Efficient and selective synthesis of e-vinylamines via tungsten(0)-catalyzed hydroamination of terminal alkynes

Article abstract of DOI:10.1002/adsc.201400568

The hydroamination of terminal alkynes (RC≡CH=phenylacetylene, 4-methylphenylacetylene, 4-fluorophenylacetylene, 1-hexyne, methyl 2-propynyl ether, prop-2-yn-1-ol) with secondary amines (piperidine, pyrrolidine, morpholine, piperazine, methylpiperazine, 4

Full text of DOI:10.1002/adsc.201400568

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DOI: 10.1002/adsc.201400568
Efficient and Selective Synthesis of E-Vinylamines via
Tungsten(0)-Catalyzed Hydroamination of Terminal Alkynes
a
a
a,
´
´
Paulina Kocie˛cka, Izabela Czelusniak, and Teresa Szymanska-Buzar *
a
Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
E-mail: teresa.szymanska-buzar@chem.uni.wroc.pl
Received: June 16, 2014; Revised: July 14, 2014; Published online: October 19, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400568.
Abstract: The hydroamination of terminal alkynes
(RC CH=phenylacetylene, 4-methylphenylacety-
piperidine),[4] as an efficient regio- and stereo-selec-
tive catalyst in the synthesis of E-vinylamines under
relatively mild conditions. Synthesis of the complex Ia
can be achieved via photochemical or thermal substi-
tution of two carbonyl groups by piperidine in the
commercially available W(CO)6.[4] The reactions of
terminal alkynes 1 (phenylacetylene 1a, 4-methylphe-
nylacetylene 1b, 4-fluorophenylacetylene 1c, 1-hexyne
1d, methyl-2-propynyl ether 1e, prop-2-yn-1-ol 1f,
but-3-yn-2-ol 1g, but-3-yn-1-ol 1h or 2-methyl-but-3-
yn-2-ol 1i) with secondary amines 2 (piperidine 2a,
pyrrolidine 2b, morpholine 2c, methylpiperazine 2d,
piperazine 2e, 4-methylpiperidine 2f and 3-methylpi-
peridine 2g) conducted in the presence of complex Ia
(1.6 mol%), in the absence of a solvent and in the
temperature range 60–908C, were completed in a max-
imum of 5 h and gave up to 99% yield of E-vinyl-
ꢀ
lene, 4-fluorophenylacetylene, 1-hexyne, methyl 2-
propynyl ether, prop-2-yn-1-ol) with secondary
amines (piperidine, pyrrolidine, morpholine, pipera-
zine, methylpiperazine, 4-methylpiperidine and 3-
methylpiperidine) was achieved in high yield (up to
99%), regioselectivity (only anti-Markovnikov
product) and stereoselectivity (only E-isomers)
within a maximum of 5 h in reactions catalyzed by
the
tungsten
tetracarbonyl
complex
cis-
[W(CO)4(piperidine)2] at 908C without any addi-
tional solvent.
Keywords: E-enamines; hydroamination reaction;
secondary amines; terminal alkynes; tungsten cata-
lyst
AHCTUNGTREGUNaNN mines 3 (Scheme 1). The hydroamination products 3
were extracted into n-hexane, from which clean trans-
vinylamines were isolated after evaporation of the
solvent and any remaining non-reacted substrates
Alkynes are widely available reagents and the catalyt- under vacuum, and characterized using 1H and
ic addition of the N H bond of amines to the triple 13C NMR spectroscopy and GC-MS analysis.
À
ꢀ
bond of alkynes, RC CH, is of great significance in
In our initial studies, the reaction of phenylacety-
the synthesis of enamines and the other nitrogen-con- lene 1a with piperidine 2a, was chosen as a model to
taining compounds.[1] This atom-economical process is optimize the hydroamination reaction conditions. The
mostly catalyzed by complexes based on such metals optimization included the selection of the most suita-
as Rh, Ru, Pd, Au, Ag, lanthanides or actinides.[1d,f,2] ble solvent, amount of the catalyst, reaction time and
The development of a catalyst for this synthetic pro- reaction temperature (Table 1).
cess that will be efficient, regio- and stereoselective,
but cheaper and easier available compared with noble
metal catalysts, has attracted significant interest.
