions.
Indoles were efficiently synthesized using micro-
wave heating through intramolecular hydroamina-
tion.
ble in such transformations due to their stability and
ease of manipulation.[9]
Gold nanoparticles (Au/NPs) have been successful-
ly applied in many organic reactions such as oxida-
tion,[10] reduction,[11] hydrosilylation,[12] and cross-cou-
pling,[13] but reports of Au/NPs-mediated hydroamina-
tion, through p bond activation, are scarce.[14] It is be-
lieved that the cationic gold species present in gold
nanoparticles is responsible for the activation of the
unsaturated bond (see also the Supporting Informa-
tion, Figure S-2).[15] Herein we are glad to report an
alkyne hydroamination catalyzed by commercially
available supported gold nanoparticles (Au/TiO2).
Through our methodology, a wide range of terminal
alkynes were regioselectively converted to amines
through intermolecular hydroamination, using molar
Keywords: Au/TiO2; hydroamination; indoles; recy-
clable; supported gold nanoparticles
Nitrogen-containing compounds such as amines and fraction amounts of catalyst and no additional sol-
nitrogen heterocycles are widely encountered in the vent; on the other hand, intramolecular hydroamina-
scaffolds of various bioactive natural products, phar- tion furnished indole derivatives in high yields.
maceuticals, and materials.[1] Therefore, many meth-
We used the hydroamination of phenylacetylene 1a
ods such as reductive amination[2] and C–N cross-cou- with aniline 2a as our model reaction (Table 1). When
pling[3] have emerged for their construction.[4] Among Au/TiO2 (0.2 mol% Au) was utilized, the correspond-
these methods, hydroamination represents an atom- ing imine product 3a’ was obtained in 55% yield after
economical strategy because of the direct addition of 711hours at 808C. Extending the reaction time to
amines to readily accessible alkenes, alkynes and al- 24 hours led to only a minor increase in yield
lenes.[5] On the one hand, intermolecular hydroamina- (Table 1, entry 1). Higher temperatures did not in-
tion affords structurally complex amines; on the other crease the yield significantly (Table 1, entry 2). In-
hand, intramolecular hydroamination provides access spired by homogeneous gold catalysis, we added acid
to a wide range of N-containing cyclic compounds. promoters into the reaction. To our delight, all of the
Various transition metal catalysts have proven their acid additives tested were able to facilitate the forma-
value in the hydroamination of alkynes,[6] of which ho- tion of 3a’ with similar efficiency (Table 1, entries 3–
mogeneous cationic gold catalysis has attracted signif- 6). The easy-to-handle solid phosphotungstic acid was
icant attention because of its excellent activity under chosen as additive in our study. An attempt to achieve
mild reaction conditions.[7] However, the non-recycla- higher yield by increasing the temperature (1008C)
bility of homogeneous gold catalysts and the decay of failed (Table 1, entry 7). A lower temperature (608C)
cationic gold greatly limit their application in organic furnished the desired product in 20% yield (Table 1,
synthesis.[8] In this regard, heterogeneous gold cata- entry 8). Finally, when only the acid additive was
lysts, especially gold nanoparticles, are highly desira- used, or the support TiO2 was used as catalyst, no
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Table 1. Screening of Au/TiO2.catalyzed intermolecular hy-
droamination.
imine product was observed, underscoring the crucial
role of gold nanoparticles (Table 1, entries 9 and 10).
With the optimized conditions in hand, the sub-
strate scope of intermolecular hydroamination was
next evaluated (Table 2). Due to the instability of the
resulting imines, they were immediately reduced to
amines after the reaction was completed. Both elec-
tron-donating and electron-withdrawing substituents
on aniline were well tolerated (Table 2, 3a–3f). Ani-
lines with substituents on sterically hindered positions
(o-methyl and phenyl) could also provide the desired
amines in high yields, although more catalyst and
longer reaction times were required (Table 2, 3g and
3h). The functional groups on phenylacetylene did
not affect the efficiency of the hydroamination
(Table 2, 3i–3l). Harsher conditions were needed to
achieve good yields when aliphatic alkynes were ex-
amined (Table 2, 3m–3p). Indole rings are one of the
most ubiquitous and important heterocycles in
nature.[16] A plethora of methodologies exist for
indole ring synthesis,[17] but among them, the cycloiso-
merization of 2-alkynylanilines is the most straightfor-
ward and atom-economical. However, this straightfor-
ward synthesis of indoles has not received much at-
tention using recyclable heterogeneous gold catalysts.
