Versatile methodology to hydrate alkynes, in the presence of a wide variety of functional groups, with mercury(II) p-toluensulfonamidate, under catalytic, mild, and neutral conditions

Article abstract of DOI:10.1080/00397911.2012.686003

A method to generate carbonylic compounds from alkynes under mild and neutral conditions, with excellent functional group compatibility and good yields, is described. Hydration takes place under catalytic conditions by using 0.1 to 0.2 equivalents of the easily available and inexpensive mercury(II) p-toluensulfonamidate in a hydroalcoholic solution. After use, the catalyst is rendered inert and/or recycled. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.686003

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Versatile Methodology to Hydrate
Alkynes, in the Presence of a Wide
Variety of Functional Groups, with
Mercury(II) p-Toluensulfonamidate,
Under Catalytic, Mild, and Neutral
Conditions
Albert Corominas a & Ángel M. Montaña a
a Unidad de Química Orgánica Industrial y Aplicada, Departamento
de Química Orgánica, Facultad de Química, Universidad de
Barcelona, Barcelona, Spain
Accepted author version posted online: 22 Jan 2013.Published
online: 28 Apr 2013.
To cite this article: Albert Corominas & Ángel M. Montaña (2013): Versatile Methodology to
Hydrate Alkynes, in the Presence of a Wide Variety of Functional Groups, with Mercury(II) p-
Toluensulfonamidate, Under Catalytic, Mild, and Neutral Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:15, 2062-2072
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Synthetic Communications1, 43: 2062–2072, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.686003
VERSATILE METHODOLOGY TO HYDRATE ALKYNES,
IN THE PRESENCE OF A WIDE VARIETY OF
FUNCTIONAL GROUPS, WITH MERCURY(II)
P-TOLUENSULFONAMIDATE, UNDER CATALYTIC,
MILD, AND NEUTRAL CONDITIONS
´
Albert Corominas and Angel M. Montana
˜
Unidad de Quımica Organica Industrial y Aplicada, Departamento de
´
Quımica Organica, Facultad de Quımica, Universidad de Barcelona,
´
´
Barcelona, Spain
´
´
GRAPHICAL ABSTRACT
Abstract A method to generate carbonylic compounds from alkynes under mild and neutral
conditions, with excellent functional group compatibility and good yields, is described.
Hydration takes place under catalytic conditions by using 0.1 to 0.2 equivalents of the easily
available and inexpensive mercury(II) p-toluensulfonamidate in a hydroalcoholic solution.
After use, the catalyst is rendered inert and/or recycled.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Acetylene; carbonylic compounds; catalyst; hydration; mercury(II) p-toluen-
sulfonamidate; regioselectivity; stereoselectivity
Received March 29, 2012.
Address correspondence to Angel M. Montan˜a, Departamento de Quımica Organica, Facultad de
´
´
Quımica, Universidad de Barcelona, c=Martı Franques 1-11, 08028, Barcelona, Spain. E-mail:
´
´
angel.montana@ub.edu
´
`
2062

