AgOTf catalyzed hydration of terminal alkynes

Article abstract of DOI:10.1002/aoc.2918

The silver catalysis towards the hydration of terminal alkynes is explored using Silver(I) triflate (AgOTf). The reaction leads to the formation of only Markovnikov addition product with excellent yield. Copyright

Full text of DOI:10.1002/aoc.2918

Full Paper
Received: 14 May 2012
Revised: 2 August 2012
Accepted: 5 August 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2918
AgOTf catalyzed hydration of terminal alkynes
Rima Das and Debashis Chakraborty*
The silver catalysis towards the hydration of terminal alkynes is explored using Silver(I) triflate (AgOTf). The reaction leads to
the formation of only Markovnikov addition product with excellent yield. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: hydration; alkyne; AgOTf; markovnikov addition; EtOAc
the conversion of amides of l,l-dialkyl-2-propynylamines to
Introduction
ketoamide.[34] Very recently Mann and Wagner reported the
use of AgSbF6 for alkyne hydration.[35] Another recent report high-
An atom-economic reaction is where all the reactant atoms
are found in the desired product.[1] Addition of water to the
lights the use of silver exchanged silicotungstic acid as hetero-
geneous catalyst for the hydration of alkynes.[36] Although,
alkynes is a 100 % atom-economic reaction that permits a
simple access to carbonyl function. In 1860, Berthelot carried
out the hydration of acetylene in the presence of sulfuric acid
and expected vinyl alcohol as the product but the actual
product was a mixture of acetaldehyde and crotonaldehyde.
Later in 1881, Kucherov discovered the concept of Hg(II)
Marsella’s report using resin bounded metal systems, attempted
one trial for alkyne hydration using AgOTf as catalyst, the reaction
took extremely long time (70h).[37] Our primary objective is to
report a convenient Ag(I) catalyzed method for Markovnikov
hydration of terminal alkynes, followed by mechanistic study.
catalyzed alkynes hydration and since then, it is
a
classical textbook reaction, which leads to the formation of
Markovnikov/anti- Markovnikov product depending upon the
substrate.[2–5] Use of water as a reagent in an organic reaction
is an environmentally benign process and alkyne hydration is
a perfect example. The traditional catalysts display Markovnikov
selectivity and hydrate terminal alkynes to methyl ketones,
which are useful chemical intermediates.[3–5] The conversion
of terminal alkynes to aldehydes has usually been performed
indirectly via hydroboration or hydrosilylation, followed by
oxidation.[6,7] However, the direct catalytic anti-Markovnikov
hydration of alkynes has recently become viable by means of
specially designed ruthenium(II) catalyst, introduced by
Wakatsuki et al.[8–10] and further improved by other
groups.[5,11–16]
Results and Discussions
Extensive investigations were done to define the best Ag(I)
salt as alkyne hydration catalyst, using phenyl acetylene as a
model substrate in EtOAc as solvent. Phenyl acetylene
(1 mmol) was allowed to react with water (3 eq.) at 80 ꢀC
under an open atmosphere, affording acetophenone in 8 h
with AgOTf as catalyst (Table 1, Entry 2). Addition of more
than 3 eq. of water does not change the reaction rate signif-
icantly. Other Ag(I) salts (Table 1, Entries 1, 3–6) were ineffec-
tive under this condition. Reaction with AgPF6 resulted in
90 % yield in 12 h (Table 1, Entry 7).[35] Less amount of cata-
lyst (5 mol %) took longer time to afford the maximum yield
where as increased amount of catalyst (15 mol %) (Table 1,
Entries 18, 19) did not improve the yield much indicating that
10 mol % of catalyst was the optimum amount. Further an
attempt to perform the reaction at room temperature was
not successful and in the absence of Ag(I), the reaction was
negligible (Table 1, Entry 17 and 20).
In general, Markovnikov addition leads to the formation of the
methyl ketones and anti-Markovnikov addition gives aldehydes
as products (Eq.1).
The appropriate solvent was optimized by varying the
classical organic solvents and EtOAc was found to be the
best. Reaction in toluene and EDC was comparable although
Use of toxic and environmentally hazardous Hg(II)-salts for
this purpose is a dramatic barrier for the application in
laboratory and industry. Other expensive and heavy metals
like Ru,[8–15] Rh,[17] Pt[18–20] and Au[21–29] are also used for
hydration of alkyne but prohibit their frequent use for labora-
tory and industrial purpose. Use of concentrated solution of
strong acids (H2SO4, H3PW12O40 and CF3SO3H) is not always
fruitful for large scale preparations.[30]
*
Correspondence to: Debashis Chakraborty, Department of chemistry, Indian
Institute of Technology Madras, Chennai-600036, Tamil Nadu, India. E-mail:
dchakraborty@iitm.ac.in
There are many reports where Ag(I) has been used as an addi-
tive with Au.[27,31–33] Alcoholic solution of AgNO3 was used for
Department of chemistry, Indian Institute of Technology Madras, Chennai-600036,
Tamil Nadu, India
Appl. Organometal. Chem. 2012, 26, 722–726
Copyright © 2012 John Wiley & Sons, Ltd.

