Reactions of polyfluorinated thietanes. Selective synthesis of 4-R-2,2-bis(trifluoromethyl)thietane-1-S-oxides and 2-substituted 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes

Article abstract of DOI:10.1016/j.jfluchem.2009.06.011

Oxidation of 4-substituted 2,2-bis(trifluoromethyl)thietanes by m-chloroperoxybenzoic acid results in selective formation of the corresponding S-oxides in 65-86% yield. Oxidation of 4-C₂H₅S-2,2-bis(trifluoromethyl)thietane under mild conditions led to selective formation of 4-C₂H₅SO₂-2,2-bis(trifluoromethyl)thietane, which under more rigorous conditions was selectively converted into trans-4-C₂H₅SO₂-2,2-bis(trifluoromethyl)thietane-1-S-oxide. Reaction of 4-substituted 2,2-bis(trifluoromethyl)thietanes with activated aluminum powder results in a highly selective ring expansion process, producing the corresponding 5-fluoro-4-(trifluoromethyl)-2,3-dihydro-2-alkoxythiophenes in 58-93% yield. These compounds were also prepared in 61-85% yield using a "one-pot" procedure, starting from sulfur, hexafluoropropene and the corresponding vinyl ether without isolation of any intermediates. Both 2-i-C₃H₇O- and 2-t-C₄H₉O- 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes were converted into 2-fluoro-3-trifluormethylthiophene by reaction with P₂O₅.

