A cationic gold(I) complex as a general catalyst for the intermolecular hydroamination of alkynes: Application to the one-pot synthesis of allenes from two alkynes and a sacrificial amine

Article abstract of DOI:10.1002/chem.200802626

A cationic gold (I) complex was reported to catalyze the addition of various types of non-tertiary amines to terminal and internal alkynes. The one-pot preparation of allenes by coupling two alkynes using a sacrificial secondary amine was also investigate

Full text of DOI:10.1002/chem.200802626

DOI: 10.1002/chem.200802626
A Cationic Gold(I) Complex as a General Catalyst for the Intermolecular
Hydroamination of Alkynes: Application to the One-Pot Synthesis of Allenes
from Two Alkynes and a Sacrificial Amine
Xiaoming Zeng, Guido D. Frey, Shazia Kousar, and Guy Bertrand*[a]
Carbon–carbon and carbon–nitrogen bond formations are
at the heart of modern organic chemistry. The development
of novel coupling strategies that facilitate the rapid and effi-
cient construction of complex molecules remains a preemi-
nent goal in synthetic chemistry. For a long time, allenes
have been considered as chemical curiosities.[1] Nowadays,
these cumulenes are not only used as building blocks for the
synthesis of complex molecules,[2] but because of the intrigu-
ing biological properties of many allenic natural products,
the C=C=C skeleton is often introduced in pharmacological-
ly active classes of compounds.[3] Consequently, despite a
number of synthetic routes known, new efficient preparative
methods are highly desirable. Of special interest are catalyt-
ic processes, which directly build the three-carbon allene
core by coupling two different fragments, thus allowing for
convergent synthesis. The first of such processes was discov-
ered in 1979, namely the Crabbꢀ homologation[4]
Scheme 1. a) Crabbꢀ homologation; b) Allene metathesis; c) Gold(I) cat-
alyzed cross-coupling of enamines and terminal alkynes; d) Synthetic
strategy; e) Synthesis of cationic gold(I) complexes A1.
(Scheme 1a). This is a CuBr mediated three-component re-
action between a terminal alkyne, formaldehyde, and diiso-
propylamine. The most important drawback of this process
is that ketones and even aldehydes cannot be used in place
of formaldehyde, and thus only mono-substituted allenes
can be produced. In 2000, Barrett reported a few examples
of allene cross metathesis,[5] which allow one of the terminal
carbon units of a preformed allene to be exchanged to yield
a new symmetrically substituted 1,2-diene, although exten-
sive polymerization side reactions occur (Scheme 1b). More
recently, we have shown that cationic AuI complex A1 effi-
ciently mediates the catalytic coupling of enamines and ter-
minal alkynes to yield allenes[6] (and not propargyl amines
as observed with other catalysts),[7] along with the corre-
sponding imines (Scheme 1c). For this process all types of
terminal alkynes can be used, as well as mono-, di-, and tri-
substituted enamines; however the latter have to be tertiary
derivatives, and one of the N-substituents an alkyl group
with a hydrogen atom next to nitrogen, in order to deliver a
hydrogen. The tertiary enamines are formally hydroamina-
tion adducts of alkynes with secondary amines.[8] Therefore,
we reasoned that if the gold complex A1 were also able to
catalyze the addition of secondary amines to alkynes,[9] it
would be possible in one-pot reactions to form allenes from
two alkynes and a secondary amine; formally, the only re-
striction would be the use of a terminal alkyne for the
second step (Scheme 1d).
[a] X. Zeng, Dr. G. D. Frey, S. Kousar, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry
University of California
Riverside, CA 92521-0403 (USA)
Fax : (+1)951-827-2725
Despite tremendous progress in identifying efficient cata-
lysts for the intermolecular addition of amine derivatives to
alkynes, real difficulties remain with secondary amines,[8–12]
partly because early transition metals cannot be used, due
E-mail: guy.bertrand@ucr.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802626.
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COMMUNICATION
Table 1. Hydroamination of alkynes a–f with primary amines 1 and 2.[a]
to the involvement of an imido complex (LxM=NR) in the
catalytic cycle.[13] Uchimaru[14] reported that a ruthenium
cluster was efficient but the scope of the reaction was strict-
ly limited to arylacetylenes and N-methyl aniline; moreover
a 10-fold excess of amine was necessary. Schmidt[15] de-
scribed the addition of morpholine (62% yield) and piperi-
dine (38% yield) to phenylacetylene in the presence of pal-
ladium(II) 3-iminophosphine complexes; in this case, a 10-
fold excess of alkyne was necessary due to the competing
cyclotrimerization of phenylacetylene. Lastly, there is only
one report concerning the hydroamination of internal al-
kynes with secondary amines. In 2005, using 10 mol% of the
Entry Amine Alkyne T [8C] t [h] Product
Yield[b]
81
1
2
1
1
a
40
90
16
12
b
86
aquapalladium complex [Pd
to[16] reported the addition of N-methylaniline to diphenyl-
acetylene and phenyl(butyl)acetylene in good yields. Clearly
ACHTUNGTRENU(GNN dppe)ACHTUNREGTG(NNUN H2O)2]ACHTNGUTRENN(UGN TfO)2, Yamamo-
3
4
1
1
c
120
140
16
12
93
94
A
ACHTUNGTRENNUNG
general catalytic systems able to promote the intermolecular
hydroamination of alkynes, especially with basic secondary
amines are still missing.
