Carbon-carbon bond forming reactions by using bistrifluoromethanesulfonimide

Article abstract of DOI:10.1055/s-2002-19329

Bistrifluoromethanesulfonimide has been used to catalyze C-C bond forming reactions such as Friedel-Crafts, Mukaiyama, 1,2-addition and 1,4-addition as well as C-glycosidation reactions.

Full text of DOI:10.1055/s-2002-19329

LETTER
45
Carbon-Carbon Bond Forming Reactions by using Bistrifluoro-
methanesulfonimide
C
arbon-Carbon Bond
n
Forming Re
i
a
ctionsne Cossy,* François Lutz, Valérie Alauze, Christophe Meyer
Laboratoire de Chimie Organique associé au CNRS, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
Fax +33(1)40794660; E-mail: janine.cossy@espci.fr
Received 16 August 2001
sufficient enough to use less than 5 mol% of TFSI-H and
acetylenic ketones 4 and 5 were obtained in 45% (volatile
compound) and 63% yield respectively (Scheme 1). In
this reaction, the anhydrides act as efficient acyl donors,
presumably after protonation with the strongly acidic
TFSI-H. To further explore the generality and scope of
TFSI-H catalyzed Friedel–Crafts reactions, acid anhy-
drides were replaced by ketals. Treatment of BTMSA 1
with bromoacetaldehyde diethylacetal 6 in the presence of
TFSI-H (5 mol%) afforded propargylic ether 7 in 47%
yield. Furthermore, when catechol 8 was treated with
dimethoxymethane 9, no protection of the diol was ob-
served, but instead a regioselective Friedel–Crafts alkyla-
tion took place and compound 10 was obtained in 60%
yield (Scheme 1).
Abstract: Bistrifluoromethanesulfonimide has been used to cata-
lyze C-C bond forming reactions such as Friedel–Crafts, Mukaiya-
ma, 1,2-addition and 1,4-addition as well as C-glycosidation
reactions.
Key words: bistrifluoromethanesulfonimide, Friedel–Crafts reac-
tions, Mukaiyama reaction, 1,2-addition, 1,4-addition, C-glycosida-
tion
Nitrogen acids are a class of compounds in which the hy-
drogen atom(s) bound to nitrogen is highly acidic. The
synthesis of imidobis(sulfuric acid) [HN(SO2OH)2],
which is the foundation for the chemistry of nitrogen ac-
ids, was first reported in 1843.1 Since then, a number
of fluorine-containing nitrogen acids have been pre-
pared including HN(SO2F)2, CF3SO2N(H)SO2C4H9,
TFSI-H
O
HNSO2(CnF2n+1 2 and bistrifluoromethanesulfonimide,
)
(5 mol%)
HN(SO2CF3)2 (TFSI-H).2,3 TFSI-H is a white crystalline
solid (mp 49–50 °C) with a pKa value of 1.7 in water4 and
Me3Si
SiMe3
(RCO)2O
+
Me3Si
CH2Cl2
1 h, rt
R
1
1
2
3
R = Me
R = Ph
4 (45%)
5 (63%)
1
7.8 in acetic acid (determined by H NMR).5 Here, we
would like to report that TFSI-H can be directly used to
catalyze C-C bond forming reactions such as Friedel–
Crafts, Mukaiyama, 1,2-addition and 1,4-addition as well
as C-glycosidation reactions.
OEt
TFSI-H
(5 mol%)
OEt
Br
+
+
Me3Si
OEt
CH2Cl2
2 h, rt
Br
7 (47%)
6
9
HO
HO
HO
HO
The Friedel–Crafts reactions, which are one of the most
important processes in organic synthesis are generally
carried out by using Lewis acid metal complexes.6,7 The
Friedel–Crafts acylation reaction often requires at least a
stoichiometric amount of Lewis acid due to the coordina-
tion with the aromatic ketone product. Although several
catalytic reactions have been reported,8–12 there is still a
great demand for strong acid catalysts, which can be used
under mild conditions. For example, p-methoxyacetophe-
none was obtained quantitatively by treatment of anisole
with Ac2O in nitromethane at room temperature in the
presence of TFSI-H.13 The ability of metallic salts, de-
rived from TFSI-H14 and N-trimethylsilylbistrifluor-
omethanesulfonimide (TMSNTf2),15 to catalyze such re-
actions has also been reported.
TFSI-H
(5 mol%)
OMe
MeO
OMe
CH2Cl2
5 d, rt
10 (60%)
8
Scheme 1
Among carbon-carbon bond forming reaction, one can
find the Mukaiyama cross-aldol reaction, which can be
catalyzed by a great variety of protic acids as well as
Lewis acid catalysts.16,17 It was hence also envisaged that
TFSI-H could efficiently catalyze the Mukaiyama aldol-
type condensations between silyl enol ethers and acetals.
Several examples of these reactions involving silyl enol
ethers 11, 12 and 13, and ketene silyl acetal 14, are shown
in Scheme 2.18 Clean reactions and complete conversions
were observed in all cases and the condensation products
19–26 were usually obtained in yields up to 50%. Trime-
thylorthoformate 18 also reacted with silyl enol ethers 11
and 13 under these conditions, and the corresponding ac-
etals 22 and 24 did not undergo further addition. Never-
theless, the double condensation product 25, could be
formed by using two equivalents of the silyl enol ether 13
(Scheme 2).
For our part, we have investigated the possibility of direct-
ly employing TFSI-H in order to catalyze Friedel–Crafts
alkynylation of acid anhydrides such as 2 and 3 with
bis(trimethylsilyl)acetylene (BTMSA) 1. Indeed, it was
Synlett 2002, No. 1, 28 12 2001. Article Identifier:
1437-2096,E;2002,0,01,0045,0048,ftx,en;G17601ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

