Bulky, spherical, and fluorinated anion BArF induces 'on-water' activity of silver salt for the hydration of terminal alkynes

Article abstract of DOI:10.1016/j.tetlet.2014.01.045

AgBAr^F displays remarkable 'on-water' activity for catalytic hydration of terminal alkynes although it is ineffective in common organic solvents. Liquid alkynes do not require additive or co-solvent whereas a small amount of ethyl acetate triggers quantitative conversions for solid alkynes.

Full text of DOI:10.1016/j.tetlet.2014.01.045

Tetrahedron Letters 55 (2014) 1444–1447
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bulky, spherical, and fluorinated anion BArF induces ‘on-water’
activity of silver salt for the hydration of terminal alkynes
⇑
Sayantani Saha, Abir Sarbajna, Jitendra K. Bera
Department of Chemistry and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
AgBArF displays remarkable ‘on-water’ activity for catalytic hydration of terminal alkynes although it is
ineffective in common organic solvents. Liquid alkynes do not require additive or co-solvent whereas a
small amount of ethyl acetate triggers quantitative conversions for solid alkynes.
Ó 2014 Elsevier Ltd. All rights reserved.
Article history:
Received 29 October 2013
Revised 12 January 2014
Accepted 13 January 2014
Available online 20 January 2014
Keywords:
Alkyne
Silver
Catalysis
Hydration
On-water
O
Water as solvent in organometallic reactions offers valuable
prospect in terms of its economic, safety, and environmental
considerations.1 Yet, chemists have long discarded water simply
because of its immiscibility with non-polar organic molecules.
Breslow introduced the concept of ‘on-water’ activity to explain
unusual rate enhancement of a reaction in water as compared to
an organic solvent.2 It is now recognized that the improved rate
is linked to enhanced interactions between the reagents and the
catalyst on the water surface.3 The high surface tension of water
favors minimal contact between hydrophobic molecules and water
allowing different components to mix efficiently. It is thus felt that
metal-based catalysts that are hydrophobic may prove beneficial
for ‘on-water’ organometallic catalysis.
Hydration of alkynes to carbonyl compounds is an important
reaction that follows 100% atom efficiency.4 Hg(II) salts in aqueous
sulfuric acid solution had been the favored catalyst for a long
time.5 However, toxicity of mercury has furnished this option ob-
solete. Since then, a variety of metal based catalysts containing
Pt,6 Ir,7 Au,8 Rh,9 Fe,10 and Pd11 have been developed. These cata-
lysts display the Markovnikov selectivity and hydrate terminal
alkynes to methyl ketones (Scheme 1). Anti-Markovnikov products
were obtained at first by Wakatsuki employing Ru catalysts with
phosphane ligands.12 Grotjahn then followed it with the use of a
bifunctional catalyst.13 A variety of Ru based catalysts have been
introduced since then which form aldehydes from alkynes.14
[Ag(I)]
H2O
R
R
Scheme 1. Silver catalyzed hydration of terminal alkynes.
Ag(I) salts are known to promote Au catalysts for alkyne hydra-
tion. Nolan reported a hybrid (NHC)AuI/AgSbF6 system for the
same at part-per-million catalyst loading.15 In 1965, Dillar
reported catalytic hydration of N-propargylamides by using a
methanolic solution of AgNO3.16 In 1993, Marsella employed AgOTf
to selectively hydrate 2-methyl-3-butyn-2-ol.17 It is only in 2012
that several reports appeared utilizing Ag based catalysts for the
hydration of a wide variety of alkynes. Ag(I)-exchanged silicotung-
stic acid is shown to be an efficient catalyst which works in the
absence of organic solvent.18 The Wagner group revealed that
AgSbF6 selectively hydrates terminal alkynes at 10 mol % catalyst
loading in methanol/water (10:1) mixture at 75 °C giving yields
greater than 90%.19 Other solvent mixtures such as tetrahydrofu-
ran/water (10:1) gave considerably lower yield (55%) for 4-phen-
ylbutyne. It was emphasized that although methanol is not
essential for the hydration process, its presence clearly improves
the reaction rate. In the same year, the Chakraborty group reported
similar activity for AgOTf but only in ethyl acetate at 80 °C.20 A host
of other organic solvents was found to be not effective. The same
reaction in neat water gave less than 10% yield in 24 h. AgBF4 in
acetic acid is recently reported as an efficient catalyst for both
terminal and internal alkynes.21
⇑
Corresponding author. Tel.: +91 5122597336.
E-mail address: jbera@iitk.ac.in (J.K. Bera).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.045