Tungsten(0) and molybdenum(0) carbonyl complexes
are very well-known catalysts for nucleophilic addi-
À
tion of O H bond of alcohols to a triple bond of alk-
A
ynes and the cyclization of alkynols.[1d,3] To date, how-
ever, no group 6 complexes have been reported as
catalysts for the hydroamination of alkynes.
Here, we report on the application of a very well-
known, easily available, simple and relatively cheap
Scheme 1. Hydroamination of terminal alkynes 1, by secon-
complex of tungsten(0), cis-[W(CO)4ACTHNUTRGNE(NUG pip)2] Ia (pip= dary amines 2, catalyzed by Ia.
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Paulina Kocie˛cka et al.
Table 1. Optimization of the reaction conditions for the addition of phenylacetylene 1a to piperidine 2a, and the formation
of 1-[(E)-(2-phenylethenyl)piperidine] 3aa.[a]
Entry
Catalyst Ia [mol%]
Temperature [oC]
Time [h]
Conversion[b] [%]
Yield[c] [%]
1
2
3
4
5
6
7
8
9
1.6[d]
1.6[e]
1.6
1.6
1.6
1.6
1.0
1.0
0.5
r.t.
r.t.
r.t.
90
90
90
90
90
90
90
24
24
24
1
2
5
5
24
5
19
14
16
58
66
90
83
93
81
84
~1
~0.1
1.5
[f]
–
–
[f]
84
80
67
77
76
10
0.5
24
[a]
Reaction conditions: Catalyst Ia, 1a (0.33 g, 3.3 mmol), and 2a (0.27 g, 3.3 mmol) under N2.
[b]
[c]
[d]
[e]
[f]
Conversion was determined by NMR on the basis of the consumption of phenylacetylene.
1
NMR yield of 3aa was based on the H NMR analysis of the crude reaction mixture.
In 30 mL of tetrahydrofuran.
In 30 mL of diethyl ether.
Not calculated.
À
The addition of the N H bond of piperidine 2a, 90 to 81% (entries 6, 7 and 9). The increase of the re-
across the triple bond of phenylacetylene 1a, cata- action time from 5 to 24 h only slightly improved the
lyzed by complex Ia was first observed in solution of conversion of an alkyne but had a negative effect on
diethyl ether and tetrahydrofuran at room tempera- the yield of an enamine (entries 8 and 10).
ture (r.t.). However, only a small amount of the en-
Attempts to replace complex Ia by W(CO)6 Ib as
amine product was detected after 24 h reaction time; the catalyst failed. In reactions of 1a and 2a in the
Table 1, entries 1 and 2. The reason was a very low presence of Ib in the molar ratio 30:30:1, respectively,
solubility of complex Ia in the solvents. Dichlorome- carried out in n-heptane solution at 908C, only traces
thane is a better solvent for complex Ia; however, as of 3aa were detected after 5 h, similarly as in reaction
has been recently observed, it reacts very easily with carried out under the same conditions but without
2a to give dipiperidylmethane.[5] Very good solubility any solvent. However, in the latter reaction, also sty-
ꢀ
of complex Ia was achieved in a 1:1 mixture of an rene, a product of hydrogenation of PhC CH, was
alkyne and amine in the temperature range 60–908C. identified by H NMR. After a 5 h reaction time and
1
The results in Table 1 illustrate the excellent per- ca. 1% conversion of the alkyne, the ratio of 3aa to
formance of the tungsten catalyst Ia in this hydroami- styrene reached 1.6:1.
nation process. The 1:30 molar ratio of the catalyst Ia
and the alkyne 1a was sufficient to give the desired conducted at room temperature in an n-heptane solu-
product 3aa, in good yield. tion containing Ib, 1a and 2a in the molar ratio
Alternatively, the photochemical reactions were
The decrease of the catalyst loading from 1.6 to 1.0 1:30:30 (Scheme 2). During photolysis of the reaction
and 0.5 mol% lowered the conversion of alkyne from mixture at least two carbonyl groups in the coordina-
Scheme 2. Reaction of phenylacetylene 1a and piperidine 2a in the presence of photochemically activated W(CO)6 Ib.