46
J. Cossy et al.
LETTER
because they usually cause protodesilylation of allylsi-
lane.26 However, we have noticed that TFSI-H catalyzed
the addition of allyltrimethylsilane to carbonyl com-
pounds. When cyclohexanone 27 was treated with allyl-
trimethylsilane 28 (1.2 equiv) in the presence of TFSI-H
(10 mol%) at –40 °C, the stable tertiary trimethylsilyl
ether 30 was formed quantitatively. After desilylation
with tetrabutylammonium fluoride, the homoallylic alco-
hol 31 was isolated in 80% yield (Scheme 4).
OSiMe3
O
OMe
MeO
MeO
R
R
R
TFSI-H
+
R
CH2Cl2
-78 °C
19 (54%)
20 (91%)
R = H
R = Me
15
16
2 h
2 h
11
11
O
OMe
MeO R'
TFSI-H
R'
+
CH2Cl2
-78 °C
30 min
MeO
H
21 (71%)
syn/anti: 60/40
R' = Ph
17
Although the introduction of an allyl side chain to an
enone has been achieved with allylcuprate27,28 and allyl-
barium reagents,29 the Sakurai reaction was considered to
effect the most practical allylation.30 Whilst trimethylsilyl
trifluoromethanesulfonate (TMSOTf) is known not to cat-
alyze enone allylation with allylsilanes, TFSI-H in the
presence of allyltrimethylsilane was reported to selective-
ly promote the 1,4-allylation of enones in good yield (93%
yield for cyclohexenone 29).31 The active species, formed
by reaction of TFSI-H with allyltrimethylsilane, is
TMSNTf2 and this compound exhibits an enhanced Lewis
acidity compared to TMSOTf.15,19 However we failed to
reproduce the 1,4-allylation of cyclohexenone 29, as the
allylated ketone 32 was obtained in low yield (10%). The
major product obtained was the allylated diketone 33
(mixture of two diastereomers, 40%), resulting from an
initial 1,4-addition of allylsilane followed by a 1,4-addi-
tion of the intermediate silyl enol ether 34 to cyclohex-
enone 29 (Scheme 4).
22 (82%)
R' = OMe 18
3 h
O
OMe
O
MeO
TFSI-H
Ph
MeO
+
17
CH2Cl2
-78 °C
3 h
Me3SiO
O
23 (80%)
d.r. = 65/35
12
O
OMe
OSiMe3
TFSI-H
+
18
Ph
OMe
24 (92%)
Ph
CH2Cl2
-78 °C
2 h
13
from 1 equiv. of 13
O
OMe O
Ph
Ph
25 (50%)
from 2 equiv. of 13
3 h
O
OSiMe3
OMe
TFSI-H
+
17
MeO
MeO
Ph
CH2Cl2
-78 °C
30 min
26 (60%)
14
The 1,4-addition of silyl enol ethers to , -unsaturated ke-
tones in the presence of TiCl4 is an important carbon-
Scheme 2
32
carbon bond forming reaction.16,33,34 Various Lewis acids
[SnCl4,35 BiCl3,36 BF3 OEt2,37 ZnCl2,38 Me2AlCl39] have
been used to catalyze this reaction, although the efficiency
is not uniformly possible due to the large differences in re-
activity between the various types of silyl enol ethers and
reagents. Furthermore, since most acid-sensitive sub-
strates can polymerize under acidic conditions, salts such
as TrClO4,40 Zn(OTf)2,41 Bu2Sn(OTf)2,42 Sc(OTf)3,43
BiCl3-xMIn,44 tris(pentafluorophenyl)boron,45 have been
used to catalyze the reaction. There is still a need for a
convenient procedure to effect additions to unsaturated
ketones. The formation of 33 as a by-product during the
allylation of cyclohexenone, suggested that TFSI-H had
