Efficient synthesis and ring-opening reactions of monofluorinated epoxides derived from α-fluorosulfoximines

Article abstract of DOI:10.1002/adsc.201000499

Monofluorinated epoxides were successfully prepared through the O-cyclization reaction between α-fluorosulfoximines and ketones. The obtained fluoroepoxides were found to readily undergo an interesting ring-opening process (involving both a C-F bond cleavage and another C-F bond formation) in the presence of titanium tetrafluoride or pyridinium poly(hydrogen fluoride) to afford α-fluorinated ketones. The later process constitutes a formal catalytic 1,2-fluorine shift reaction.

Full text of DOI:10.1002/adsc.201000499

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DOI: 10.1002/adsc.201000499
Efficient Synthesis and Ring-Opening Reactions of
Monofluorinated Epoxides Derived from a-Fluorosulfoximines
Wei Zhanga and Jinbo Hua,*
a
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Ling-Ling Road, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; phone: (+86)-21-5492-5174; e-mail: jinbohu@sioc.ac.cn
Received: June 28, 2010; Published online: October 26, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000499.
Abstract: Monofluorinated epoxides were success-
fully prepared through the O-cyclization reaction
between a-fluorosulfoximines and ketones. The ob-
tained fluoroepoxides were found to readily under-
go an interesting ring-opening process (involving
both a C F bond cleavage and another C F bond
formation) in the presence of titanium tetrafluoride
or pyridinium poly(hydrogen fluoride) to afford a-
fluorinated ketones. The later process constitutes a
formal catalytic 1,2-fluorine shift reaction.
reactions with nucleophiles.[3c,7,9] When monofluoroep-
oxides were treated with fluoride ion, a-fluorinated
carbonyl compounds were obtained through a ring-
À
opening process involving both a C F bond cleavage
[3a,8]
À
and another C F bond formation.
This type of
À
À
transformation also proceeded smoothly in the pres-
ence of triethylamine-tris(hydrogen fluoride)[9] or
boron trifluoride[10]. Futhermore, the acid-catalyzed
isomerizations of polyfluorinated epoxides into the
corresponding ketones have been previously docu-
mented.[7]
Keywords: epoxides; fluorine; fluorine shift; ring-
opening; sulfoximines
As part of our efforts to develop new synthetic ap-
proaches for organofluorine compounds,[11] we recent-
ly developed some unique synthetic methods for the
preparation of di- and monofluorinated compounds
by using fluorinated sulfoximine reagents.[12,13] In par-
ticular, the a-difluoromethylsulfoximine was used as a
Due to the high electronegativity and small size of novel and efficient difluorocarbene reagent,[12] and
the fluorine atom, the replacement of hydrogen the a-fluorosulfoximines could readily react with
atom(s) by fluorine in organic compounds often re- simple nitrones to give monofluoroalkenes with excel-
sults in a profound change in their physical, chemical lent Z/E stereocontrol.[13] In this paper, we wish to
and biological properties.[1] As a consequence, over report another interesting synthetic application of flu-
the past decade, organofluorine compounds have orinated sulfoximines, that is, the efficient synthesis of
found broad applications in both life sciences and ma- a-monofluorinated epoxides derived from a-fluoro-
terials science.[2] To date, many endeavours have been sulfoximines and ketones, and the subsequent ring-
made to develop new efficient synthetic methods for opening reaction of the monofluoroepoxides by using
the selective introduction of fluorine atom(s) or fluo- TiF4 or pyridinium poly(hydrogen fluoride) (Olahꢀs
rinated moieties into organic molecules.[2] Although reagent, pyridine-9HF) to afford a-fluoro ketones.
epoxides are highly useful building blocks in organic
a-Fluorosulfoximines (2a–2c) could be readily pre-
synthesis, the reports on the preparation and synthetic pared from non-fluorinated sulfoximines (1a–1c) by
utilizations of monofluorinated epoxides are electrophilic fluorination reactions. When compounds
scarce.[3–5] Monofluorinated epoxides are usually pre- 1a–1c were treated with n-BuLi at À788C and then
pared by the epoxidation of monofluoroolefins,[3] the reacted with N-fluorodibenzenesulfonimide (NFSI),
treatment of fluorohalohydrins with a base,[4] or the the corresponding a-fluorosulfoximines (2a–2c) were
Darzens reaction between a ketone and dibromo(tert- obtained in 54–71% yields (Scheme 1).
butyldimethylsililyl)fluoromethane[5] (or ethyl dibro-
With the a-fluorosulfoximines in hand, we next at-
tempted their O-cyclization reaction with ketones to
mofluoroacetate[6]).