CATALYTIC HYDRATION OF ALKYNES
2063
INTRODUCTION
The addition of water to alkynes to obtain carbonyl compounds is a very useful
reaction in organic chemistry.[1] The traditional methods based on mercury(II) cat-
alysts[2] hydrate internal and terminal alkynes to give the Markovnikov adduct.
Usually acid is required to form the carbonyl product in typical methods. The use
of an acid considerably reduces the functional group compatibility and narrows
the scope of the methodology. When using a buffered system such as mercury(II)
triflate–N,N,N0,N0-tetramethylurea (TMU) complex,[3] good functional group
compatibility is observed except for substrates containing acetal or benzyl groups.[4]
Other methods have been reported that use catalysts based on gold(I),[5] gold
(III),[6] platinum(II),[7] palladium(II),[8] rhodium (III),[9] and ruthenium(II)[10] to
form the anti-Markovnikov product. These are very efficient catalysts but their main
disadvantage is the cost of the catalysts based on noble metals.
This article reports a catalytic method under neutral conditions based on
the inexpensive reagent mercury(II) p-toluensulfonamidate.[11] The efficiency of this
reagent in the hydration of terminal alkynes has been proven, under stoichiometric
conditions, for the total synthesis of 1,7-epoxycyclononanes,[12] affording intermedi-
ate diketones in excellent yields (Table 1). Under these reaction conditions, polymer-
ization and epimerization of the substrates (especially at the a position of ketones)
were avoided due the neutral and mild conditions used.
Herein we report that mercury(II) p-toluensulfonamidate is a highly effective
and inexpensive catalyst with an excellent functional group tolerance for the
hydration of terminal and internal alkynes, even with acetal or benzyl substrates.
Preparation of mercury(II) p-toluensulfonamidate is six times cheaper than
Hg(OTf)2 ꢀ TMU and approximately 100 times lower in cost than Au(I) or Au(III)
catalysts. Furthermore we report the chemoselective hydration of the most accessible
alkyne group in some dialkyne substrates. On the other hand, the use of small
catalytic amounts of this catalyst, together with its inertization and=or recovery,
avoids possible toxic and=or environmental impacts.
RESULTS AND DISCUSSION
In our initial assays, we worked under stoichiometric conditions of Hg(TsNH)2
with the 8-oxabicyclo[3.2.1]octane substrates (Table 1). The main characteristic of
these substrates is the presence of a protected propargylic alcohol group and one
stereocenter in a to the carbonyl group (on C-3), sensitive to epimerization.
Upon treatment of 1 with 1.3 eq. of mercury(II) sulfonamidate in a (85:15) mix-
ture of ethanol–water as a solvent for 24 h, the dicarbonyl product 2 was obtained in
quantitative yield. It is important to note that the benzyl group (Table 1, entry 1)
does not cleave at all upon the hydration reaction. Also the acetalic group methox-
ymethyl remains stable (Table 1, entry 4). Thus, the sensitive substrate 7 that con-
tains the acetalic protecting group methoxymethyl (OMEM) was converted into 8
in quantitative yield.
The methoxy and ethoxy substrates 3 and 5 also afforded the hydration
products 4 and 6 with good yields. Finally, the hydration reaction of 9 also gave
the carbonylic product 10 with good yield.

´
˜
A. COROMINAS AND A. M. MONTANA
2064
Table 1. Hydration of terminal alkynes of 1,7-epoxycyclononane substrates with Hg(TsNH)2 under
stoichiometric conditions (1:1.3 eq.)
Entry
R group
Substrate
Time (h)
Product
Yield (%)a,b,c
1
2
3
4
5
OBn
OMe
OEt
1
3
5
7
9
24
23
18
24
24
2
4
92–100
85
85
6
OMEM
H
8
10
98
85
aIsolated yields.
bThe reaction yields are quantitative by NMR or GC (using internal standard).
cConversion in all cases is 100%.
In all these reactions the epimerization product at the a carbons of both ketone
groups, in the final product, was not observed. However, this problem was detected
previously by us working with other hydration methods that need the use of an
acid (i.e., HgSO4=H2SO4). Moreover, by working with Hg(TsNH)2 under neutral
conditions the R group at the propargylic position was not cleaved at all.
The next step in our project was to investigate how the reaction took place
under catalytic conditions and the functional group compatibility (Table 2). Pheny-
lacetylene 11 reacted under catalytic conditions, using 0.1 eq of Hg(TsNH)2, to form
acetophenone 12 in excellent yield, which indicates that the electron delocalization of
the triple bond towards the aromatic ring does not interfere in the reaction.
Hydration of 1-heptyne 13 to afford 2-heptanone 14 demonstrates that the reaction
works with good yield for substrates with alkyl linear chains. A dialkyne substrate 15
was also dihydrated with 0.17 eq. into 2,7-octadienone 16. Methyl ester 17, nitrile 19,
and chloride 21 groups did not interfere with the catalytic cycle and completely con-
verted into the ketone products 18, 20, and 22, respectively.
Because of the poor solubility of substrates 23 and 27 in methanol, the reac-
tions were carried out in (85:15) ethanol–water as a solvent. The hydration of both
products gave 24 and 28 in good yields. The dimethoxyacetal substrate 25 was
efficiently converted into product 26, without observing any degree of hydrolysis
of this acid-sensitive group, under these mild neutral conditions. Finally, alcohol
29 was converted into the ketone 30 in good yield, and no anchimeric influence of
the hydroxyl group was observed on the yield and regioselectivity of the reaction.
Most of substrates (Table 2) also reacted with mercury(II) p-toluensulfonami-
date under stoichiometric conditions (1:1.3 eq.), with similar reaction times, conver-
sions, and yields. The reaction also converted substrates with internal CꢁC bonds
into carbonyl compounds (Table 3). Because of the characteristics of the catalyst,
the selective hydration of the terminal alkyne group in front of the internal triple
bonds is accomplished, under reflux conditions, on substrates in which one of the
CꢁC bonds is sterically hindered. This steric-controlled chemoselectivity could be
of interest in synthetic organic chemistry.