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Efficient and Selective Synthesis of E-Vinylamines
Table 2. Effect of alkynes 1 on the yield of trans-vinylpiperi-
dines 3.[a]
tion sphere of W(CO)6 Ib are easily substituted by
molecules of substrates. In this way the substrates are
activated. After 2 h of photochemical reaction, the
conversion of 1a reached 100%. However, the yield
of 3aa was not high (43%). The major side product
detected by GC-MS (Mr =289.4) was the three-mole-
cules coupling product (1-[1,4-diphenylbuta-1,3-dien-
2-yl]piperidine 5a2a), involving two molecules of the
alkyne 1a and one molecule of the amine 2a. Com-
pound 5a2a was detected by NMR spectroscopy as
a mixture of cis (34%) and trans (22%) isomers.
The dimerization of the alkyne 1a and the forma-
tion of trans-enynes 4a2 was also observed by GC-MS
and NMR but in very low yield (ca. 1%).
The above results show that using complex Ia,
which contains two loosely coordinated amine ligands
in the cis position, as catalyst has pronounced effects
on the efficiency and selectivity of the hydroamina-
tion reaction.
Based on the promising catalytic activity displayed
by Ia in the hydroamination reaction of 1a, we next
investigated the scope of this reaction with a variety
of terminal alkynes (Table 2).
In general, we observed good conversion in 5 h
when 1.6 mol% of Ia was used as catalyst (Table 2).
Phenylacetylene is hydroaminated with high yield,
84% (entry 1); substituents on the phenyl ring of the
arylacetylenes induced evident electronic effects (en-
tries 1–3). The presence of an electron-withdrawing
fluorine atom in the 4-position of arylacetylene 1c de-
creased the reactivity of the alkyne and the yield of
the enamine 3ca, whereas an Me group in the 4-posi-
tion of the arylacetylene 1b, increased the yield of the
ꢀ
[a]
enamine 3ba. The electron-rich arylalkynes RC CH
Reaction conditions: Ia (0.05 g, 0.11 mmol), 1 (3.3 mmol),
(R=Ph 1a, C6H4-4-Me 1b) react more smoothly than
the electron-poor arylalkyne 1c. The highest conver-
sion was achieved for 1b (100%) with a selectivity of
99% to the target product, trans-vinylamine 3ba
(Table 2, entry 2). The aliphatic alkyne 1-hexyne 1d is
also transformed to the corresponding product 3da,
but in low yield, 8% (entry 4). Under similar condi-
tions, a methyl propargyl ether 1e is converted to the
and 2a (0.27 g, 3.3 mmol) without any solvent, under N2.
[b]
[c]
[d]
1
Conversion was determined by H NMR on the basis of
the consumption of the alkyne 1.
NMR yield of 3 was based on the H NMR analysis of
1
the crude reaction mixture.
Only the three-molecules coupling product 5h2a was de-
tected.
E-enamine 3ea with a much higher yield (entry 5). It nal alkyne 1-phenyl-1-propyne 1j failed (Table 2,
is also worth noting that an alkyne with a free hy- entry 10). Apparently, the catalytic activity of Ia is
À
droxy group, prop-2-yn-1-ol 1f, but-3-yn-2-ol 1g, but- strongly affected by the presence of an alkyne C H
3-yn-1-ol 1h or 2-methylbut-3-yn-2-ol 1i reacted with bond.
