Zn(ODf)2: preparation and application in asymmetric alkynylation of
aldehydes
Zili Chen, Wennan Xiong and Biao Jiang*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China. E-mail: jiangb@pub.sioc.ac.cn
Received (in Cambridge, UK) 16th July 2002, Accepted 29th July 2002
First published as an Advance Article on the web 8th August 2002
A new Lewis acid, Zn(ODf)2, was first prepared from
commercially available 3,3,4,4-tetrafluoro[1,2]oxathietane
2,2-dioxide in four steps with 56% yields and also was
applied to catalyze highly enantioselective alkynylation of
aldehydes in the presence of ligand (1S,2S)-3-(tert-butyldi-
methylsilyloxyl)-2-N,N-dimethylamino-1-(p-nitrophenyl)-
propane-1-ol or ligand (2)-N-methylephedrine to afford the
corresponding propargylic alcohols in high yields with up to
99% ee.
Fluorosulfonyldifluoroacetic acid 2, prepared from the hydrox-
ylation of compound 1, was decarboxylated with Na2SO4 in
CH3CN+H2O (1+1, v/v) to provide difluoromethanesulfonyl
fluoride 3, which was then treated in THF+H2O (1+1, v/v) to
afford difluoromethanesulfonic acid 4 (HCF2SO2OH) in 80%
yield (Scheme 1).
Zn(ODf)2 was easily obtained in quantitative yield by
treating difluoromethane sulfonic acid with ZnO. With
Zn(ODf)2 in hand, we sought to determine the value of this
inexpensive new Lewis acid.6
Organometallic compounds of weakly coordinating anions,
e.g., BF42, PF62, and CF3SO32, have been the object of many
studies.1 These Lewis acids contain a hard anion coordinated to
a soft metal center in a low oxidation state, in which the anions
are excellent leaving groups and can easily be replaced by other
chiral ligands (to give a chiral Lewis acid intermediate) and by
many s- and p-donors (such as alkene, alkyne and carbonyl
group, etc.; to give ionic complexes) under very mild condi-
tions, so they are highly reactive and have proven to be useful
materials either to synthesize other organometallic complexes
or to act as catalysts for asymmetric reaction. In this
contribution, we report the first example for the preparation of
zinc difluoromethanesulfate (zinc diflate, Zn(ODf)2), which
contains soft metal Zn2+ and hard anion HCF2SO32, and
demonstrating its application in asymmetric alkynylation of
aldehydes as a Lewis acid catalyst for the first time.
A preliminary study quickly revealed that Zn(ODf)2 readily
participated in the enantioselective alkynylation of isobutyr-
aldehyde to give the optically active propargylic alcohol in
excellent enantioselectivity (97% ee) and yield in the presence
of Et3N and ligand (1S,2S)-3-(tert-butyldimethylsilyloxyl)-
2-N,N-dimethylamino-1-(p-nitrophenyl)propane-1-ol 6 in tolu-
ene (Table 1, entry 1).4 The use of CH2Cl2 or THF instead of
toluene resulted in a notable decrease in enantioselectivity (93%
ee in CH2Cl2, 92% ee in THF), whereas in CH3CN, the optical
purity of the adduct is only 47% ee.10
The reactions with various aliphatic and aromatic aldehydes
using Zn(ODf)2 as additive are summarized in Table 1.† The
ligands which were used here included ligand (1S,2S)-6 and
(2)-N-methylephedrine 7. The representative results were
summarized in Table 1. For aliphatic aldehydes (entries 1–16),
the reaction was faster than that with aromatic aldehyde (entries
17–21) and was completed within 2 h. Based on comparison
with the literature,11 (R)-absolute configuration propargylic
alcohols were obtained with ligand (1S,2S)-6, while the
opposite configuration was obtained with ligand (1R,2S)-N-
methylephedrine 7. All of the propargylic alcohols were
obtained with excellent ee (up to 99%) and in as high a chemical
yield as with Zn(OTf)2 (Scheme 2).
A catalytic condition (0.22 equiv. ligand and 0.2 equiv.
Zn(ODf)2 as additive) was then studied and proved to be
effective to this reaction too, while the ee and the chemical
yields were slightly decreased (entry 22–26, Table 1).
In conclusion, a new Lewis acid, Zn(ODf)2, was first
prepared from commercially available 3,3,4,4-tetrafluoro-
[1,2]oxathietane 2,2-dioxide in four steps with 56% yields.
Highly enantioselective alkynylation of aldehydes was carried
out in the presence of Zn(ODf)2 and Et3N, catalyzed by ligand
(1S,2S)-3-(tert-butyldimethylsilyloxyl)-2-N,N-dimethylamino-
1-(p-nitrophenyl)propane-1-ol 6 or ligand (2)-(1R,2S)-N-
A previous study indicated that difluoromethanesulfonic acid
(colog KA = 5.7) is more acidic than sulfuric acid (colog KA
7.0) but weaker than trifluoromethanesulfonic acid (colog KA
=
=
4.7).2 However, there is still a very limited knowledge about the
synthesis, the application of its metal salts.
Asymmetric alkynylation of aldehydes in the presence of
Zn(OTf)2, Et3N and some amino alcohol ligands is a highly
selective reaction that can be performed under very mild
conditions.3,4 The reaction mechanism involves a hydrogen-
bonded s-complex of Zn2+, which undergoes tautomerization to
the hydrogen-bonded p-complexes of zinc cation.5 The struc-
tural similarity and the character nuance stimulated us to
speculate that zinc difluoromethanesulfate (zinc diflate,
Zn(ODf)2), a more economical6,7 and mild zinc salt, might be
attractive as a potential equivalent of Zn(OTf)2 for converting
terminal alkynes to the corresponding zinc alkynylides.
Cost-effective difluoromethanesulfonic acid (diflic acid) can
be easily prepared under mild conditions from commercially
available 3,3,4,4-tetrafluoro[1,2]oxathietane 2,2-dioxide 1,8 a
key monomer in the preparation of Nafion ion membrane resin.9
Scheme 1
Scheme 2
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CHEM. COMMUN., 2002, 2098–2099
This journal is © The Royal Society of Chemistry 2002