First isolation of enantiopure perfluoroalkylated sulfilimines and sulfoximines

Article abstract of DOI:10.2533/chimia.2014.410

Mono-, di- and trifluoromethyl sulfilimines and sulfoximines have been isolated for the first time in enantiopure form by separation of the racemate by supercritical fluid chromatography. The electrophilic trifluoromethylating Shibata reagent has been prepared as a single enantiomer. Schweizerische Chemische Gesellschaft.

Full text of DOI:10.2533/chimia.2014.410

410 CHIMIA 2014, 68, Nr. 6
Fluorine Chemistry
doi:10.2533/chimia.2014.410
Chimia 68 (2014) 410–413 © Schweizerische Chemische Gesellschaft
First Isolation of Enantiopure
Perfluoroalkylated Sulfilimines and
Sulfoximines
Thanh-Nghi Leab, Emilie Kolodziejb, Patrick Ditera, Bruce Pégota, Chloée Bournaudb, Martial Toffanob,
Régis Guilotb, Giang Vo-Thanh*b, and Emmanuel Magnier*a
Abstract: Mono-, di- and trifluoromethyl sulfilimines and sulfoximines have been isolated for the first time in
enantiopure form by separation of the racemate by supercritical fluid chromatography. The electrophilic
trifluoromethylating Shibata reagent has been prepared as a single enantiomer.
Keywords: Enantiomers · Fluorine · SFC · Sulfilimines · Sulfoximines
In recent years, a pronounced revival sulfoxides 1 via sulfilimine intermediates
of the chemistry of fluorinated sulfilimi- 2 (Scheme 1). Variations of the aromatic
Introduction
nes and sulfoximines has appeared. They
substituents (electron-donating and elec-
Sulfilimines and sulfoximines, which
contain a stereogenic sulfur (iv) or (vi)
center respectively, are very particular
substrate classes in organic chemistry.
Numerous studies have demonstrated their
relevance as ligands for catalysis or as chi-
have been identified as versatile perfluo- tron-withdrawing groups), and of the fluo-
roalkylating reagents (as source of radical, rinated chain (monofluoro- to perfluoroal-
electrophilic and nucleophilic fluorinated kyl chains) were feasible.[5e] Our versatile
entities)[5] and our group (vide infra) along methodology allowed the straightforward
with others have improved their synthesis synthesis of a wide range of racemic per-
but only for racemic versions.[6] Examples fluoroalkyl sulfur (iv) and (vi) compounds.
ral auxiliaries.[1,2] Their introduction for
of enantiopure fluorinated sulfur (iv) or
(vi) molecules reported in literature are optically active fluorinated sulfilimines
very scare. To the best of our knowledge, and sulfoximines, we were wondering if
there are only four articles which refer to the key step of our methodology, the for-
In our quest for the preparation of
life sciences purposes, namely in medici-
nal chemistry or crop protection, is further-
moreanemergingandpromisingdomain.[3]
This increased interest for these moieties
the preparation and the use of enantiopure mation of sulfilimine, could be realized
has been facilitated by the major progress
realized in their synthesis. Efficient, safe
and metal-free preparative methods have
been published in recent years and have
made them easily accessible, including in
monofluoro[7] – and difluoromethyl[8] sulf- without racemization of the sulfur center.
oximines. These optically pure compounds
To test this hypothesis, synthesis of per-
have always been constructed by late fluo- fluoroalkylated sulfoxides of pure optical
rination of the corresponding enantiopure form was therefore a first requirement.
particular their enantiomeric form.[1,4] The
sulfoximines. Furthermore, noenantiomer- The only examples of fluorinated chiral
ically pure trifluoromethylated sulfilimine sulfur (iv) arylsulfinylacetic acids (RS(O)
fluorinated version of such functions, i.e.
with a perfluoroalkyl chain attached to
the sulfur atom, is a very unusual mem-
or sulfoximine has been described so far. CF2COOH) were described byYagupolskii
In this context, the development of highly et al.[9] They were obtained by crystal-
stereoselective synthesis of perfluoroalkyl- lization of diastereomers with (R)-(+)-
ber of this family. Their synthesis has in-
ated sulfoximines is still a challenge.
methylbenzylamine. Several methods have
deed been restricted for a long time to a
small number of targets and to very tedious
methodologies.
In this article, we describe our work nevertheless been reported for the highly
devoted to the preparation of optically en- stereoselective synthesis of alkylated sulf-
riched sulfur derivatives with emphasis on oxides,[10] but they were never applied to
perfluoroalkylated sulfilimine and sulfoxi- fluorinated series. One of the most effi-
mine.
cient conventional approaches for the ste-
reoselective synthesis of sulfoxides is the
asymmetric oxidation by an hydroperoxide
in the presence of a stoichiometric or cata-
lytic amount of a chiral tetra-isopropoxide
Result and Discussion
Our group has recently unlocked the titanium complex bearing tartrate ligands
access of various perfluoroalkylated aryl following Kagan’s[11] or Modena’s[12] pro-
sulfoximines 3 starting from fluorinated tocols. Unfortunately all the attempts with
*Correspondence: Prof. E. Magniera,
Prof. G. Vo-Thanhb
aInstitut Lavoisier de Versailles (ILV)
UMR CNRS 8180
Université de Versailles St Quentin en Yvelines
45, avenue des Etats Unis
Scheme 1.
Straightforward syn-
thesis of perfluoro-
alkylated sulfilimines
and sulfoximines.
O
78035 Versailles Cedex, France
E-mail: emmanuel.magnier@uvsq.fr
bLaboratoire de Catalyse Moléculaire
Institut de Chimie Moléculaire et des Matériaux
d’Orsay (ICMMO)
O
S
N
S
O N
H
R
R
R
S
1) KMnO
CH3CN
4
R
F
R
F
R
F
UMR CNRS 8182
Université Paris-Sud, 91405 Orsay Cedex, France
E-mail: giang.vo-thanh@u-psud.fr
Tf O
2
2) HCl
1
2
3