d
Recently, we have shown that the cationic gold(I) com-
5
6
7
1
1
2
e
f
100
90
10
24
12
94
82
81
plex A1 (Scheme 1),[17,18] featuring a bulky cyclic (alkyl)-
ACHTUNGTRENNUNG
(amino)carbene (CAAC),[19] as ancillary ligand, efficiently
promotes the addition of ammonia to non-activated
alkynes.[20] These first examples of homogeneous catalytic
d
140
hydroamination with ammonia hinted at the possibility of a
general catalyst for the hydroamination of alkynes with sec-
ondary amines.
[a] A1 (5 mol%), amine (0.5 mmol), alkyne (0.5 mmol), C6D6 (0.4 mL).
[b] Yields are determined by H NMR using benzylmethyl ether as an in-
ternal standard.
1
Here we report that complex A1 catalyzes the addition of
many types of non-tertiary amines to terminal as well as in-
ternal alkynes, and the first examples of intermolecular hy-
droamination of internal alkynes with secondary alkyl
amines. Moreover, we show that addition of a terminal
alkyne to the in situ formed enamines, allows for the synthe-
sis of a variety of allenes without any additional catalyst.
Since several catalytic systems are known to promote the
hydroamination of alkynes with primary amines, we briefly
investigated if complex A1, prepared by mixing the corre-
sponding [(CAAC)AuCl] A complex with one equivalent of
Table 2 shows that diarylamine 3, arylalkylamine 4, benzocy-
clic amine 5, and even dialkylamine 6 add to terminal al-
kynes a, as well as internal alkynes b–d at temperatures be-
tween 60 to 1208C, and reaction times between 7 to 24 h,
using 5 mol% of A1. The only noticeable difficulties have
been found with phenylacetylene a (except entry 1), because
of competitive oligomerization processes. With phenylacety-
lene a (entries 1, 3, 7, 11) and diphenylacetylene b (entries 4,
8, 12) the Markovnikov adduct and the E isomer, respective-
ly, were exclusively formed. With methylphenylacetylene c,
in most cases, the expected mixture of Markovnikov and
anti-Markovnikov products were obtained (entries 2, 5, 9,
13) but more surprising are the results observed with diethyl-
acetylene d (entries 6, 10, 14). Indeed, we observed that the
expected hydroamination adduct was only the minor prod-
uct (21–43%) of the reaction, the major product (79–57%)
being an isomer in which the unsaturation has been shifted.
So far, we have no explanation for this isomerization. How-
ever, as can be seen below, this is not a hurdle for the next
step of the cascade reaction leading to allenes.
KBACHTUNGTRENNUNG(C6F5)4 (Scheme 1e) was efficient as well. We chose the
bulky mesitylamine 1, as an example of arylamine, and six
different alkynes a–f, which are representative of all types of
simple acetylene derivatives (Table 1). In all cases
(entries 1–6), clean reactions occurred (81–94% yield) at
temperatures between 40 and 1408C, and a reaction time
between 10 to 24 h, using 5 mol% of A1. Not surprisingly, a
mixture of Markovnikov and anti-Markovnikov products
were obtained when methylphenylacetylene c and tert-butyl-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
obtained (entry 6). Since the reaction with 3-hexyne re-
quired the most drastic conditions, this substrate was used to
test a primary alkylamine, and we chose the bulky tert-butyl-
amine (entry 7). We were pleased to observe that after 12 h
at 1408C, the hydroamination adduct was obtained in 81%
yield (as a 55/45 mixture of E and Z isomers), indicating the
broad applicability of our catalytic system.
Then, having demonstrated that complex A1 allowed for
the synthesis of tertiary enamines, with alkyl substituents at
nitrogen, we investigated the one-pot preparation of allenes
by coupling two alkynes, using a sacrificial secondary amine
(Scheme 1d). Our synthetic strategy was first checked by
studying the homocoupling of tert-butylacetylene e. A deu-
terated benzene solution of alkyne with 0.5 equivalent of
various amines was heated at 1208C for 16 h, in the pres-
ence of 5 mol% of A1. As shown in Scheme 2, (methyl)-
These quite promising results prompted us to investigate
the hydroamination of alkynes with secondary amines.