The only reported work by Helaja and co-workers
used home-made gold nanoparticles, which limits its
application.[18] Capitalizing on our success with inter-
molecular hydroamination of alkynes, we envisioned
that commercially available Au/TiO2 would enable
Entry Au/TiO2 [mol% Additive (mol%)
Au]
T
Yield
[8C] [%][a]
1
2
3
4
5
6
0.2
0.2
0.2
0.2
0.2
0.2
–
–
80 55
100 62
HOTf (0.1%)
HNTf2 (0.1%)
HCTf3 (0.1%)
H3PO4·12WO3
(0.1%)
80
80
80
80
88
88
83
86
7
0.2
0.2
–
H3PO4·12WO3
(0.1%)
H3PO4·12WO3
(0.1%)
H3PO4·12WO3
(0.1%)
H3PO4·12WO3
(0.1%)
100 71
8
60
80
80
20
0
9
10[b]
–
0
[a]
Yields were determined by 1H NMR using 1,3,5-trime-
thoxybenzene as internal standard.
TiO2 (50 mol%) was used.
[b]
Table 2. Au/TiO2-catalyzed intermolecular hydroamination of terminal alkynes.[a]
[a]
Reaction conditions: 1 (1 mmol), 2 (1.2 mmol), Au/TiO2 (0.2 mol% Au), H3PO4·12WO3 (0.1 mol%), neat at 808C. Isolated
yields ae shown in parentheses.
Au/TiO2 (0.5 mol% Au), H3PO4·12WO3 (0.5 mol%) were used in 24 h.
[b]
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Table 3. Au/TiO2-catalyzed indole synthesis through intramolecular hydroamination.[a]
[a]
Reaction conditions: 4 (0.1 mmol), Au/TiO2 (5 mol% Au), in toluene (1 mL) at 1208C for 1 h; Isolated yields.
At 1408C.
H3PO4·12WO3 (5 mol%) was used.
[b]
[c]
the intramolecular version. Indeed, when microwave
The hydroamination of basic amines catalyzed by
heating was applied, various 2-alkynylanilines were cationic gold is a challenge. This is because a basic
easily converted to the corresponding indole deriva- amine (e.g., morpholine) has much stronger affinity
tives. As shown in Table 3, different substituted 2-al- towards cationic gold than towards the alkyne sub-
kynylanilines furnished the corresponding indole strate, thus, impairing the cationic gold.[19] This is one
products in very good to excellent yields (Table 3, 5a– reason why most homogeneous gold catalysts cannot
5d). The halogens (F and Cl) and methyl group at the work in the strong basic environment created by basic
4- or 5- positions of aniline proved beneficial, allow- amines.[7c,d] We speculated that the cationic gold spe-
ing indole products to be obtained in excellent yields cies present in Au/NPs might be more tolerant to-
(Table 3, 5e–5i). When R2 was replaced by an aliphat- wards bases and, consequently, they could efficiently
ic group, an acid promoter was required (Table 3, 5j). activate alkynes in the presence of basic amines. To
examine this assumption, the hydroamination of phe-
nylacetylene with morpholine was carried out using
Au/TiO2 as catalyst. To our delight, after reduction,
the desired amine product was obtained in 76% yield.
In contrast, a commonly used L-AuCl/AgOTf or an
otherwise efficient imido gold pre-catalyst completely
lost their activities in the presence of morpholine
(Scheme 1).[20] However, when thiomorpholine and
pyrrolidine were used as basic amine partners, the hy-
droamination reaction did not occur; piperidine gave
Scheme 1. Comparison of homogeneous gold catalyst and
gold nanoparticles in the hydroamination of basic amines.
Scheme 2. Gram-scale intermolecular hydroamination of 1a
and 2a.