AgOTf catalyzed hydration of terminal alkynes
Table 1. Hydration of phenyl acetylene using various Ag(I) salts and solvents
Entry
Ag(I) Salt (10 mol %)
Solvent
Time (h)a
Yield (%)b
1
AgOAc
AgOTf
AgI
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
Toluene
THF
20
8
10
92
20
-
2
3
24
28
30
24
12
12
10
15
18
20
21
24
15
12
24
24
10
24
4
AgBr
5
AgCl
-
6
AgNO3
AgPF6
5
7
90
60
5
8
AgOTf
9
AgOTf
10
11
12
13
14
15
16
17
18
19
20
AgOTf
CH3CN
CH3NO2
EtOH
40
20
10
20
15
5
AgOTf
AgOTf
AgOTf
DMF
AgOTf
DMSO
1,4-dioxane
EDC
AgOTf
AgOTf
70
5
-
EtOAc
EtOAc
EtOAc
EtOAc (rt)
AgOTf (5 mol %)
AgOTf (15 mol %)
AgOTf
70
93
5
aMonitored using TLC,
bIsolated yield after column chromatography of the crude product
there was formation of halogenated side product in case of Ethylene
dichloride (EDC), consistent with the report by Darcel et al.[38]
Other common solvents (Table 1, Entries 8–15) gave very less
amount of product formation even after 24 h. The reaction in water
as solvent gave < 10 % product in 24 h. Table 2 shows the successful
formation of Markovnikov hydration product i.e. methyl ketones in
excellent yield for a variety of terminal alkynes. The reaction is accel-
erated by electron-donating groups in the phenyl moiety (Table 2,
Entries 2–6) where as presence of electron-withdrawing groups in
the phenyl moiety retards the reaction considerably (Table 2, Entry
8). Substrates having a hetero atom in the ring also yielded methyl
ketone successfully (Table 2, Entries 10, 11). Napthyl systems worked
well under the same reaction condition (See supporting information
for spectral data).
The substrate scope was further extended using a range of
aliphatic and a, b unsaturated alkynes and the results are
depicted in Table 3. All the substrates gave the corresponding
carbonyl compounds in reasonable time with good to excel-
lent yield. Substrates having propagylic alcohol, does not
hinder the product formation. Again, a, b unsaturated alkynes
also tolerate the reaction conditions without having decom-
position of the a, b unsaturated ketone (See supporting
information for spectral data).
It seems likely that the generation of silver acetylide is
necessary for carrying out the reaction as shown by the
following mechanism (Fig. 1). A similar mechanism with gold
has been proposed in Laguna’s report.[23] Generally hydration
of alkyne may follow two pathways: one is proposed based
on Hg(II) salts as catalysts and another is proposed using
acid-Brønsted catalysts.[5] Ag(I) salts show Lewis acidity, so
AgOTf is able to accept the electron pair easily. Based on
the above two facts, a mechanism was speculated for the
hydration of alkyne (Fig. 1). Further studies have been done
to confirm the formation of silver acetylide species. We
performed the reaction in an NMR tube using benzene-d6
and recorded the spectra after addition of each reagent. After
the addition of AgOTf, there was broadening of the peaks as
indicated in the literature[39] ensuring the generation of silver
acetylide species in situ via the activation of the triple bond
through a short lived p-complex.[40] Then addition of water
to the more stable carbocation like centre leads to the forma-
tion of methyl ketone.[41,42]
A controlled experiment have been carried out to under-
stand whether the reaction is catalyzed by hidden Brønsted
acid such as TfOH[43] in the presence of 1 mol % AgOTf and
t-butyl chloride under the same reaction conditions. The reac-
tion took 20h for completion and resulted in 78 % isolated
yield of the product, indicating the lower catalytic effect of
in situ generated Brønsted acid. This also proves that Ag(I)
has a definite role in such catalysis.
To check the catalytic ability of this system with internal alkynes,
we performed a reaction with diphenyl acetylene under the
same conditions and unfortunately, we were not able to get the
product (Scheme 1).
Appl. Organometal. Chem. 2012, 26, 722–726
Copyright © 2012 John Wiley & Sons, Ltd.
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R. Das and D. Chakraborty
Table 2. Hydration of aromatic alkynes using AgOTf as catalyst
Entry
1
Alkyne
Time (h)a
8
Product
Yield (%)b
92
2
7
90
3
4
7.5
95
90
7
5
6
7
5
98
95
92
6
7.5
8
9
12
80
94
5.5
10
11
10
7
82
90
aMonitored using TLC,
bIsolated yield after column chromatography of the crude product
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 722–726

