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R. Mello et al. / Tetrahedron Letters 51 (2010) 4281–4283
Table 1
1-ethynylcyclohexanol (1d) and 1,1,1-trifluoro-2-phenyl-3-buty-
nol (1e) react in 24 h (Table 1, entries 4 and 5). Internal alkynes re-
act more slowly than their terminal counterparts, which is probably
due to the steric or hydrophobic shielding of the substituents to
the interaction of the C–C triple bond with the catalysts on the
hydrophilic silica surface (Table 1, entries 10 and 11).
The hydration of 1,6-heptadiyne (1g) led to 3-methyl-2-cyclo-
pentenone (2g) as the main product with small amounts of 2,6-
heptadione (Table 1, entry 7). These results could be attributed
to either Hg-catalyzed diyne cyclization or the intramolecular
acid-catalyzed aldol condensation of the corresponding diketone
on the acidic silica surface. The absence of acid-catalyzed side reac-
tions for the tertiary alcohol 1d or the enyne 1i suggest that 3-
methyl-2-cyclopentenone (2g) might be formed mainly through
a Hg-catalyzed C–C-bond formation.
Hydration of alkynes (1) with hydrated silica-supported HgSO4/H2SO4 (3)a
Entry
1
2
t
(h)
Yieldb
(%)
O
1
2
91
91
1a
2a
O
2
2
1b
2b
HO
HO
O
3b
4
2
90
87
2c
1c
OH
OH
O
24
The results show that the water molecules coordinated with the
acidic sites of the silica surface (7.2 water molecules per sulfate an-
ion) are sufficiently nucleophilic to react with the electrophilic spe-
cies involved in the reaction. Higher loads of water on the
supported reagent led to slower reactions. For instance, the reac-
tion of phenylacetylene (1a) with a sample of 38% w/w hydrated
3 (molar ratio HgSO4/H2SO4/H2O 1:5.5:79) required 6 h for com-
pletion. These results suggest that excess water diminishes the
mass transfer efficiency at the water/dichloromethane interface
which acts as a barrier by inhibiting the access of the substrate
to the catalytic sites on the silica surface. The induction period ob-
served for substrates 1h, 1j, and 1k (entries 8, 10, and 11) was
attributed to this factor. Higher loads of HgSO4/H2SO4 did not im-
prove the reaction efficiency, but led to an extensive reduction of
the metal on the silica surface. Lower HgSO4/H2SO4 ratios led to
the formation of HgO on the silica surface and to the formation
of an inactive yellow solid. Silica-supported HAuCl4 and FeCl3
(20% hydrated, 1 and 3 mmol per gram of material, respectively)
were found inactive under the same conditions (conversions 6%
and 7%, respectively, after 24 h). Brönsted acids were more effec-
tive, thus the reaction of phenylacetylene (1a) with silica-sup-
ported H2SO4 (2.5 mmol per gram of material) under the same
conditions led to the complete conversion of the substrate after
120 h. The strongly hygroscopic nature of this solid material pre-
vented a precise determination of water content. It is likely that
the highly hydrated silica surface both diminished the acidity of
the catalysts and hindered the approach of the substrate to the cat-
alytic sites.
1d
2d
CF3
CF3
OH
OH
5
6
24
5
89
86
O
2e
1e
O
O
O
OCH3
OCH3
2f
1f
O
7
6
85c
1g
2g
COOCH3
COOCH3
O
COOCH3
COOCH3
8
9
17d
20e
95
1h
2h
O
57e
1i
2i
O
O
Ph
Ph
10
11
16d
99f
99f
1j
2j
2j’
O
O
1k
15d
In summary, hydrated silica-supported HgSO4/H2SO4 is an ade-
quate and inexpensive reagent to perform the hydration of alkynes
(1). The reaction proceeds efficiently in dichloromethane under
mild conditions, and the isolation of products involves filtering
and evaporating the solvent. The results show that supported
aqueous-phase catalyst (SAPC) is a suitable approach to perform
the hydration of alkynes with water-soluble mercury salts in an
acid medium.
2k
2k’
a
Reactions carried out in dichloromethane at 40 °C with hydrated 3 (5 mol % of
HgSO4) except where noticed; substrate conversion was complete in all cases.
b
Isolated yield after removing the solvent.
The reaction mixture contained a 7% of 2,6-heptadione.
c
d
10 mol % of HgSO4.
e
Reaction with 10 mol % of HgSO4 at room temperature in a closed vial; GC
analysis showed complete conversion to give 2i as the only product.
f
Ratios 2j/2j0 and 2k/2k0 8.1:1 and 1.8:1, respectively.
Acknowledgments
The results show that silica-supported HgSO4/H2SO4 (3) is very
efficient to perform the hydration of alkynes under these condi-
tions. The reaction work-up significantly improved compared to
those procedures which use soluble Hg(II) salts under biphasic
conditions or in aqueous solvents. The solid which was recovered
once the reaction had been completed is olive green in color with
a loose appearance and can be reused at least three times with a
similar activity.
Financial support from the Spanish Dirección General de Inves-
tigación and Fondos Feder, (CTQ2007-65251/BQU) and Consolider
Ingenio 2010 (CSD2007-00006) is gratefully acknowledged. A.A.A.
thanks the Spanish Ministerio de Educación y Ciencia for fellow-
ships. We also thank the SCSIE (Universidad de Valencia) for allow-
ing us access to their instruments and facilities.
The reaction is very efficient for the alkynes with electron-with-
drawing or sterically hindered substituents (Table 1, entries 4 and
5). For instance, phenylacetylene (1a) reacts in 2 h while methyl
2-pentinoate 1h reacts in 5–6 h (Table 1, entries 1 and 6), and both
Supplementary data
Supplementary data associated with this article can be found, in