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Scheme 3. Leaching experiment in the hydroamination with Au/TiO2.
a low yield (25%) of imine product according to nipulate, and reusable. Au/TiO2 catalyzed the inter-
1H NMR. This result indicated that gold nanoparticles molecular hydroamination of terminal alkynes afford-
might be a better activator for triple bonds in the ing amines under solvent-free condition. Whatꢀs
presence of some specific strong bases such as mor- more, a series of indole derivatives were synthesized
pholine.
The scalability of the present method was also wave heating.
investigated through gram-scale experiment
(Scheme 2). The hydroamination proceeded smoothly,
through intramolecular hydroamination under micro-
a
giving the corresponding imine product 3a’ in 89% Experimental Section
yield.
A
leaching experiment was also carried out General Procedure for Hydroamination of Terminal
(Scheme 3). The hydroamination of 1a and 2a provid- Alkynes 1 and Anilines 2
ed the desired imine 3a’ in 43% yield after 4 hours
Au/TiO2 (39.4 mg, 0.2 mol% Au) and H3PO4·12WO3
under the optimized conditions; then a portion of the
supernatant (150 mL) was transferred to another vial.
Both the reaction mixture with Au/TiO2 and the su-
pernatant without Au/TiO2 were allowed to react for
another 2 hours. We found that the mixture contain-
ing Au/TiO2 afforded 3a’ in 85% yield; In contrast,
the supernatant without Au/TiO2 did not produce ad-
ditional 3a’. These results demonstrate that the hydro-
amination is catalyzed by gold species present in gold
nanoparticles rather than species leached into solu-
tion.
The recyclability of gold nanoparticles was also in-
vestigated through five straight runs of cycloisomeri-
zation (Scheme 4). After each run Au/TiO2 was fil-
tered out and washed with fresh toluene. We found
that gold nanoparticles were efficiently reused with-
out loss of catalytic activity.
(2.88 mg, 0.1 mol%) were placed in a dry and clean vial
which was then vacuumed and purged with argon three
times. Then a mixture of alkyne 1 (1 mmol) and aniline 2
(1.2 mmol) was added into the vial. The reaction mixture
was stirred in an oil bath at 808C for the designated time
and cooled down to room temperature. Then Au/TiO2 was
filtered, the residual was dissolved in DCM (6 mL). NaB-
H(OAc)3 (424 mg, 2 equiv.) and AcOH (114 mL, 2 equiv.)
were then added into the solution. The mixture was allowed
to stir at room temperature for 24 hours. The reaction was
quenched with 1M NaOH aqueous solution, extracted with
DCM (5 mLꢂ2), the combined organic phase was dried
over Na2SO4, and after filtration of Na2SO4 the filtrate was
concentrated to dryness and the residue was subjected to
flash chromatography (silica gel; EA/hexane).
General Procedure for Indole 5 Synthesis through
Cycloisomerization of 2-Alkynylanilines 4
In conclusion, we have developed a highly efficient
hydroamination protocol using commercially avail-
able gold nanoparticles (Au/TiO2). Compared with
commonly used homogeneous gold catalysts, this het-
erogeneous gold catalyst is cost-effective, easy-to-ma-
2-Alkynylaniline 4 (0.1 mmol) was placed in a clean and dry
microwave vial and dissolved in toluene (1 mL). Au/TiO2
(98.5 mg, 5 mol% Au) was then added into the solution. The
mixture was reacted in the microwave reactor for 1 hour at
1208C and cooled down to room temperature. Then Au/
TiO2 was filtered, the filtrate was concentrated to dryness
and subjected to flash chromatography (silica gel; EA/
hexane).
Acknowledgements
Scheme 4. Recyclability test of Au/TiO2 in indole 5a synthe-
sis.
We are grateful to the National Science Foundation (CHE-
1401700) for financial support. BX is grateful to the National
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Science Foundation of China (NSFC-21472018) for financial
support.
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Commercial Supported Gold Nanoparticles Catalyzed
Alkyne Hydroamination and Indole Synthesis
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Adv. Synth. Catal. 2016, 358, 1 – 7
Shengzong Liang, Luisa Hammond, Bo Xu,*
Gerald B. Hammond*
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