piperidine 2a to give the expected hydroamination
We later found that also other secondary amines,
products 3fa, 3ga, 3ha, and 3ia, but the yield of the pyrrolidine 2b, morpholine 2c, methylpiperazine 2d,
enamine was very low (entries 6, 7, 8, and 9) com- piperazine 2e, 4-methylpiperidine 2f and 3-methylpi-
pared with the yield of the three-molecules coupling peridine 2g, could be effectively reacted with phenyl-
product 5, and the product formed in the nucleophilic
ACHTUNGTRENaNUNG cetylene and the other terminal alkynes under the
À
ꢀ
addition of O H bond to a C C triple bond of the current reaction conditions (Table 3). Morpholine 2c,
alkyne.[6] The enamines 3fa, 3ga, 3ha and 3ia were methylpiperazine 2d and piperazine 2e with the elec-
identified by NMR spectroscopy but not isolated. It tron-withdrawing oxygen or nitrogen atoms in the
should, however, be pointed out that only terminal al- ring gave significantly higher yields of enamines than
kynes are reactive under these conditions. The at- piperidine 2a or pyrrolidine 2b with electron-donating
tempted Ia-catalyzed addition of piperidine to inter- CH2 groups (entries 3, 4 and 5). However, in the reac-
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Paulina Kocie˛cka et al.
Table 3. Effect of amines 2 on the yield of trans-vinylamines 3.[a]
[a]
Reaction conditions: Ia (0.05 g, 0.11 mmol), 1 (3.3 mmol), and 2a (3.3 mmol) without
any solvent.
[b]
Conversion was determined by NMR on the basis of the consumption of the alkyne
1.
[c]
[d]
NMR yield of 3 was based on the 1H NMR analysis of the crude reaction mixture.
Two- and three-molecules coupling products 3ae and 3a2e were formed in a 58:42
molar ratio, respectively.
[e]
Reaction was carried out at the molar ratio of 1a:2e=2:1. The hydroamination prod-
ucts 3ae and 3a2e were formed in a 17:83 molar ratio, respectively.
tion of piperazine 2e and alkyne 1a, the three-mole-
A methyl group in the 4-position or 3-position of
cules coupling product 3a2e (Scheme 3), involving two the piperidine, 2f and 2g, increased the reactivity of
molecules of the alkyne 1a and one molecule of the amine and the yield of the enamine 3af and 3ag (en-
amine 2e was isolated as a major product in the reac- tries 7 and 8) compared to that with piperidine 2a.
tion carried out at alkyne to piperazine molar ratio
2:1 (entry 6).
The major side products, which were detected
during prolonged reaction (908C, 5 h) of 1 and 2 in
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Efficient and Selective Synthesis of E-Vinylamines
ever, with the catalyst Ib, the yield of the enamine is
much lower than with Ia under comparable reaction
conditions. The application of Ib under photochemical
conditions was very promising. Unfortunately, unde-
sirable reaction pathways became more favourable di-
minishing the yield of the desired product 3aa to
43%.
The mechanism of the tungsten(0)-catalyzed addi-
tion of secondary amines to terminal alkynes is un-
clear at this stage. However, based on the anti-Mar-
Scheme 3. Three-molecules coupling product (1,4-bis[(E)-2-
phenylethenyl]piperazine 3a2e) isolated in the reaction of
phenylacetylene 1a and piperazine 2e.
the presence of Ia were identified by GC-MS as con- kovnikov regioselectivity of the reaction and on the
taining two molecules of the alkyne and one molecule result that no hydroamination of internal alkynes was
of the amine. The NMR analysis of these products observed, the reaction may proceed via a vinylidene-
(5a2b, 5b2c, 5h2a and 5i2a) revealed their existence as tungsten intermediate. Nucleophilic attack of the
a mixture of cis and trans isomers (see Scheme 2).
amine at the a-carbon of the vinylidene ligand would
The initial experiments carried out with primary lead to the formation of the anti-Markovnikov prod-
amines (aniline and cyclohexylamine) revealed that, ucts observed here.[8] The formation of vinylidene li-
under comparable reaction conditions which were ap- gands in reaction of terminal alkynes with tungsten(0)
plied for secondary amines, the reactivity of primary d6 complexes is very well documented in literature.[9]
amines was markedly lower. The yield of the major
In conclusion, a facile, catalytic hydroamination re-
hydroamination product of phenylacetylene by aniline action of terminal alkynes with secondary amines has
reached 7% and by cyclohexylamine 3%, after 5 h re- been revealed yielding selectively E-enamines. The
action at 908C in the presence of 1.6 mol% of Ia.
scope and mechanism of this reaction are currently
The reaction was not selective and gave isomers of under investigation.
two-molecules and three-molecules coupling products.