Fluorinated epoxides are known to be relatively un- afford the monofluorinated epoxides (Table 1). It was
stable, and tend to undergo a variety of ring-opening found that n-BuLi and LDA were more suitable than
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Table 2. O-Cyclization of ketones with 2.
Scheme 1. Synthesis of a-fluorosulfoximines 2a–2c (Ts=p-
toluenesulfonyl group).
Table 1. Survey of O-cyclization reaction conditions.
[a]
Determined by 19F NMR spectroscopy using PhCF3 as in-
ternal standard.
NaH and t-BuOK for this addition-elimination reac-
tion; after deprotonation with 1.5 equivalents of n-
BuLi, compound 2a was able to react with ketone 3a,
giving the monofluoroepoxide 4a in 94% yield
(Table 1, entry 2).
However, it should be noted that the above-ob-
tained product 4a was found to be unstable, which de-
composed during the purification process using silica
gel flash column chromatography. To further evaluate
the stability of monofluorinated epoxides, we synthe-
[a]
[b]
Isolated yield.
The two isomers can be separated by column chromatog-
raphy.
[c]
Determined by 19F NMR spectroscopy.
sized a variety of structurally diverse monofluoroep- while to mention that the current epoxide formation
oxides by using the a-fluorosulfoximines and ketones reaction with a-fluorosulfoximines were not only suc-
under the optimized reaction conditions (Table 2). Al- cessfully applied to a variety of acyclic ketones (see
though benzophenone 3b and ketone 3c showed good Table 1, Table 2 and Table 4), it can also be extended
reactivity towards a-fluorosulfoximine 2a, the corre- to cyclic ketones such as 1-tetralone (the reaction be-
sponding fluoroepoxides 4b and 4c could not be ob- tween 2a and 1-tetralone gave the corresponding fluo-
tained after passage through a silica gel column rinated epoxide in 60% yield, determined by
(Table 2, entries 1, 2). Further investigation indicated 19F NMR). However, a similar reaction between fluo-
that the monofluorinated epoxides 4d–4i, which con- rosulfoximine 2a and aldehydes (such as benzalde-
tain a strong electron-withdrawing substituent on the hyde) only gave a complex reaction mixture.
aryl group, were more stable than 4b and 4c, and the
Considering that the monofluoroepoxides could be
two diastereomers of the epoxides were separated by conveniently prepared by addition-elimination reac-
silica gel column chromatography without decomposi- tions between fluorosulfoximines and ketones, we
tion (Table 2, entries 3–8). We also noticed that the n- then examined their ring-opening reactions to give a-
butyl group-substituted fluorosulfoximine 2c can be fluorinated carbonyl compounds. By using the mono-
readily converted to monofluorinated epoxide 4i in fluoroepoxide 4a as a model compound,[14] the ring-
good yield (76%, Table 2, entry 8). These fluoroepox- opening reaction was assessed by tuning the reaction
ides (4d–4i) were obtained with low diastereoselectiv- parameters such as additive, solvent, temperature and
ity; however, the two isomers of each product could reaction time (Table 3). After the O-cyclization reac-
be separated by column chromatography. It is worth- tion, the resulting crude product 4a was obtained by
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Efficient Synthesis and Ring-Opening Reactions of Monofluorinated Epoxides
Table 3. Survey of ring opening reaction conditions.
by simple treatment with pyridine-9HF (Olahꢀs re-
agent) was also explored. When pyridine-9HF
(65.0 equiv. of HF) was added to the crude product
4a, derived from the reaction mixture of the fluorosul-
foximine 2a and ketone 3a, the corresponding product
5a was obtained with good yield (Method B, Table 3,
entry 14).
With these two optimized reaction conditions
(Methods A and B), we then examined the substrate
scope of this ring opening reaction. As shown in
Table 4, the first O-cyclization reactions were carried
out under standard conditions (Table 1, entry 2), and
the yields of monofluoroepoxides are shown in the
parentheses, which were determined by 19F NMR
spectroscopy. It was found that a variety of structural-
ly diverse ketones showed high reactivity with the a-
fluorosulfoximines 2a–2c to afford the corresponding
monofluoroepoxides (Table 4, entries 1–17). However,
the phenyl-substituted a-fluorosulfoximines 2d gave a
low yield (30%, Table 4, entry 18). When aryl-substi-
tuted epoxides, derived from aryl ketones (3a, 3h, 3k–
3n), were treated by TiF4 or pyridine-9HF reagent
(Method A or B), a-fluorinated ketones were ob-
tained in good isolated yields (Table 4, entries 1–4, 9–
18). However, the monofluoroepoxides bearing three
alkyl groups (derived from ketone 3i and 3j) showed
lower reactivity with both methods A and B (Table 4,
entries 5–8). Furthermore, the ring-opening reactions
with n-butyl- and phenyl-substituted monofluoroepox-
ides proceeded smoothly with Method A or B
(Table 4, entries 17, 18).