Fluorine Chemistry
CHIMIA 2014, 68, Nr. 6 411
Scheme 2. Attempts
of enantioselective
synthesis of fluori-
nated sulfoxides.
fluorinated sulfides by these methods have
led only to the formation of a racemic mix-
ture of sulfoxides in very low yield (13%).
We assumed that the tamed reactivity of
the non-bonding electrons of the sulfur,
due to the presence of the fluorine atoms,
could account for this result. Nucleophilic
substitution is one of the other main strat-
egies usually explored. For this reaction,
we used the Andersen sulfinate[13] which
usually reacts with nucleophiles through
total inversion of configuration. However,
though the reaction of menthyl (S)-p-
tolylsulfinate with Ruppert-Prakash re-
agent CF3SiMe ,[14] initiated by menthyl
alkoxide or fluo3rine anion, was acceptable
in terms of conversion, it gave rise only to
racemic p-tolyltrifluoromethylsulfoxide
O
S
°
°
O
nucleophilic R
ee 0%
S
F
R
F
O
yield :52-54%
nucleophilic R =CFSiMe ,CF3MgBr,CFSiMe
3
F
3
3
6 1
2 5
attributed without any ambiguity by X-ray
mer of sulfoxides or of sulfoximines. The
analysis (Fig. 1).[23]
next step was the adaption of this analyti-
We then extended our separation proto-
col by SFC to some other fluoroalkylated
cal method to a preparative one, to check
the feasibility of the separation and the
enantiomeric stability. Another technique
has shown a high ability for the separa-
tion of enantiomer on chiral phase: super-
critical fluid chromatography (SFC).[21]
Generally the results obtained from HPLC
are easily transposable. Although the chi-
ral column Chiralpak IA at our disposal
was not adapted to the separation of fluo-
rinated sulfoxides, it was hoped that this
compounds (Table 1). Each enantiomer of
dichlorofluoromethyl- and bromodifluoro-
methyl sulfoximines 7 and 8 was separated
(entries 3 to 6) and independently trans-
formed into their corresponding sulfur
(Scheme 2). The use of other fluorinated
(vi) derivatives. The enantiomeric purity
nucleophiles (C2F5SiMe3, C6F13MgBr) or
chiral sulfinate[15] cannot change the total
lack of enantioselectivity for the nucleo-
of each sulfoximine 9a,b and 10a,b was
ensured by analytical SCF or HPLC. Table
1 summarizes the optical rotation of the 12
philic substitution. The racemization of
enantiomers obtained in our study. The
absolute configuration of the monofluoro-
would not be the case for the sulfilimines.
the sulfur center of chiral sulfinates has al-
ready been reported in nucleophilic substi-
tutions with lithium nucleophiles[16] or in
acidic conditions,[17] but never with fluori-
nated nucleophiles. Faced with these quite
disappointing results, we next turned our
attention to the oxidation step of sulfilimi-
nes. As these compounds are oxidized
into sulfoximines by the relatively mild
potassium permanganate reagent, we envi-
sioned the possibility to develop oxidative
chiral resolution.[18] Unfortunately kinetic
resolution for this last oxidation with the
Kagan’s reagent proved inefficient as no
conversion occurred.
To our delight, we succeeded in the separa-
and difluoromethyl sulfur derivatives was
attributed by analogy with the data (reten-
tion of the two enantiomers of sulfilimine
5 using 20% methanol as the co-solvent
with carbon dioxide (Scheme 3).[22] This
tion time and X-ray analysis) collected for
the trifluoromethyl series.
procedure gave us the enantiomers 5a and
5b which are fully separated. Each stereo-
isomer was afterwards oxidized to produce
the corresponding sulfoximines 6a and 6b.
The first isolation of the trifluorometh-
yl phenyl sulfoximine 6b strongly encour-
aged us to prepare an enantiomerically pure
version of Shibata’s reagent 12 (Scheme
Their enantiomeric purity was secured by
4). Compound 6b was thus transformed
into the electrophilic perfluoroalkylating
chiral HPLC analysis. We were pleased to
confirm that the oxidation proceeded with
retention of relative configuration.
agent 12-(S) following the same procedure
as described for the racemic reagent.[5b]
Compounds 6a and 6b were stored for
two months at room temperature on the
bench and their enantiomeric stability was
confirmed by HPLC measurements. After
this period, no erosion of the enantiomeric
excess had been detected. Lastly, the abso-
lute configurations of the trifluoromethyl
sulfilimines 5a and sulfoximines 6a were
The absolute configuration of compounds
11 and 12 was confirmed thanks to the op-
tical data (Table 1).
We then examined the reaction of this
chiral electrophilic trifluoromethylating
reagent with β-keto ester 13 as nucleophile,
under standard conditions as described by
Gais and coworkers[19] described the
separation of a racemic mixture of methyl-
phenylsulfoximine by crystallization with
0.5 equiv of (–)-10-camphorsulfonic acid
in acetone. Both enantiomers were separat-
ed in good yield and enantiomeric excess,
after basic hydrolysis. The same operation
with a racemic mixture of trifluoromethyl-
°
phenylsulfoximine does not allow any
crystallization of diastereoisomeric salts.
We assumed that the nitrogen atom of the
sulfoximine was not basic enough to react
with the camphorsulfonic acid. Other chi-
ral acids were tested but without further
success.
°
O
F3C
HN
F3C
HN
S
S
NH
S
5a
6a
6b
F3C
SFC
CO2 /
MeOH 20%
KMnO4
(85% yield)
°
°
HN
CF3
O
CF3
S
S
5
HN
At this stage of our studies, reliable
chiral separation methods became neces-
sary to assess the stereochemical integrity
of enantiomers as well as to examine the
potential for interconversion of individual
stereoisomers. In a first approach, all the
chiral sulfur derivatives (sulfoxides, sul-
filimines and sulfoximines) were analyti-
cally separated by high performance liquid
chromatography (HPLC) showing that it is
possible to differentiate these chiral com-
5b
ee > 99%
Scheme 3. Separation of enantiomers of sulfilimines and transformation into sulfoximines.
(R)-
(S)-sulfilimine
pounds.[20] The clear separation of signals
sulfoximine
for each enantiomer proves that there is no
rapid equilibrium between each enantio- Fig. 1. X-ray structures of enantiopure sulfilimine 5a and sulfoxime 6a.