CATALYTIC HYDRATION OF ALKYNES
2065
Table 2. Chemoselective hydration of alkynes having different functional groups using catalytic amounts
of Hg(TsNH)2
Eq. Hg(TsNH)2
(mmol=mmol
of substrate)
Conversion
(%)
Yield
(%)a,b,c
Entry
Substrate
Conditions
Product
MeOH=H2O
Reflux, 20 h
d
1
2
3
0.10
0.20
0.17
100
100
100
80c
78c
82c
d
d
MeOH=H2O
Reflux, 14 h
MeOH=H2O
Reflux, 39 h
MeOH=H2O
Reflux, 22 h
d
4
0.11
100
98
d
MeOH=H2O
Reflux, 21 h
5
6
7
0.08
0.11
0.10
100
100
100
93
88
80
MeOH=H2O
Reflux, 20 h
c
c
EtOH=H2O
Reflux, 66 h
c
MeOH=H2O
Reflux, 48 h
8
0.21
100
85
c
c
EtOH=H2O
Reflux, 24 h
9
0.20
0.20
100
100
80
93
MeOH=H2O
Reflux, 38 h
10
aIsolated yields (by column chromatography).
bThe reaction yields are quantitative by NMR and GC (using internal standard).
cThe reactions have been performed several times at different scales (20–300 mg), showing robustness.
However, a loss of mass has been observed in the case of low-boiling products during workup, especially
for those products that form azeotropes with the solvents. Alternative isolation procedures for these pro-
ducts are described in the experimental section.
dCommercially available substrates.
eSynthesized substrates.
The nonhindered dialkyne substrate 31 was used as a standard substrate and it
was hydrated under catalytic conditions to afford the dicarbonyl products 32 and 33
in a 1:1 ratio. In this case, as expected, no chemoselectivity was observed, and both
CꢁC bonds were hydrated, but with complete regioselectivity for the terminal CꢁC
bond. 1,1,3-Triphenylpropargyl alcohol 34 did not react under reflux of solvent even
with 1.3 eq. of mercury(II) p-toluensulfonamidate, even for long reaction times,
because of the steric hindrance around the CꢁC bond. The reaction conditions were
forced by using microwave irradiation and then the hydration took place regioselec-
tively, by insertion of the hydroxyl group on the carbon atom a to the aromatic ring.
Product 35 and the dehydration product 36 were obtained (in a ratio of 1:6), because
35 is a b-hydroxyketone that dehydrates to afford the highly conjugated a,b-unsatu-
rated ketone 36.

´
˜
A. COROMINAS AND A. M. MONTANA
2066
Table 3. Chemoselective hydration catalyzed by Hg(TsNH)2 of dialkynes having two differently hindered
CꢁC bonds
Eq.
Product 2 Hg(TsNH)2
Conversion Yield
(%)a
Entry
Substrate
Conditions
Product 1
(%)
b
EtOH=H2O
Reflux, 16 h
1
2
3
0.2
100
0
95
—
93
—
MeOH=H2O
Reflux, 24 h
c
—
—
1.3
0.2
c
MeOH=H2O,
microwave,
130 ꢂC,
96
8 bar, 27 h
MeOH=H2O
Reflux, 24 h
b
4
5
0.2
0.2
82
80
98
MeOH=H2O,
microwave,
130 ꢂC,
b
100
8 bar, 20 h
aIsolated yields (by column chromatography).
bSynthesized substrates.
cCommercially available substrates.
Then, this hydration methodology was studied for substrate 37, a diacetylene
having a terminal -CꢁCH bond and a hindered -CꢁC-CR3 triple bond. This study
was performed under two different reactions conditions: a) Under reflux of solvent,
only the methyl ketone was obtained and the internal triple bond was not hydrated
at all, affording 38 in 80% yield. No other products were detected by NMR or gas
chromatogrphy (GC), which demonstrates chemoselective discrimination. (b) Under
microwave conditions, substrate 37 was completely converted; 62% of the substrate
was dihydrated and formed the 1,5-dicarbonylic compound 39, which lost the
Scheme 1. General overview of the alkyne hydration with mercury(II) p-toluensulfonamidate under mild
and neutral conditions.