[a]
For entries 1, 3 and 4-.8, the starting material 4a was
We also extended this ring-opening reaction to pro-
duce a-chlorinated ketone 6 (Scheme 2). When mono-
fluoroepoxides 4a was used to react with 4.0 equiva-
lents of TiCl4, the corresponding a-chlorinated ketone
6 was obtained in 82% yield after 2 steps.
Although a detailed investigation is needed to get
insight into the mechanism of the current interesting
formal 1,2-fluorine shift reaction, we propose that it
mostly unreacted. For entries 3 and 4, 4a was consumed
to give a complex product mixture.
Two steps, isolated yield of 5a.
[b]
[c]
H2SO4 (10%) was used.
Determined by 19F NMR spectroscopy.
[d]
[e]
Method A.
Method B.
[f]
extraction, drying over MgSO4 and evaporation of the may proceed through a Lewis acid- or Brønsted acid-
solvent without further purification. Thereafter, the mediated 1,2-fluorine shift[18] process via transition
epoxide 4a was heated in CH2Cl2 or THF without states 7A or 7B, respectively (Scheme 3). A coordina-
other additive, however, the thermal fluorine shift re- tion of the Lewis acid or Brønsted acid on the oxygen
action did not occur (Table 3, entries 1, 2). Further- atom of 4 may facilitate the fluoride ion to attack the
À
more, in the presence of H2SO4, BF3·Et2O or TBAF, a-carbon atom and the subsequent C F bond cleav-
the reaction was unsuccessful and no desired product age. Although Elkik and co-workers reported the
5a was detected (Table 3, entries 3–5). When a cata- thermal rearrangement of 2-fluoroepoxides leading to
lytic amount of SbF3 or TiF4 was added to the reac- a-fluorocarbonyl compounds,[10] a direct 1,2-fluorine
tion mixture, we obtained a trace amount of a-fluori- shift mechanism appears to be unlikely for the trans-
nated ketone 5a (Table 3, entries 6, 7). Thereafter, formation from 2 to 5 (see Table 3, entries 1–8 and
various solvents and the amount of TiF4 were exam- Scheme 2). It should be noted that similar ring-open-
ined. The best product yield (90%, two steps from 2a) ing reactions with a-nitro- or chloro-substituted epox-
was obtained when 0.4 equivalent of TiF4 was used as ides have been reported, and in those cases, both
an additive by using THF as a solvent, and the mix- nitro and chlorine act as leaving groups during the
ture was heated at 608C for 24 h (Method A, Table 3, acid-mediated ring-opening process.[15–17]
entry 13). The other method of transforming the
In summary, we have demonstrated an efficient O-
monofluoroepoxide 4a to the a-fluorinated ketone 5a cyclization reaction between a-fluorosulfoximines and
ACHTUNGTRENNUNG
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Wei Zhang and Jinbo Hu
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Table 4. Ring-opening reactions of monofluoroepoxides.
ketones to afford monofluorinated epoxides, and the
subsequent ring-opening/formal catalytic 1,2-fluorine
shift reaction of monofluoroepoxides was also devel-
oped by simply treatment of catalytic amount of TiF4
or pyridine-9HF reagent. Not only do our results
present a useful method for the synthesis of mono-
fluoroepoxides and a-fluorinated ketones, they also
provide another important synthetic aspect of fluori-
nated sulfoximines.[12,13] Further explorations of fluori-
nated sulfoximine chemistry and fluorine shift reac-
tions are currently underway in our laboratory.
Experimental Section
Typical Procedure for O-Cyclization Reactions of a-
Fluorosulfoximines and Ketones
Under an N2 atmosphere, n-BuLi (0.23 mL, 1.6M,
1.5 equiv.) was added via a needle to the solution of a-fluo-
rosulfoximine 2a (124 mg, 0.36 mmol, 1.5 equiv.) in dry THF
(4 mL) at À788C, then the mixture was stirred for 30 min,
followed by addition of a solution of 3d (40 mg, 0.24 mmol,
1.0 equiv.) in THF (1 mL). The temperature was maintained
for 0.5 h at À788C, then the dry-ice bath was removed.