S. Saha et al. / Tetrahedron Letters 55 (2014) 1444–1447
1445
Table 2
Ag
AgBArF catalyzed hydration of terminal alkynesa
F3C
CF3
Entry Alkyne
Product
Time
(h)
Conversionb
(%)
CF3
O
1
12
6
>99
>99
B
F3C
CF3
CF3
O
2
MeO
MeO
CF3
O
F3C
3
6
>99
O
O
O
Scheme 2. Silver tetrakis 3,5-bis(trifluoromethyl)phenyl borate.
O
O
4
5
6
7
12
12
12
12
84
82
75
70
Me
Table 1
Hydration of phenyl acetylene using different Ag(I) catalysts and solventsa
Me
O
Entry
Catalyst
Solventb
Time
(h)
Conversionc
(%)
1
2
AgSbF6, AgNO3, AgOAc, AgPF6,
AgOTf, Ag2O, AgBF4, Ag2CO3
AgBArF
H2O
12
24
<1
37
O
MeOH/
H2O
Br
N
Br
N
(10:1)
EtOAc/
H2O
O
3
AgBArF
24
0
(10:1)
H2O
H2O
4
5
6
7
AgBArF
AgBArF
AgBArF
AgBArF
6
12
6
86
99
50d
40
O
8
12
85
D2O
(H2O)e
6
S
S
a
All reactions were carried out using phenylacetylene (1 mmol), and catalyst
9
—
—
0
(0.1 mmol) at 80 °C.
N
b
Total volume 3 mL.
Determined by GC–MS analysis using n-dodecane as an internal standard.
C6H5COCD3.
O
c
10
24
48
d
e
1 equiv with respect to alkyne.
O
11
12
12
12
>99c
>99c
MeO
The counter-anion of the Ag(I) salts and the solvent medium ap-
pears to influence the hydration efficiency. We employed a bulky,
spherical, and highly fluorinated anion BArF [tetrakis 3,5-bis(tri-
fluoromethyl)phenyl borate] (Scheme 2) in an anticipation that
the non-coordinating anion would enhance metal Lewis acidity, in-
crease hydrophobicity and thus improve the catalytic efficiency.22
Although AgBArF turned out to be ineffective in common organic
solvents for alkyne hydration, it showed remarkable activity in
neat water. Herein, we report the chemo-selective ‘on-water’
hydration of a wide range of solid and liquid terminal alkynes cat-
alyzed by AgBArF.
Initial exploratory reactions were carried out using phenyl acet-
ylene as a model substrate. All results related to optimization stud-
ies are summarized in Table 1. AgNO3, AgOAc, AgSbF6, AgPF6,
AgOTf, Ag2O, AgBF4, and Ag2CO3 (10 mol %) were inactive for the
model substrate phenyl acetylene (1 mmol) at 80 °C in neat water
(3 mL). AgBArF afforded a sluggish conversion in MeOH/H2O med-
ium (10:1) after 24 h whereas no product could be detected in
EtOAc/H2O. Remarkably, AgBArF gave acetophenone in 86% yield
in neat water within 6 h and complete conversion occurred after
12 h. The desired product was isolated by simple extraction with
ethyl acetate. Switching the solvent to D2O drastically lowered
the yields to 50% in 6 h. D2O has higher cohesive pressure and vis-
cosity than H2O which makes mixing of the substrates difficult and
thus lowers the rate of product formation.23 Reaction was also
carried out in neat condition with 1 equiv of water with respect
to alkyne. Only 65% conversion was recorded after 12 h which is
considerably lower compared to quantitative conversion recorded
MeO
O
O
13
12
>99c
87c,d
CHO
CHO
O
O
14
15
24
24
62c,d
50c,d
O
16
17
18
24
24
—
3a, 3b, 3c
—
48, 30, 10c,d,e
0
a
All reactions were carried out using alkynes (1 mmol), catalyst (0.1 mmol), and
water (3 mL) at 80 °C.
b
Determined by GC–MS analysis using n-dodecane as an internal standard.
10 lL of EtOAc was added as an additive.
Carried out inside a sealed glass tube.
See Scheme 4.
c
d
e
in bulk water (3 mL). A control experiment was carried out using
KBArF. No product was formed in the absence of AgBArF confirming
that Ag(I) is the critical reagent in this reaction. In situ generated
AgBArF from 1:1 AgNO3 and KBArF afforded 75% conversion for