Full text of DOI:10.1016/j.jfluchem.2009.06.011

Journal of Fluorine Chemistry 130 (2009) 780–787
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Reactions of polyfluorinated thietanes
Selective synthesis of 4-R-2,2-bis(trifluoromethyl)thietane-1-S-oxides
and 2-substituted 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes
b
Viacheslav A. Petrov a, , William J. Marshall
*
a DuPont Central Research and Development,1 Experimental Station, PO Box 80500, Wilmington, DE 19880-0500, United States
b DuPont Corporate Center for Analytical Sciences, Experimental Station, PO Box 80500, Wilmington, DE 19880-0500, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Oxidation of 4-substituted 2,2-bis(trifluoromethyl)thietanes by m-chloroperoxybenzoic acid results in
selective formation of the corresponding S-oxides in 65–86% yield. Oxidation of 4-C2H5S-2,2-
bis(trifluoromethyl)thietane under mild conditions led to selective formation of 4-C2H5SO2-2,2-
bis(trifluoromethyl)thietane, which under more rigorous conditions was selectively converted into
trans-4-C2H5SO2-2,2-bis(trifluoromethyl)thietane-1-S-oxide. Reaction of 4-substituted 2,2-bis(trifluor-
omethyl)thietanes with activated aluminum powder results in a highly selective ring expansion process,
producing the corresponding 5-fluoro-4-(trifluoromethyl)-2,3-dihydro-2-alkoxythiophenes in 58–93%
yield. These compounds were also prepared in 61–85% yield using a ‘‘one-pot’’ procedure, starting from
sulfur, hexafluoropropene and the corresponding vinyl ether without isolation of any intermediates.
Both 2-i-C3H7O- and 2-t-C4H9O- 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes were converted
into 2-fluoro-3-trifluormethylthiophene by reaction with P2O5.
Received 14 March 2009
Received in revised form 17 June 2009
Accepted 18 June 2009
Available online 26 June 2009
Keywords:
4-Alkoxy-2,2-bis(trifluoromethyl)thietanes
Oxidation
4-Alkoxy-2,2-bis(trifluoromethyl)thietane-
1-S-oxides
Reductive defluorination
2-Alkoxy-5-fluoro-4-(trifluoromethyl)-2,3-
dihydrothiophenes
ß 2009 Elsevier B.V. All rights reserved.
2-Fluoro-3-trifluoromethyl-thiophene
1. Introduction
Additional information on reactions and synthetic applica-
tions of HFTA and its cyclic dimer, along with chemical
Discovered by W. Middleton in 1960s, polyfluorinated thietanes
were originally prepared by [2 + 2] cycloaddition reaction of
monomeric hexafluorothioacetone (HFTA) with vinyl ethers or
vinyl sulfides [1]. Later the Ishikawa group introduced a system
based on a readily available [2] 2,2,4,4-tetrakis(trifluoromethyl)-
1,3-dithietane (cyclic dimer of HFTA)/dry KF and vinyl ether,
which significantly simplified the synthesis of 4-alkoxy-2,2-bis(tri-
fluoromethyl)thietanes [3]. So far, only four representatives of
4-R-2,2-bis(trifluoromethyl)thietanes (R = CH3O–, C2H5O–, CH3S–,
(CH3)3CS–) [1,3], and one perfluorinated analog along with 2,2,4,4-
tetrakis-(trifluoromethyl)-3,3-difluorothietane [4] were prepared.
Examples of polycyclic fluorinated thietanes include adducts of
HFTA with dihydropyrane and dioxene [1] or by the cycloadducts of
fluorinated thiocarbonyls and quadricyclanes. For example, the
reaction of HFTA with quadricyclane gives 3-thia-4,4-bis(trifluor-
omethyl)-tricyclo[5.2.1.02,5]non-7-ene in a high yield [5]; bis(tri-
fluoromethyl)thioketene also undergoes cycloaddition with
quadricyclane and its derivatives forming norbornenethietanes
containing exocyclic double bond [6,7].
transformations of other polyfluorinated sulfur containing
heterocycles can be found in recently published review article
[8].
Despite the fact that fluorinated thietanes have been known for
a long time, the information on chemical transformation of these
materials is limited to ring opening reactions of 4-RO-2,2-
bis(trifluoromethyl)thietanes (R = CH3 and C2H5) under action of
R0–M (M = Li or –MgX) resulting in the formation of ring opening
products CF2 = C(CF3)CH2CH(OR)S–R0 [3] and a similar transforma-
tion of 3-thia-4,4-bis(trifluoromethyl)-tricyclo[5.2.1.02,5]non-7-
ene resulting in high yield formation of 3-[(perfluoroprop-1-en-
2-yl)bicyclo[2.2.1]-hept-5-en-2-yl]sulfanes in reaction with
organo-lithium or magnesium derivatives [9]. The only reported
example of ring expansion is a base catalyzed insertion of sulfur
into of 4-alkoxy-2,2-bis(trifluoromethyl)thietanes leading to the
formation of 5-alkoxy-3,3-bis(trifluoromethyl)-1,2-dithiolanes in
moderate yield [3].
This paper describes two new transformations of 4-R-2,2-
bis(trifluoromethyl)thietanes—a highly selective oxidation by m-
chloroperoxybenzoic acid (MCPBA) with the formation of the
corresponding S-oxides and a new reductive ring expansion
process, leading to 2-substituted 5-fluoro-4-(trifluoromethyl)-
2,3-dihydrothiophenes.
* Corresponding author.
1
Publication No 8922.
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.06.011