412 CHIMIA 2014, 68, Nr. 6
Fluorine Chemistry
Table 1: Optical rotation data for sulfilimines and sulfoximines.
Acknowledgments
Thanh-Nghi Le is grateful to the Vietna-
mese Government, the National Institute of
Medicinal Materials in Hanoi for a doctoral
fellowship. Yohan Macé is acknowledged for
seminal experiments and Lucy Cooper for im-
provement of the English manuschript.
( )n
O
N R
S
R
F
20a
Entry Cpd
RF
R
H
n
0
0
0
0
0
0
1
1
1
1
1
1
1
1
c [g/mL]
0.0113
0.0112
0.0303
0.0302
0.0445
0.0453
0.0229
0.0223
0.0151
0.0138
0.0178
0.0177
0.0311
0.0150
Solvent
CH2Cl2
CH2Cl2
acetone
acetone
acetone
acetone
CH2Cl2
CH2Cl2
acetone
acetone
acetone
acetone
acetone
CHCl3
[a]D
Config.
(S)b
(R)b
(S)c
(R)c
(S)c
(R)c
(R)b
(S)b
(R)c
(S)c
Received: March 27, 2014
1
2
5a
5b
7a
CF3
+89.6
–88.3
+45.0
–44.9
+22.7
–23.0
+16.6
–16.3
+4.6
CF3
H
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3
CFCl2
CFCl2
CF2Br
CF2Br
CF3
Ac
Ac
Ac
Ac
H
4
7b
8a
5
6
8b
6a
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6b
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CF3
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9
CFCl2
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CF2Br
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H
10
11
12
13
14
9b
10a
10b
11
H
–4.8
H
+3.9
(R)c
(S)c
(S)c
H
–3.8
Me
(Me)2
–22.1
–3.5
12
CF3
(S)c
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O
O
O
N
CH3I 5equiv
K2CO3 5equiv
N
H
N
S
S
CF SO OMe
S
3
2
TfO
CF
3
CF
CF
3
3
THF, Δ 7h
94%
r.t. 6h
90%
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11-(S)
3b-(S)
12-(S)
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CF
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2
2
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14
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pound 14 in racemic form.
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sulfilimines was separated by SFC, fully

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