1446
S. Saha et al. / Tetrahedron Letters 55 (2014) 1444–1447
significant conversions were observed when reactions were carried
O
OH
R
out inside sealed glass tube at 80 °C (entries 14–16). For 1,7-octa-
diyne (entry 17), conversion was high. Cyclized compounds of
monohydrated product were obtained with minor amount of
double hydrated 2,7-octadione (Scheme 4). Hydration of internal
alkyne was also investigated under both conditions—only water
R = Ph
CH3
H
R
> 99 %
Scheme 3. Meyer–Schuster rearrangement.
and water +10 lL of ethyl acetate. But no product was detected
for diphenyl acetylene (entry 18). The fact that internal alkynes
do not hydrate under the reaction conditions led us to conclude
that the likely mechanism for terminal alkynes involves Ag-acety-
lide species.
the model reaction in 6 h that is lower in yields compared to
AgBArF (86%).
Subsequently, the scope of AgBArF was explored under opti-
mized conditions (10 mol % catalyst loading at 80 °C) in neat
water (3 mL) with a wide variety of alkynes. Liquid aromatic al-
kynes were efficiently converted to the corresponding ketones
with high conversions. Electron rich aromatic alkynes (Table 2,
entries 1–5) showed better conversions in comparison to alkynes
that contain electron withdrawing groups (entries 6 and 7). For
4-ethynylbenzonitrile (entry 7), selective hydration occurred for
the terminal alkyne whereas the nitrile group remained
unaffected. The reaction was extended to alkynes containing
heterocyclic rings. No product was observed for 2-ethynylpyri-
dine (entry 9) but high yields (85%) were obtained for 3-ethynyl-
thiophene (entry 8). The lack of reactivity for the pyridine
analogue could be attributed to catalyst deactivation via metal
coordination to the pyridine nitrogen. In case of 1,3-diethynyl
benzene (entry 10), conversion was less (48%) and only mono
hydration product was obtained.
AgBArF does not hydrate alkynes in most organic solvents. A
possible explanation could be that organic solvents dilute the silver
catalyst to such an extent that the reaction becomes impractical.
To rule out the possibility of dilution effect, we carried out model
reaction in 3 mL of MeOH/H2O medium (10:1) at different catalyst
loadings (20, 30 and 50 mol %). A maximum conversion of 41% was
recorded after 24 h. Considering a very similar conversion (37%) at
10 mol % catalyst loading under identical conditions, it can be
safely assumed that the lower activity of AgBArF in organic solvent
is not due to dilution. It is also shown that the use of bulk water
(3 mL) leads to higher conversions than when 1:1 alkyne–water
mixture was employed. Minimal ion pair interactions in water
may be linked to better activity. However, it is not relevant here
since AgBArF is insoluble in water. The remarkable alkyne-hydra-
tion activity of AgBArF could only be explained in terms of ‘on-
water’ effect. Catalyst AgBArF and liquid alkyne form a hydropho-
bic secondary phase on the water surface. The enforced
hydrophobic interactions that bring together the reagents and
the catalyst are responsible for the ‘on-water’ activity. Clearly, such
congregation is favored by bulky, spherical, and fluorinated anion
BArF where other silver salts fail. For solid alkynes, a small amount
of ethyl acetate is needed which dissolves the alkyne and the cat-
alyst to form an oily phase on the water surface. Ethyl acetate also
promotes hydration for aliphatic alkynes only under pressured
condition. It is postulated that ethyl acetate is hydrolyzed in the
presence of bulk water and Ag(I) forming acetic acid, a Brønsted
acid that favors the hydration process. This work thus suggests that
increase in catalyst hydrophobicity might improve its ‘on-water’
activity.
For ‘on-water’ reactions, the hydrophobic reactants form a het-
erogeneous phase on water surface where transformation takes
place. To form a secondary phase, one of the reactants needs to
be a liquid. This has been a major hindrance to broaden the scope
of ‘on-water’ reaction. We addressed this issue during the course of
this investigation while dealing with solid alkynes. A heteroge-
neous phase could be attained on water by the addition of a little
amount of liquid substance featuring some intrinsic properties—
immiscible with water, lighter than water, moderately high boiling
point, and being inert under the reaction conditions. We screened a
large number of organic solvents with solid alkynes among which
only ethyl acetate gave the desired results. Addition of 10 lL of
environmentally benign ethyl acetate afforded quantitative con-
version in 12 h for solid alkynes (entries 11–13). For propargylic
alcohol (Scheme 3), hydration reaction led to the Meyer–Schuster
rearrangement product instead of the expected methyl ketones.24
It could be argued at this point that ethyl acetate is the actual sol-
vent and water has no positive effect except being a reagent. It was
however discarded since reaction of solid alkyne 1-ethynyl-2,4,5-
trimethylbenzene in 3 mL of EtOAc/H2O (10:1) mixture gave only
marginal yield (<5%) after 24 h.
We further attempted less reactive aliphatic alkynes under
identical conditions but it was not successful. Different additives
were added to activate the triple bond including inorganic acid
H2SO4 and organic tosylic acid but it did not improve the outcome.
Addition of Et3N to facilitate the formation of silver acetylide, a
possible intermediate in this reaction, was also futile. Addition of
Acknowledgments
This work is financially supported by the Department of Science
and Technology (DST), India, and the Council of Scientific and
Industrial Research (CSIR) of India. We thank Prof. Y.D. Vankar
for useful discussions and Prof. D.H. Dethe for providing one alkyne
derivative. J.K.B. thanks DST for the Swarnajayanti fellowship. S.S.
thanks CSIR, India, and A.S. thanks UGC, India for fellowships.
Supplementary data
Supplementary data (full experimental details) associated with
this article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.01.045.
10 lL of ethyl acetate did not afford the desired products, but
O
O
O
O
3a
3b
3c
10%
48%
30%
Scheme 4. Hydration of 1,7-octadiyne.

S. Saha et al. / Tetrahedron Letters 55 (2014) 1444–1447
1447
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