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G. Bertrand et al.
Table 2. Hydroamination of alkynes a-d with secondary amines 3–6.[a]
Entry Amine Alkyne T [8C] t [h] Product
Yield[b]
1
2
3
4
3
3
4
4
a
c
80
7
95
48
70
98
120
70
16
8
a
b
120
16
Scheme 2. Influence of the amine on the homo-coupling of tert-butyl-
acetylene.
5
4
c
120
16
98
6
7
8
4
5
5
d
a
b
120
60
16
12
24
86
46
96
90
9
5
5
c
120
110
16
20
67
85
10
d
11
12
6
6
a
100
90
12
20
23
98
Scheme 3. Gold-catalyzed homo-coupling of various alkynes in the pres-
ence of 1,2,3,4-tetrahydroisoquinoline 5.
b
Table 3. Gold-catalyzed hetero-coupling of various alkynes in the pres-
13
14
6
6
c
120
110
20
16
89
94
ence of 1,2,3,4-tetrahydroisoquinoline
5
or dibenzylamine 7.
d
[a] A1 (5 mol%), amine (0.5 mmol), alkyne (0.5 mmol), C6D6 (0.4 mL).
1
[b] Yields are determined by H NMR using benzylmethyl ether as an in-
ternal standard.
Entry
Amine
R1
R2
R4
10 [%]
11 [%]
Yield[b]
1
2
3
4
5
6
7
7
5
5
5
7
7
7
Et
Et
Et
Et
Ph
Ph
Ph
Et
Et
Et
Et
Me
Me
Me
tBu
100
100
100
100
57
0
0
0
0
43
35
40
83
95
82
73
61
64
59
ACHTUNGTRENNUNG(benzyl)amine, dibenzylamine 7 and 1,2,3,4-tetrahydroiso-
quinoline 5 were the most efficient amines, the expected
allene 8a being formed in 59, 61 and even 89% yield,
nBu
cHex
Ph
tBu
nBu
cHex
ACHTUNGTRENNUNG
65
60
the amine to the tert-butylacetylene e; a similar regioselec-
tivity was observed with trimethylsilylacetylene, as expected
because of the steric hindrance of both reactants. With n-
butylacetylene, the allene 9, resulting from the Markovnikov
addition of the amine, was formed in 68% yield, whereas
with cyclohexylacetylene a 14/86 mixture of the two isomer-
ic allenes 8 and 9 was obtained in 79% yield (Scheme 3).
To expand the scope of our catalytic process, we investi-
gated the cross-coupling reaction of alkynes (Table 3). Ben-
zene solutions of an internal alkyne and 0.9 equivalent of
1,2,3,4-tetrahydroisoquinoline 5 or dibenzylamine 7 were
first heated at 1208C, in the presence of 5 mol% of A1. The
reactions were monitored by NMR spectroscopy, and after
complete conversion of the amine, 0.9 equivalent of a termi-
nal alkyne was added to the reaction mixtures. After heating
for 16 h at 1308C, we were pleased to find that the expected
allenes were formed in good to excellent yields. Indeed, the
cross-coupling of 3-hexyne and a set of four different termi-
nal alkynes afforded the corresponding allenes 10 in 73 to
1
[a] Yields are determined, based on the amine, by H NMR using benzyl-
methyl ether as an internal standard.
95% yield (Table 3, entries 1–4). Note that although two iso-
meric hydroamination products are formed (see, Table 2, en-
tries 6, 10 and 14), both of them lead, as expected,[21] to only
one allene. When the unsymmetrical methylphenylacetylene
was used, two isomeric allenes 10 and 11, arising from the
Markovnikov and anti-Markovnikov hydroamination prod-
ucts, were obtained in reasonable yields (Table 3, entries 5–
7).
These results, in addition to those previously reported on
the hydroamination with ammonia,[20] demonstrate that
ACTHNUTRGNEUNG
[(CAAC)Au]+ [B(C6F5)]À complex A1 is a very general pre-
catalyst for the addition of amines to alkynes. It is of note
that Tanaka[22] reported that a cationic gold(I) complex, sim-
ilar to A1, but bearing triphenylphosphine as an ancillary
ligand (prepared from [(Ph3P)AuCH3] and an acidic promot-
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Gold Catalyst for the Hydroamination of Alkynes
COMMUNICATION
[2] K. M. Brummond, J. E. DeForrest, Synthesis 2007, 795–818.
