Hg(OTf)2-catalyzed instantaneous hydration of β- and δ-hydroxy internal alkynes with complete regioselectivity

Article abstract of DOI:10.1055/s-0028-1088153

The hydration of β- and δ-hydroxy internal alkynes catalyzed by Hg(OTf)₂ took place instantaneously to give ketones with complete regioselectivity under mild conditions, whereas the hydration of internal alkyne without hydroxy moiety was very slow and gave a mixture of ketones. If the hydroxy group is located more than five carbons from the triple bond it has no significant effect upon the hydration reaction. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1088153

LETTER
1175
Hg(OTf)2-Catalyzed Instantaneous Hydration of b- and d-Hydroxy Internal
Alkynes with Complete Regioselectivity
H
ydration of
b
-
and
d
-
u
H
ydroxy Inte
g
rna
l
A
lkyne
i
s
o Nishizawa,* Takayuki Takemoto, Ikuo Sasaki, Mayo Nakano, Elisabeth Ho, Kosuke Namba,1
Hirofumi Yamamoto, Hiroshi Imagawa
Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan
Fax +81(88)6553051; E-mail: mugi@ph.bunri-u.ac.jp
Received 13 January 2009
C4H9
O
Abstract: The hydration of b- and d-hydroxy internal alkynes cat-
H2O (3 equiv)
alyzed by Hg(OTf)2 took place instantaneously to give ketones with
complete regioselectivity under mild conditions, whereas the hydra-
tion of internal alkyne without hydroxy moiety was very slow and
gave a mixture of ketones. If the hydroxy group is located more than
five carbons from the triple bond it has no significant effect upon the
hydration reaction.
C4H9
C4H9
C5H11
MeCN–CH2Cl2 (3:1)
r.t., 24 h
1
3
2
Hg(OTf)2⋅2TMU (5 mol%) 42%
Hg(OTf)2 (5 mol%)
73%
O
Hg(OTf)2
(1 mol%)
Key words: Hg(OTf)2, homogeneous catalysis, hydroxy internal
alkyne, regioselectivity, neighboring-group effects
OH
C5H11
OH
H2O (3 equiv)
MeCN, r.t., 5 min
4
C4H9
90%
Hydration of terminal alkynes giving rise to methyl ke-
tones is an important functional-group transformation in
modern organic synthesis.2 This reaction is usually carried
out by heating with a HgII salt and sulfuric acid.3 A variety
of metal salts such as PdII,4 PtII,5 AuIII,6 AuI,7 RhIII,8 CuII,9
and FeIII,10 as well as Brønsted acid11 have also been in-
vestigated. However, these alternative reagents did not
yield sufficiently good results to merit adoption as a new
standard procedure. A method involving RuII-complex-
catalyzed hydration of terminal alkyne leading to alde-
hyde via anti-Markovnikov addition has also been report-
ed.12 In 1983 we developed Hg(OTf)2 as a highly efficient
olefin cyclization agent.13 In 2002, the Hg(OTf)2–bistetra-
methylurea (TMU) complex was found to show highly ef-
ficient catalytic activity for the hydration of terminal
alkynes leading to methyl ketones under neutral mild con-
ditions.14 Since then a variety of Hg(OTf)2-catalyzed reac-
tions, such as hydroxylative enyne cyclization, arylyne
cyclization, biomimetic tandem cyclization, indole syn-
thesis, furan synthesis, cyclic carbonate synthesis, glyco-
sylation, SN2-reaction, hydroxylative enone synthesis, E-
selective conjugate ester synthesis, furanoyne cyclization,
aryl allyl alcohol cyclization, and N-allyl alcohol cycliza-
tion have been described.15a–15n,16 Very recently, the first
solid-supported mercuric salt, silaphenylmercuric triflate,
was developed that also acts as a powerful catalyst for
most Hg(OTf)2-catalyzed reactions.17
Scheme 1 Hg(OTf)2-Catalyzed hydration of alkyne
neously to give g- and d-hydroxy ketones, respectively.
The reaction proceeds in excellent yield, involving a
hydroxy-group participation, with complete regioselectiv-
ity. In contrast, hydration of an internal alkyne without
hydroxy moiety requires a longer reaction time and af-
fords a mixture of ketones.
When the hydration of dec-5-yne (1) was examined using
conditions described for the hydration of a terminal
alkyne using 3 equivalents of H2O in the presence of 5
mol% of Hg(OTf)2·2TMU complex in MeCN–CH2Cl2
(3:1) at room temperature,14 ketone 2 was obtained in only
42% yield after 24 hours (Scheme 1). Although Hg(OTf)2
itself showed higher reactivity, yield was still 73% after
24 hours. Then we examined the reaction of dec-5-yn-1-
ol (3) using 1 mol% of Hg(OTf)2 in the presence of H2O
(3 equiv, Scheme 1). The reaction in MeCN–CH2Cl2 (3:1;
the original solvent system employed for the hydration of
terminal alkyne)14 afforded ketone 4 in 79% yield within
10 minutes as a sole product (Table 1, entry 1).19 Al-
though MeNO2, CH2Cl2, and THF afforded poor results
for this hydration, toluene afforded 4 in 80% yield within
10 minutes. Acetonitrile was the solvent of choice afford-
ing 4 in 90% yield after only 5 minutes reaction followed
by column chromatography on silica gel (entry 6).20a
A
catalyst loading of 0.1 mol% was also enough to give 4 in
94% yield within an acceptable reaction time. However, a
catalyst loading of 0.05 mol% required 20 hours to give 4
in 95% yield.
Here, we investigated Hg(OTf)2-catalyzed hydration of
internal alkynes. Very few studies have been conducted
on this reaction, mainly due to the poor regioselectivity.18
We found that Hg(OTf)2-catalyzed hydration of b- and d-
hydroxy internal alkynes takes place almost instanta-
The reaction involves an equilibrium between exo-
cyclized product 5 and endo-cyclized product 6, and the
favored 5 produces ketone 4 selectively via rate-determin-
ing protonation by in situ generated TfOH leading to 7,
SYNLETT 2009, No. 7, pp 1175–1179
x
x.
x
x.
2
0
0
9
Advanced online publication: 20.03.2009
DOI: 10.1055/s-0028-1088153; Art ID: U00409ST
© Georg Thieme Verlag Stuttgart · New York