CATALYTIC HYDRATION OF ALKYNES
2067
Scheme 2. Proposed mechanism for the regioselective formation of 1,5-diketones by anchimeric assistance
of the initially formed carbonyl group.
alcohol group. The reaction also afforded the monohydrated product 38 in 36% yield.
In this case, an unexpected regioselective hydration of the internal alkyne was
observed, and a 1,5-dicarbonyl compound was obtained, instead of a 1,6-diketone,
which is the product expected from the usual hydration mechanism, because of the
insertion of the Hg-ligand entity on the less hindered acetylenic carbon (Scheme 2).[13,14]
The explanation for this result could be based on the fact that the hydration
of the terminal alkyne is faster than that of the internal one, and thus the product 38
is obtained as a reaction intermediate. In a second step, 1,5-dicarbonyl compound
39 is formed instead of the 1,6-diketone by the anchimeric assistance of the
initially formed carbonyl group, according to a mechanism previously reported[15]
(Scheme 2).
The general reaction mechanism proposed (Scheme 3) consists of a catalytic
cycle.[16] The first step is the coordination of Hg(II) to the triple bond (I) fol-
lowed by the addition of water to the carbon, which better supports the partial
Scheme 3. Proposal of a catalytic cycle for the hydration process.

´
˜
A. COROMINAS AND A. M. MONTANA
2068
positive charge (II), avoiding interactions of the R group with the Hg-ligand
entity.
Then the b-hydroxyethynyl-mercury intermediate (III) is formed, which after
a proton exchange forms the oxonium cation (IV). Intermediate (IV) could afford
the a-mercuriated carbonyl compound (V), which has been characterized by x-ray
diffraction analysis in previous studies.[13]
CONCLUSIONS
We may conclude that we have developed a catalytic method of hydrating
terminal and internal triple bonds under mild and neutral conditions. The hydration
reaction with mercury(II) p-toluensulfonamidate does not cleave the acid-sensitive
groups: dimethoxyacetal, benzyl, and propargylic methoxymethyl, methoxy and
ethoxy groups. The reaction conditions are compatible with a wide variety of other
functional groups as azides, alcohols, nitriles, esters, ketones, or terminal double
bonds. These neutral conditions also avoid epimerization of enolizable substrates.
This catalytic approach and the recovery of the insoluble mercury by-products by
filtration or centrifugation considerably minimize the environmental and safety
impacts of the process.
Last but not least, this methodology is also much more economical than other
hydration reactions of CꢁC triple bonds. Further efforts are under way in our lab-
oratory to study the reaction under conditions very different from those reported
here and using much smaller amounts of catalysts.
EXPERIMENTAL
NMR spectra were recorded on a Varian Inova 300 and=or a Mercury 400
apparatus. Chemical shifts (d) are expressed in parts per million (ppm) versus
tetramethylsilane (TMS) as an internal standard. Infrared (IR) spectra were
recorded on a Nicolet 6700 fourier transform (FT)–IR by film, KBr pellet, or
ATR (attenuated total reflectance) methods. Mass spectrometry was performed
on a Hewlett-Packard 5890 apparatus, generally under a CI (chemical ioniza-
tion) method using NH3 or CH4 or by direct insertion under electron impact
at 70 eV and 150 ꢂC. The elemental analyses were obtained in a Fisons elemental
analyzer, model Na-1500. The samples were previously pyrolized at 1000 ꢂC
under an oxygen atmosphere, and the contents of carbon and hydrogen were
determined by evaluation of the combustion gases by gas chromatography
using a flame ionization detector. All solvents were dried according to standard
procedures and distilled prior to use. All other major chemicals were obtained
from commercial sources and used without further purification. Gas chromato-
graphy was perfomed by using a Shimatzu AOC-20i apparatus with a capilary
column HP-5 crosslinked 5% Ph-Me-siloxane(L ¼ 30 m, (/i ¼ 0.32 mm, film
thickness ¼ 0.25 mm) under the following experimental conditions: Ti ¼ 35 ꢂC,
ti ¼ 1 min, rate ¼ 5 ꢂC=min, Tf ¼ 250 ꢂC, tf ¼ 5 min, split 1:100; gas brands and
´
preasures are He ¼ 5.5 bar (Linde, Helio 5.0), air ¼ 3 bar (Linde, Aire Sintetico),
H2 ¼ 3 bar (Linde, H2 5.0).