AgOTf catalyzed hydration of terminal alkynes
Table 3. Hydration of aliphatic alkynes using AgOTf as catalyst
Entry
1
Alkyne
Time (h)a
5
Product
Yield (%)b
90
2
3
5.5
9.5
88
80
4
5
6
8
10
9
89
82
92
aMonitored using TLC,
bIsolated yield after column chromatography of the crude product
AgOTf (10 mol %)
water as an eco-friendly reagent and these reactions are 100 %
atom-economic, so there is no side product formation. Moreover,
no additive like presence of acid or other metal was needed.
Ph
Ph
no product
H2O (3 eq.)
EtOAc,
80 oC
Scheme 1. Hydration with internal alkyne.
Experimental Section
To a stirred solution of alkyne (1 mmol) in EtOAc (3 ml) was added
successively AgOTf (25 mg, 10 mol %) and water (3 mmol, 0.1 ml).
The reaction mixture was heated up to 90 C and progress of the
ꢀ
Ph
Ph
CH3
CH2
reaction was monitored by TLC. After completion of the reaction,
the crude mixture was allowed to cool at room temperature and
the solvent was removed under vacuum. The crude product was
extracted in dichloromethane, dried over Na2SO4 and concentrated,
followed by flash column chromatography of the crude, resulting in
pure products. The characterization data of the products matched
well with those reported in the literature.
HO
O
Ph
H
Ag+
H/H2O
Ag
Ph
CH
HO
Ag
H
Ph
H
Ag
H
Ph
O
Acknowledgments
CH
H
This work was supported by the Department of Science and
Technology, New Delhi. R.D. thanks the Council of Scientific and
Industrial Research, New Delhi for a research fellowship.
H
H
Ph
Ag
H2O
Figure 1. Proposed mechanism of hydration of terminal alkyne.
References
[1] B. M. Trost, Angew. Chem. Int. Ed. 1995, 34, 259.
[2] M. Berthelot, C. R. Hebd, Seances Acad. Sci. 1862, 50, 805.
[3] R. C. Larock, W. W. Leong in Comprehensive Organic Synthesis, Vol. 4
(Eds.: B. M. Trost, I. Fleming, M. F. Semmelhack), Pergamon, Oxford,
U. K, 1991, pp. 269.
[4] J. March, Advanced Organic Chemistry, 4th edn, Wiley, New York,
1992, pp. 76.
[5] L. Hintermann, A. Labonne, Synthesis 2007, 1121.
[6] J. March, Advanced Organic Chemistry, 4th edn, Wiley, New York,
1992, pp. 787.
Conclusions
In summary, this is the first report of gold-free silver catalysis for
the hydration of terminal alkynes which leads to the formation of
Markovnikov’s addition product i.e. ketone with excellent yield. A
number of aromatic, heteroaromatic, aliphatic, alicyclic alkynes
tolerate the reaction conditions. However, internal alkynes were
inactive under these conditions since mechanistic study indicates
the generation of silver acetylide species that is crucial for the
product formation. The present study highlights the utility of
[7] H. Sakurai, M. Ando, N. Kawada, K. Sato, A. Hosomi, Tetrahedron Lett.
1986, 27, 75.
[8] M. Tokunaga, Y. Wakatsuki, Angew. Chem. Int. Ed. 1998, 37, 2867.
Appl. Organometal. Chem. 2012, 26, 722–726
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