1176
M. Nishizawa et al.
LETTER
In contrast, the hydration of the protected hydroxyalkyne
required long reaction time as is the case of dec-5-yne (1).
The reaction of acetate 3b with H2O (3 equiv) in the pres-
ence of 1 mol% of Hg(OTf)2 in MeCN at room tempera-
ture for 24 hours afforded a 1:1 mixture of ketones 4b and
9b in only trace amounts. The hydration of Boc-protected
3c also took place very slowly to give a similar result. We
reasoned that the observed significant acceleration of the
hydration of 3 originated from the participation of the hy-
droxy moiety thereby producing a cyclic vinyl ether inter-
mediate, such as 5. By using 5 mol% of Hg(OTf)2, a 1:1
mixtures of 4b and 9b, as well as 4c and 9c were obtained
in 65% and 60% yields, respectively, after 24 hours reac-
tion time (Scheme 3). The ratios were determined by
HPLC as well as 13C NMR.
Table 1 Hg(OTf)2-Catalyzed Hydration of Dec-5-yn-1-ol (3)
Entry
Hg(OTf)2 Solvent
(mol%)
Time
Yield 4
(%)a
1
2
3
4
5
6
7
8
1
MeCN–CH2Cl2 (3:1)
10 min
3 h
79
63
76
54
80
90
94
95
1
MeNO2
CH2Cl2
THF
1
2 h
1
24 h
1
toluene
MeCN
MeCN
MeCN
10 min
5 min
2.5 h
20 h
1
0.1
0.05
a Isolated yield.
OR
Hg(OTf)2
H2O (3 equiv)
C4H9
3b,c
MeCN, r.t., 24 h
Hg(OTf)2
3
O
O
TfOHg
O
O
TfOHg
OR
C4H9
C5H11
OR
C4H9
C4H9
5
6
4b,c
9b,c
TfOH
b: R = Ac, c: R = Boc
H2O
4
+
O
TfOHg
O
Scheme 3 Hg(OTf)2-Catalyzed hydration of protected 5-alkyn-1-ol
H
C4H9
7
8
C4H9
Next we investigated the hydration of a variety of alkynyl-
1-ol, and the results are summarized in Table 2. The reac-
tion of hept-2-yn-1-ol (10) with 3 equivalents of H2O in
the presence of 5 mol% of Hg(OTf)2 in MeCN at room
temperature was very slow, and g-keto alcohol 11 was ob-
Scheme 2 Mechanism of the selective hydration of 3 to 4
subsequent demercuration to give 8, and following hydra-
tion.
Table 2 Hg(OTf)2-Catalyzed Hydration of Alkynyl Alcoholsa
Substrate
Hg(OTf)2 Time Product
(mol%)
Yield (%)b
O
O
O
OH
5
1
1
1
24 h
60
C4H9
C4H9
OH
10
11
OH
5 min
5 min
24 h
82
OH
OH
C4H9
C4H9
12
13c
O
OH
85 (15/16 = 6:1)
75 (18/19 = 3:1)
C4H9
C4H9
OH
C5H11
15c
16c
14
O
O
OH
OH
C4H9
C4H9
OH
C5H11
17
18
19
O
OH
O
1
5
24 h
24 h
51 (21/22 = 1:1)
98 (21/22 = 1:1)
OH
C4H9
C4H9
C5H11
OH
20
21
22
a Reaction by using Hg(OTf)2 (1 mol%) and H2O (3 equiv) in MeCN at r.t.
b Yield after column chromatography on SiO2.
c Characterized by the derivatization to acetate.
Synlett 2009, No. 7, 1175–1179 © Thieme Stuttgart · New York