CATALYTIC HYDRATION OF ALKYNES
2069
Preparation of Catalyst
Preparation of p-toluensulfonamide.
Commercial tosyl chloride (117.73 g, 0.61 mol) dissolved in acetonitrile (590 mL) was
slowly added to a magnetically stirred solution of ammonium hydroxide (197 mL,
3.09 mol) under an Ar atmosphere. The reaction mixture was stirred for 30 min.
After completion of the reaction, the excess of ammonium hydroxide was neutralized
by using concentrated HCl. Then, the solvent was evaporated, and a white solid was
formed. The solid was washed with water (to remove NH4Cl) and dried under vac-
uum, affording pure product as white solid (87.78 g, 83%). Mp: 137–138 ꢂC (EtOH).
1H NMR (300 MHz, CDCl3) d ppm: 7.82 (d, J ¼ 8.1 Hz, 2H, H2, H6), 7.32 (d,
J ¼ 8.1 Hz, 2H, H3, H5), 4.72 (br s, 2H, H10), 2.44 (s, 3H, H7).13C NMR
(100 MHz, CDCl3) d ppm: 143.71 (C4), 139.03 (C1), 129.70 (C3,C5), 126.44
(C2,C6), 21.51 (C7). IR (KBr) n: 3329 (N-H, st), 3232 (N-H, st), 3017 (Csp2-H,
3
=
st), 2919 (Csp -H, st), 1911–1793 (comb), 1588 (N-H, d), 1322 (S O, st as),
1015 cmꢃ1. MS [DIP-EI, 70 eV, 150 ꢂC, m=z (%)]: 171 (Mþ, 40), 169 (Mþ-H, 10),
156 (Mþ-Me, 16), 155 (Mþ-NH2, 50), 91 (Mþ-SO2NH2, 100), 65 (43). HRMS: Calcd.
for C7H9NO2S: 171.03540. Found: 171.03536.
Preparation of mercury(II) p-toluensulfonamidate.
p-Toluensulfonamide (29.66 g, 173.2 mmol) and HgO (18.28 g, 83.1 mmol) were
ground together in a mortar to get a very fine and homogeneous powder. The
mixture was placed in a crucible and warmed up to 200 ꢂC in an oven for 1.5 h.
The mixture was stirred with a glass bar every 15 min. Once the orange color of
HgO disappeared, the reaction was complete. The crucible was cooled down to room
temperature and the solid was transferred to a round-bottomed flask fitted with a
reflux condenser. EtOH (150 mL) was added, and the solution was refluxed for
30 min to dissolve unreacted p-toluensulfonamide excess. Then the mixture was
filtered when hot. After lixiviation, the resulting residue was dried in a desiccator
containing phosphorus pentoxide for 1 day to obtain pure product as a white solid
(36.12 g, 89%). Mp: 249.8–250.7 ꢂC (in closed capillary tube, previously purged with
Ar). IR (KBr): 3307 (N-H, st), 3036-3024 (Csp2-H, st), 2978 (Csp3-H, st), 1910–1652
(comb), 1276 (S O, st, as), 1139 (S O, st, sym), 945 (S-O, st), 812 cmꢃ1.1H NMR
(400 MHz, DMSO-d6) d ppm: 7.68 (d, J ¼ 8.0 Hz, 4H; H2, H6, H20, H60), 7.34 (d,
J ¼ 8.0 Hz, 4H; H3, H5, H30, H50), 2.35 (s, 6H, H7 and H70). Calcd. for
C14H16HgN2O4S2: C, 31.08; H, 2.98; N, 5.18; S, 11.85%. Found: C, 31.11; H,
3.01; N, 5.15; S, 11.83%.
=
=