also catalyzed the 1,4-addition of the intermediate silyl
enol ether 34 to the starting enone 29. Indeed, when TFSI-
H (5 mol%) was added to a mixture of silyl enol ether 11
and cyclohexenone 29 at –78 °C, a smooth 1,4-addition
took place and after desilylation of the intermediate trim-
ethylsilyl enol ether 35 with tetrabutylammonium fluo-
ride, the corresponding 1,5-diketone 36 was obtained in
55% yield and with a diastereoisomeric ratio of 60/40
(Scheme 4).46
A
comparison of the catalytic activity of TFSI-H
with N-trimethylsilylbistrifluoromethanesulfonimide
(TMSNTf2)19a was carried out using 2,2-dimethoxypro-
pane 16 and the cyclohexanone silyl enol ether 11
(Scheme 3). From these results, it emerges that TFSI-H
exhibits a catalytic activity similar to that of TMSNTf2.
Indeed, it is plausible that TFSI-H could induce an initial
protodesilylation of the silyl enol ether resulting in the in
situ formation of TMSNTf2 as an active catalytic spe-
cies.19b
O
OMe
OSiMe3
catalyst
(5 mol%)
MeO
MeO
+
CH2Cl2
2 h, -78 °C
16
21
11
TFSI-H
TMSNTf2
91%
80%
Scheme 3
Carbon-carbon bonds can also be formed by addition of
allylsilanes to aldehydes and ketones. This addition is a
mild method for the preparation of homoallylic alco-
hols,20–22 and is usually accomplished in the presence of
either a stoichiometric amount of Lewis acid23,24 or a cat-
alytic amount of fluoride ion.25 Brønsted acids have sel-
dom been used as catalysts for this type of transformation
Finally, the addition of allylsilane onto glycals has also
been studied. The Lewis acid promoted reaction known as
the “carbon Ferrier” process,47,48 is a synthetically useful
method for the introduction of an axially disposed allyl
group at the C-2 position of the pyran ring. In general
these reactions afford C-allylated glycosides with moder-
Synlett 2002, No. 1, 45–48 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Carbon-Carbon Bond Forming Reactions by using Bistrifluoromethanesulfonimide
47
ly catalyst, producing no metallic wastes, which is a major
advantage for industrial applications.
O
HO
Me3SiO
Bu4NF
THF
TFSI-H
AllylTMS
+
CH2Cl2
-40 °C
30 min
Acknowledgement
31 (80%)
27
28
30
Rhodia is gratefully acknowledged for financial support.
O
O
O
O
References
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mmol) in CH2Cl2 at –78 °C, was added dropwise TFSI-H
(500 L, 0.5 M solution in CH2Cl2, 0.025 mmol, 5 mol%).
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a sat. aq solution of NaHCO3 and extracted with ether. The
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MgSO4, filtered and concentrated under reduced pressure.
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OAc
OAc
O
O
R
TFSI-H
10 mol%
+
R-SiMe3
CH2Cl2
AcO
AcO
OAc
OAc
37
38 (80%) α/β = 10/1
28 R = Allyl
R =
-78 °C
39 (36%)
SiMe3
1
-78°C to rt
Scheme 5
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