LETTER
Hydration of b- and d-Hydroxy Internal Alkynes
1177
tained in 60% yield after 24 hours.20b In sharp contrast, the The result suggested that the equilibrium between endo-
reaction of oct-3-yn-1-ol (12) was completed within 5 14 and exo-14 is energetically comparable to producing
minutes to give 1-hydroxyoctan-4-one (13) in 82% both 15 and 16, respectively. The structures of 15 and 16
yield.20c Because 13 was obtained as a mixture of keto al- were confirmed after derivatization to their acetates. The
cohol and hemiacetal forms, the structure was confirmed reaction of dodec-6-yn-1-ol (17) was slow, and a 3:1 mix-
by the derivatization to the corresponding acetate upon ture of 18 and 19 was obtained in 75% yield after 24
treatment with acetic anhydride in pyridine. The high hours. Undec-7-yn-1-ol (20) also afforded a 1:1 mixture
regioselectivity should be induced from the favored five- of 21 and 22 in 51% and 98% yields by using 1 mol% and
membered ring intermediate endo-12 than the less favored 5 mol% catalyst, respectively, after 24 hours.21
four-membered ring intermediate exo-12 (Scheme 4). The
The Hg(OTf)2-catalyzed hydration of sec- as well as tert-
hydration of g-hydroxyalkyne, non-3-yn-1-ol (14), also
b-, g-, and d-hydroxyakynes were also examined as shown
appeared to take place instantaneously, although in this
in Table 3. Hydration of non-4-yn-2-ol (23) using 1 mol%
of Hg(OTf)2 and 3 equivalents of H2O in MeCN took
case a 6:1 mixture of 1-hydroxynonan-4-one (15)20d and
1-hydroxynonan-5-one (16)20e was obtained in 85% yield.
place smoothly to give 2-hydroxynonan-5-one (24) in
Table 3 Hg(OTf)2-Catalyzed Hydration of sec- and tert-Alkynyl Alcoholsa
Substrate
Product
Yield (%)b
93
O
C4H9
OH
OH
C4H9
OH
OH
23
24
O
82
C4H9
C4H9
25
26
OH
O
O
O
OH
OH
78 (28/29 = 3:1)
77 (31/32 = 4:1)
C5H11
C4H9
C4H9
OH
29c)
28c
27
OH
O
C5H11
C4H9
C4H9
OH
32
31
30
OH
OH
O
O
95
89
OH
OH
C4H9
C5H11
33
34
C5H11
C4H9
36
35
OH
OH
O
O
87
97
C5H11
OH
OH
C4H9
38
37
C5H11
C4H9
39
40
a Reaction by using Hg(OTf)2 (1 mol%) and H2O (3 equiv) in MeCN at r.t. for 10 min.
b Isolated yield after column chromatography on SiO2.
c Characterized after derivatization into acetate.
Synlett 2009, No. 7, 1175–1179 © Thieme Stuttgart · New York