R. Das and D. Chakraborty
[9] T. Suzuki, M. Tokunaga, Y. Wakatsuki, Org. Lett. 2001, 3, 735.
[10] M. Tokunaga, T. Suzuki, N. Koga, T. Fukushima, A. Horiuchi, Y. Wakatsuki,
J. Am. Chem. Soc. 2001, 123, 11917.
[25] Y. Fukuda, K. Utimoto, J. Org. Chem. 1991, 56, 3729.
[26] P. Nun, R. S. Ramón, S. Gaillard, S. P. Nolan, J. Organomet. Chem.
2010, 696, 7.
[11] F. Chevallier, B. Breit, Angew. Chem. Int. Ed. 2006, 45, 1599.
[12] M. M. T. Khan, S. B. Halligudi, S. Shukla, J. Mol. Catal. 1990, 58, 299.
[13] D. B. Grotjahn, D. A. Lev, J. Am. Chem. Soc. 2004, 126, 12232.
[14] D. B. Grotjahn, C. D. Incarvito, A. L. Rheingold, Angew. Chem. Int. Ed.
2001, 40, 3884.
[15] A. Labonne, T. Kribber, L. Hintermann, Org. Lett. 2006, 8, 5853.
[16] F. Boeck, T. Kribber, L. Xiao, L. Hintermann, J. Am. Chem. Soc. 2011,
133, 8138.
[27] A. Almássy, C. E. Nagy, A. C. Bényei, F. Joó, Organometallics 2010, 29, 2484.
[28] C. E. Czégéni, G. Papp, A. Kathó, F. Joó, J. Mol. Catal. A: Chem. 2011, 340, 1.
[29] S. Sanz, L. A. Jones, F. Mohr, M. Laguna, Organometallics 2007, 26, 952.
[30] J. Halpern, B. R. James, A. L. W. Kemp, J. Am. Chem. Soc. 1961, 83, 4097.
[31] N. Marion, R. S. Ramón, S. P. Nolan, J. Am. Chem. Soc. 2009, 131, 448.
[32] A. Leyva, A. Corma, J. Org. Chem. 2009, 74, 2067.
[33] A. S. K. Hashmi, T. Hengst, C. Lothschütz, F. Rominger, Adv. Synth.
Catal. 2010, 352, 1315.
[17] J. Blum, H. Huminer, H. Alper, J. Mol. Catal. 1992, 75, 153.
[18] W. Hiscox, P. W. Jennings, Organometallics 1990, 9, 1997.
[19] J. W. Hartman, W. C. Hiscox, P. W. Jennings, J. Org. Chem. 1993,
58, 7613.
[20] W. Baidossi, M. Lahav, J. Blum, J. Org. Chem. 1997, 62, 669.
[21] W. Wang, B. Xu, G. B. Hammond, J. Org. Chem. 2009, 74, 1640.
[22] P. Roembke, H. Schmidbaur, S. Cronje, H. Raubenheimer, J. Mol.
Catal. A: Chem. 2004, 212, 35.
[34] N. R. Easton, D. R. Cassady, R. D. Dillard, J. Org. Chem. 1965, 30, 3084.
[35] M. B. T. Thuong, A. Mann, A. Wagner, Chem. Commun. 2012, 48, 434.
[36] K. T. V. Rao, P. S. S. Prasad, N. Lingaiah, Green Chemistry 2012, 14, 1507.
[37] I. K. Meier, J. A. Marsella, J. Mol. Catal. 1993, 78, 31.
[38] X.-F. Wu, D. Bezier, C. Darcel, Adv. Synth. Catal. 2009, 351, 367.
[39] U. Létinois-Halbes, P. Pale, S. Berger, J. Org. Chem. 2005, 70, 9185.
[40] S. Li, Y. J. Chang, Y. Wang, L. Y. Dai, Chin. Chem. Lett. 2011, 22, 393.
[41] K. Y. Lee, D. Y. Park, T. H. Kim, J. N. Kim, Bull. Korean Chem. Soc. 2005,
26, 1617.
[23] R. Casado, M. Contel, M. Laguna, P. Romero, S. Sanz, J. Am. Chem. Soc.
2003, 125, 11925.
[42] E. S. Stoyanov, I. V. Stoyanova, C. A. Reed, J. Am. Chem. Soc. 2011,
133, 8452.
[43] T. T. Dang, F. Boeck, L. Hintermann, J. Org. Chem. 2011, 76, 9353.
[24] E. Mizushima, K. Sato, T. Hayashi, M. Tanaka, Angew. Chem. Int. Ed.
2002, 41, 4563.
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