´
˜
A. COROMINAS AND A. M. MONTANA
2070
Hydration Procedures
Procedure for the hydration reaction with mercury(II) p-toluensulfona-
midate under stoichiometric conditions. To a stirred suspension of mercury(II)
p-toluensulfonamidate (892 mg, 1.65 mmol) in a mixture of MeOH–H2O (85:15)
(30 mL) under Ar atmosphere, methyl hept-6-ynoate (17) (178 mg, 1.27 mmol) in
the same solvent system (5 mL) was added. The mixture was warmed up to reflux
of solvent and stirred for 13 h.
Procedure for the hydration reaction with mercury(II) p-toluensulfona-
midate under catalytic conditions. To a stirred solution of mercury(II) p-toluen-
sulfonamidate (16.4 mg, 0.03 mmol) in a mixture of MeOH–H2O (85:15) (20 mL)
under Ar atmosphere and warmed up to reflux, 11-chloroundec-1-yne (21) (51 mg,
0.28 mmol) in the same solvent system (5 mL) was added. The mixture was stirred
under reflux of solvent for 20 h.
Workup Procedures
Usual workup procedure for isolation of low-boiling-point products.
To a stirred solution of mercury(II) p-toluensulfonamidate (225 mg, 0.42 mmol) in
a mixture of MeOH–H2O (85:15) (50 mL), 1-heptyne (13) (200 mg, 2.08 mmol) in
the same solvent system (10 mL) was added, under an Ar atmosphere. The mixture
was warmed up to reflux of solvent and stirred under these conditions for 14 h. After
completion of the reaction (control by TLC, CG), the mixture was cooled down to
room temperature and the mercury(II) catalyst precipitated with an aqueous 10%
(w=w) solution of (NH4)2S (0.6 mL). The precipitate of HgS was removed by centri-
fugation. Then, the mother liquor was extracted with n-pentane (ꢄ6), the organic
layers were combined together, dried over MgSO4, and filtered. Pentane was
removed by slow simple distillation at atmospheric pressure, using a Vigreux
column, affording pure product (14)(186.2 mg, 78%).
Usual workup for high-boiling-point products. To a stirred solution of
mercury(II) p-toluensulfonamidate (74.4 mg, 0.14 mmol) in a mixture of MeOH–
H2O (85:15) (40 mL), 5-hexynenitrile (19) (165 mg, 1.72 mmol) in the same solvent
system (6 mL) was added, under an Ar atmosphere. The mixture was warmed up
to reflux of solvent and stirred under these conditions for 21 h. After completion
of the reaction (control by TLC, CG), the crude mixture was cooled down to room
temperature and an aqueous 10% (w=w) solution of (NH4)2S (0.2 mL) was added.
The resulting mixture was filtered through a short column of silica gel and alumina
(1:1) to remove HgS, which at the end was collected and processed properly. The
hydro-alcoholic solution was concentrated to remove alcohol (MeOH or EtOH),
and the resulting residue was dissolved in Et2O. The ethereal solution was extracted
with NaOH (2 M) (ꢄ3). The aqueous layers were combined and extracted again with
diethyl ether. The organic layers were combined together, extracted with brine, dried
over MgSO4, filtered, and concentrated to dryness. The crude product was then
submitted to a flash column chromatography on silica gel, eluting with mixtures
of hexane and ethyl acetate (or diethyl ether) of increasing polarity, obtaining with
H=AcOEt 50:50 pure product (20) (178.4 mg, 93.3%).

CATALYTIC HYDRATION OF ALKYNES
2071
SUPPORTING INFORMATION
Synthetic procedures for the acetylenic substrates and complete spectroscopic
characterization of acetylenes and their hydration products are included in the
1
Supplementaly Material (S1), available online. In addition, copies of H NMR,
13C NMR, and IR spectra of acetylenic substrates and hydration products are
included also as Supplementarly Material (S2) and are available at the Web page
of this journal.
ACKNOWLEDGMENTS
We acknowledge the financial support from the Spanish Ministry of
Science and Innovation (Project CTQ-2007-64843). We also thank the University
of Barcelona for financial support (UBVRR=2012: AR000126).
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