After stirring for 3 h at room temperature, the reaction was
quenched by adding an excess amount of H2O, followed by
extraction with diethyl ether. The organic phase was washed
with brine and then dried over anhydrous MgSO4. After the
solution had been filtered and the solvent evaporated under
vacuum, the residue was subjected to silica gel column chro-
matography using petroleum ether as eluent to give product
4d [yield: 82%, cis-4d (21 mg), trans-4d (21 mg)].
cis-4d; white solid; mp 54–558C). IR (film): n=1606,
1
1523, 1350, 1191, 1115, 840, 758 cmÀ1; H NMR: d=8.17 (d,
J=9.3 Hz, 2H), 7.43 (d, J=8.7 Hz, 2H), 1.74 (s, 3H), 1.36
(d, J=16.5 Hz, 3H); 19F NMR: d=À123.9 (q, J=16.6 Hz,
1F); 13C NMR: d=147.8, 145.5, 127.0, 124.0, 99.4 (d, J=
266.5 Hz), 66.5 (d, J=19.3 Hz), 18.8, 16.1 (d, J=32.8 Hz);
MS (EI): m/z (%)=211 (M+, 2.12), 149 (100.00); HR-MS
(EI): m/z=211.0648, calcd. for C10H10NO3F: 211.0645.
trans-4d: white solid; mp 87–888C. IR (film): n=1603,
[a]
Method A: TiF4 (0.4 equiv.), THF, 608C, 24 h, THF.
Method B: pyridine-9HF (65.0 equiv.), 608C, 4 h, THF.
[b]
[c]
Yields of monofluorooxiranes 4 are shown in the paren-
1
theses, which were determined by 19F NMR spectroscopy.
1523, 1350, 1187, 1112, 860, 706, 572 cmÀ1; H NMR: d=8.24
[d]
Two steps, isolated yield of product 5.
Additional pyridine-9HF (40.0 equiv.) was added after
4 h, then stirring for 3 h.
Additional 0.6 equiv. of TiF4 was added after 24 h, then
(d, J=8.1 Hz, 2H), 7.59 (d, J=8.7 Hz, 2H), 1.86 (d, J=
16.5 Hz, 3H), 1.71 (s, 3H); 19F NMR: d=À125.2 (q, J=
16.6 Hz, 1F); 13C NMR: d=147.6, 144.6, 127.6, 123.4, 98.6
(d, J=261.3 Hz), 65.8 (d, J=20.1 Hz), 19.5, 16.2 (d, J=
33.5 Hz); MS (EI): m/z (%)=211 (M+, 3.50), 149 (100.00);
HR-MS (EI): m/z=211.0647, calcd. for C10H10NO3F:
211.0645.
[e]
[f]
stirring for 42 h.
Typical Procedure for Ring-Opening Reactions of
Monofluoroepoxides
Method A: The crude product 4a (without purification by
silica gel column chromatography) was prepared by the O-
cyclization reaction between a-fluorosulfoximine 2a and
ketone 3a under the above-mentioned reaction conditions.
The product yield of 4a was 94%, which was determined by
19F NMR spectroscopy. Therefore, the reaction was
quenched by adding an excess amount of H2O, followed by
Scheme 2. Synthesis of a-chlorinated ketone 6.
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Efficient Synthesis and Ring-Opening Reactions of Monofluorinated Epoxides
Scheme 3. Proposed reaction mechanism.
extraction with diethyl ether. The organic phase was washed
Acknowledgements
with brine and then dried over anhydrous MgSO4. After the
solution had been filtered and the solvent evaporated under
vacuum, crude product 4a was then obtained.
Support of our work by the National Natural Science Foun-
dation of China (20772144, 20825209, 20832008) and the
Chinese Academy of Sciences (Hundreds-Talent Program
and Knowledge Innovation Program) is gratefully acknowl-
edged.
Under an N2 atmosphere, TiF4 (0.4 equiv.) was added to
the solution of the epoxide 4a in dry THF (5 mL). After stir-
ring for 24 h at 608C, the reaction was quenched by adding
an excess amount of H2O, followed by extraction with dieth-
yl ether. The organic phase was washed with brine and then
dried over anhydrous MgSO4. After the solution had been
filtered and the solvent evaporated under vacuum, the resi-
due was subjected to silica gel column chromatography
using petroleum ether as eluent to give product 5a as a col-
orless liquid; yield: 90%.
Method B: The crude product 4a (without purification by
silica gel column chromatography) was prepared by the O-
cyclization reaction between a-fluorosulfoximine 2a and
ketone 3a under the above-mentioned reaction conditions.
The product yield of 4a was 94%, which was determined by
19F NMR spectroscopy. Therefore, the reaction was
quenched by adding excess amount of H2O, followed by ex-
traction with diethyl ether. The organic phase was washed
with brine and then dried over anhydrous MgSO4. After the
solution had been filtered and the solvent evaporated under
vacuum, crude product 4a was then obtained.
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Adv. Synth. Catal. 2010, 352, 2799 – 2804