1178
M. Nishizawa et al.
LETTER
References and Notes
O
O
TfOHg
13
(1) Present address: Division of Chemistry, Graduate School of
Science, Hokkaido University, Kita-ku, Saporo 060-0810,
Japan.
TfOHg
endo-12
C4H9
C4H9
exo-12
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15
16
O
C4H9
TfOHg
O
TfOHg
endo-14
C4H9
exo-14
Scheme 4 Equilibrium affording 13 selectively and a mixture of 15
and 16
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93% yield selectively within 10 minutes at room temper-
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dration of b-alkynyl tert-alcohol 25 using 1 mol% of
Hg(OTf)2 also took place smoothly to give 26 in 82%
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yield, respectively, based on the energetically comparable
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ly and instantaneously in 95% yield via exo-mode
participation.20h The tert-alcohol, 2-methyldodec-6-yn-2-
ol (35) was also quickly hydrated in the presence of 1
mol% of Hg(OTf)2 giving rise to 36 in 89% yield selec-
tively. Phenyl-substituted 37 similarly hydrated quickly to
give 38 in 87% yield as a sole product. Cyclobutanol de-
rivative 39 also instantaneously provided single keto alco-
hol 40 in 97% yield.
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In conclusion, we found that although the Hg(OTf)2-cata-
lyzed hydration of internal alkynes requires long reaction
time, a hydroxy group located in the b-, g-, and d-position
can significantly accelerate the reaction through the inter-
mediate formation of cyclic enol ethers. Although b-, and
d-hydroxyalkyne selectively generate a single keto alco-
hol, g-hydroxyalkyne affords a mixture of keto alcohols
via energetically comparable exo- and endo-mode cy-
clization. A hydroxy group located in more than five car-
bons from the alkyne does not participate in the hydration
reaction. Indeed, these alkynols behave like an internal
alkyne to give a mixture of ketones without regioselectiv-
ity. The sec- and tert-hydroxy groups also accelerate the
hydration of alkyne when they are located in the b- and d-
position to give keto alcohols in complete regioselectivity
via endo- and exo-mode participations, respectively.
Acknowledgment
This study was financially supported by a Grant-in-Aid from the
Ministry of Education, Culture, Sports, Science, and Technology of
the Japanese Government, and a MEXT.SENRYAKU, 2008.
(14) (a) Nishizawa, M.; Skwarczynski, M.; Imagawa, H.;
Sugihara, T. Chem. Lett. 2002, 12. (b) When the hydration
of terminal alkyne was carried out using Hg(OTf)2 as
catalyst, mercuric acetylide (i) is formed to some extent,
corresponding to catalyst suicide (Figure 1).
R
Hg
i
R
Figure 1
Synlett 2009, No. 7, 1175–1179 © Thieme Stuttgart · New York

LETTER
Hydration of b- and d-Hydroxy Internal Alkynes
1179
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(19) Typical Experimental Procedure
To a stirred mixture of dec-5-yn-1-ol (3, 280 mg, 2 mmol)
and H2O (108 mg, 6 mmol) in MeCN (6.6 mL) was added a
solution of Hg(OTf)2 (0.1 M MeCN soln, 0.2 mL, 0.02
mmol) at r.t., and the mixture was stirred for 5 min at the
same temperature. After addition of Et3N (30 mL) and then
brine (6 mL), the organic materials were extracted with
Et2O. Dried and concentrated extract was subjected to a
column chromatography on SiO2 using hexane and EtOAc
(2:1) as an eluent to give 1-hydroxydecan-5-one (4, 275 mg,
90% yield) as a colorless oil.20a
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spectroscopic methods.
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