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amines, but the protocol did not tolerate alkylamines and
secondary amines.[23] The comparison of Tanaka’s results
with our findings, clearly demonstrates the specific proper-
ties of the CAAC ancillary ligand.
The coupling of two alkynes to form allenes, using an
amine as a two hydrogen donor, is one of the very few
known processes[24] that combines two very distinct chemical
reactions, yet relies on a single catalyst. Since gold com-
plexes[25] feature little oxophilic character, and thus display
excellent functional group tolerance and low air and mois-
ture sensitivity, this one-pot catalytic protocol should find
applications in the synthesis of complex molecules.
[7] See for examples: a) C. Wei, C. Li, J. Am. Chem. Soc. 2003, 125,
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Experimental Section
All manipulations were performed under an inert atmosphere of argon
by using standard Schlenk techniques. Water- and oxygen-free solvents
were employed.
General procedure for the hydroamination reactions: In a dried J-Young-
Tube, [(CAAC)AuCl] complex A (15 mg, 0.025 mmol) and KBACHTUNRGTNEUNG(C6F5)4
(16 mg, 0.025 mmol) were loaded under an argon atmosphere. C6D6
(0.4 mL) and the internal standard, benzyl methyl ether, were added and
after shaking the tube, the alkyne (0.5 mmol) and amine (0.5 mmol) were
loaded. The tube was sealed, placed in an oil bath behind a blast shield,
and heated at the specified temperature. The reaction was monitored by
NMR spectroscopy. The products were purified by removal of the solvent
and extraction with n-hexane.
[9] For recent reviews on gold-catalyzed hydroamination, see: a) R. A.
Widenhoefer, X. Han, Eur. J. Org. Chem. 2006, 4555–4563; b) I.
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[10] Lanthanide complexes have been shown to promote the addition of
À
secondary amines to terminal alkynes, as a part of tandem C N and
À
C C bond forming processes. Y. Li, T. J. Marks, J. Am. Chem. Soc.
General procedure for the catalytic homocoupling of alkynes: Same ex-
perimental conditions as above, but 1.0 mmol of terminal alkyne, and
heating at 1208C for 16 h. The products were purified by column chroma-
tography.
1998, 120, 1757–1771.
[11] It has been reported that, in the presence of cationic ruthenium hy-
dride, benzocyclic amines can be used in a hydroamination/CH
bond activation process. C. S. Yi, S. Y. Yun, I. A. Guzei, J. Am.
Chem. Soc. 2005, 127, 5782–5783.
General procedure for the catalytic cross-coupling of alkynes: In a dried
J-Young-Tube, [(CAAC)AuCl] complex A (15 mg, 0.025 mmol) and KB-
[12] Several examples of hydroamination of terminal alkynes with secon-
dary (aryl)alkyl>amines using mercury and thallium compounds
have been reported, but the high toxicity of the catalyst is a major
drawback; a) J. Barluenga, F. Aznar, R. Liz, R. Rodes, J. Chem. Soc.
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Odom, J. Am. Chem. Soc. 2004, 126, 1794–1803.
ACHTUNGTRENNUNG(C6F5)4 (16 mg, 0.025 mmol) were loaded under an argon atmosphere.
C6D6 (0.4 mL) and the internal standard benzyl methyl ether were added
and after shaking the tube, internal alkyne (0.55 mmol) and amine
(0.5 mmol) were loaded. The tube was sealed, placed in an oil bath
behind a blast shield, and heated at 1208C, and the reaction was moni-
tored by NMR spectroscopy. After complete conversion of the reactants,
a terminal alkyne (0.5 mmol) was added, and the reaction mixture heated
at 1308C for 16 h. The products were purified by column ACTHNUGRTNEUNGchromato-
graphy.
[14] Y. Uchimaru, Chem. Commun. 1999, 1133–1134.
[15] A. R. Shaffer, J. A. R. Schmidt, Organometallics 2008, 27, 1259–
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Acknowledgements
[17] G. D. Frey, R. D. Dewhurst, S. Kousar, B. Donnadieu, G. Bertrand,
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Financial support from the NIH (R01 GM 68825) is gratefully acknowl-
edged. Thanks are also due to the China Scholarship Council (CSC)
(X.Z.), the Higher Education Commission of Pakistan (S.K.) for Gradu-
ate Fellowships, and the Alexander von Humboldt Foundation for a Post-
doctoral Fellowship (G.D.F.).
[18] It should be noted that a few similar complexes, bearing very bulky
phosphine ligands, have been isolated. E. Herrero-Gꢃmez, C. Nieto-
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À
Keywords: carbenes
coupling · gold · hydroamination
·
C C coupling reactions
·
cross-
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Received: December 13, 2008
Published online: February 13, 2009
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