In conclusion, the regio- and stereoselective hydro-
amination of terminal alkynes with secondary amines Experimental Section
can be achieved with good to excellent yields in reac-
tions catalyzed by the relatively inexpensive, simple Synthesis of Complex [W(CO)
and readily available tungsten complex cis-[W(CO)4
4ACHTUNGTERN(NUNG pip)2] Ia
The piperidine (pip) complex [W(CO)4ACHTNUTRGNE(UNG pip)2] Ia was pre-
ACHTUNGTRENNUNG(pip)2] Ia. Corresponding additions of secondary
pared in
a photochemical reaction of W(CO)6 (0.7 g,
amines to the terminal alkynes take place at 908C
under solvent-free conditions at low catalyst loading
(1.6 mol%). In all of the terminal alkynes and secon-
dary amines tested, only anti-Markovnikov selectivity
of hydroamination reactions was detected.
A small disadvantage of the catalyst Ia is constitut-
ed by the impurities of the hydroamination product
with E-vinylpiperidines 3 formed in reaction of the
alkyne 1 and piperidine 2a derived from the catalyst.
This problem can be omitted by the replace catalyst
Ia with a tungsten complex containing the appropriate
secondary amines 2b–g in place of piperidine 2a as li-
gands. The diamine complexes of the type [W(CO)4
2.0 mmol) and piperidine (0.6 mL, 6.1 mmol) in n-hexane
(50 mL). During this reaction (ca. 8 h) the tungsten complex
containing two piperidine ligands settled out as a light
yellow powder, which was separated, washed several times
with n-hexane and dried under vacuum to give ca. 0.6 g of
analytically pure and sufficiently stable in the solid state
complex Ia. Photochemical reactions were carried out under
an atmosphere of nitrogen in a glass reactor with a quartz
window. The photolysis source was an HBO 200 W medium-
pressure Hg lamp.
General Procedure for Hydroamination of Terminal
Alkynes
ACHTUNGTRENNUNG
The reactions were carried out under a nitrogen atmosphere
in Schlenk tubes equipped with a condenser and a magnetic
In summary, this work represents a major step for- stir bar. In a typical run, tungsten complex (Ia, 0.05 g,
0.11 mmol) was loaded into the tube. The tube was evacuat-
ed and filled with nitrogen. The corresponding liquid re-
agents: alkyne 1 (3.3 mmol) and the corresponding amine 2
(3.3 mmol) in a 1:1 molar ratio were then introduced to the
tube via syringes and needles. The solid reagents were
loaded into the tube together with the catalyst. Next the re-
action mixture was stirred in a silicon oil bath at the suitable
temperature (slightly lower than the boiling temperature of
reagents but not exceeding 908C) for a maximum 5 h. The
ward in the development of regio- and stereoselective
hydroamination catalysts. To the best of our knowl-
edge, this is the first example of the hydroamination
of alkynes with such a very well-known and simple
tungsten complex as catalyst. It should be noted that
the catalytic activity of Ia meets or exceeds the activi-
ty of other catalytic systems based on Rh and
Ru.[2d,e,f,h] It is also worth mentioning that the hydroa-
mination reaction can be realized in the presence of
course of the reaction was monitored by regular sampling
1
W(CO)6 Ib, as the precursor of the catalyst Ia. How- (0.1 mL every one hour) and analysis by H NMR. The re-
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Paulina Kocie˛cka et al.
sulting products were extracted into n-hexane, from which
they were isolated as clean compounds after the evaporation
of the solvent and any remaining non-reacted substrates
under vacuum. Purity of the compounds has been checked
by 1H NMR spectroscopy and GC-MS analysis. Only the
three-molecules coupling product 3a2e was isolated in the
solid state and purified by crystallization in dichlorome-
thane/n-hexane mixture. All products were characterized
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Acknowledgements
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Gꢆrski, A. Kochel, T. Szymanska-Buzar, Inorg. Chem.
This work was financed by the Polish National Science
Centre (NCN) under Grant No. 2013/09/N/ST5/00402. The
authors thank Dr. J. Skonieczny for measurements of NMR
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