V.A. Petrov, W.J. Marshall / Journal of Fluorine Chemistry 130 (2009) 780–787
781
2. Results and discussion
2.1. Oxidation of 4-R-2,2-bis(trifluoromethyl)thietanes
4-R-2,2-Bis(trifluoromethyl)thietanes 2a–i used in this study
were prepared using a reported KF catalyzed reaction of 2,2,4,4-
tetrakis-(trifluoromethyl)-1,3-dithietane with vinyl ethers [3]. The
reaction was extended and used for the synthesis of new thietanes
thietanes 2b–h and 2i (Eq. (1)).
Duetothepresenceoftwostrongelectronwithdrawinggroupsin
-position a sulfur atom in 2,2-bis(trifluoromethyl)thietanes carries
a
significant positive charge, which makes possible a direct attack of
softnucleophiles, suchasGrignardreagents,onsulfurofthethietane
ring [3]. Despite the electron-deficiency of sulfur in these materials,
they still have an ability to react with oxidizers. We found that the
reaction of thietanes 2a–c, e–f with m-chloroperoxybenzoic acid
(MCPBA) proceeds rapidly and is surprisingly selective. The addition
of thietanes 2a–c, e–f in CH2Cl2 to a solution of an excess of MCPBA
results in an exothermic reaction, leading to the formation of the
corresponding thietane-1-S-oxides 3a–f (Eq. (2); Tables 1 and 2).
ð2Þ
(1)
The oxidation of bicyclic thietane 2i results in rapid and
selective formation of S-oxide 3f (mixture trans-, cis- isomers,
Eq. (3), Table 1).
(3)
In the process of synthesis of thietanes 2a–i it was discovered
that the course of reaction is significantly affected by the nature of
the catalyst and reaction conditions and a detailed study of this
reaction will be the subject of separate publication.
Table 1
Yields, boiling points, IR and MS data for new materials.
Entry
Comp. no.
Yield (%)
B.p., 8C/mmHg (m.p.)
IR (cmꢀ1
)
MS (m/z)
1
2
3a
3b
3c
3d
3e
3f
77
79
41–45/1.7
51–55/0.7
88–91/0.8a
58–62/0.7
(59–61)c
1064, 1090 (S5O)
1069, 1090 (S5O)
1076, 1090b (S5O)
1073, 1090 (S5O)
1070, 1092 (S5O)
1031, 1072 (S5O)
1422 (SO2)
241 [C5H3F6O2S+, M–C2H5)+], 222[C7H8F6O+, (M–SO)+]
236 [C8H10F6O+, (M–SO)+]
–
250 [C9H12F6O+, (M–SO)+]
250 [C9H12F6O+, (M–SO)+]
220 [(M–SO)+, C7H6F6O+]
302 (M+, C7H8F6O2S+)
–
3
76
4
86
5
65
6
88
64–67/1
7
3g
3h
4a
4b
4c
4d
4e
4f
60
(112.5–113)c
(84–84.5)c
66–67/20
80–82/50
57–58/6
8
75
1078 (S5O), 1422 (SO2)
1678 (C5C)
9
76
216 (M+, C7H8F4OS+)
230 (M+, C8H10F4OS+)
270 (M+, C11H14F4OS+)
230 (M+, C8H10F4OS+)
244 (M +, C9H12 F4OS+)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
63
1678 (C5C)
74
1677 (C5C)
58
27–28/0.2
36–37/0.01
32–33/0.02
50–52/1.8
116–118/0.8 (80–80.5)c
76–77/2
1677 (C5C)
80
1678 (C5C)
85
1678 (C5C)
244 (M +, C9H12 F4OS+)
232 (M+, C7H8F4S2
)
+
4g
4h
4i
73
1677 (C5C)
264 (M+, C7H8F4O2S2
)
+
68
1454 (SO2), 1677 (C5C)
1679 (C5C)
83
250 (M+, C7H7ClF4OS+)
214 (M+, C7H6F4OS+)
236 (M+, C10H8F4S+)
286 (M+, C11H8F6S+)
–
6a
6b
6cd
7de
7ef
8
71
77–78/20
60–61/0.2
–
1670 (C5C)
93
1668 (C5C)
50
1638, 1655 (C5C)
1745 (C5C)
12
–
11
100–105/0.05
99–100
1746 (C5C)
–
35–50
1611 (sh), 1589
170 (M+, C5H2F4S+)
a
Sample crystallized upon standing at ambient temperature.
In CH2Cl2 solvent.
b
c
d
e
f
Crystallized form CCl4.
Reaction was run using 1 g of 5c, purity of 6c-95%.
Purity 95%, mixture of diastereomers, ratio 1:1.
Purity 99%, mixture of diastereomers, ratio 2:1.

Table 2
1H, 19F, 13C NMR data for new materials.
Entry
1
Comp. no.
1H NMRa
19F NMRa
13C NMRa
3ab
1.29 (q), 2.31 (1H, m), 2.82 (3H, m), 3.7–4.1 (2H, m), 6.14 (t, J = 5.4), 6.48 (dd, J = 7.3, 4.6)
Major: ꢀ64.04 (3F, q, J = 9.2), ꢀ66.70 (3F, q, J = 9.2);
minor: ꢀ63.00 (3F, q, J = 9.9), ꢀ66.45 (3F, q, J = 9.9)
Major: ꢀ64.00 (3F, q, J = 9.2), ꢀ66.70 (3F, q, J = 9.2);
minor: ꢀ63.00 (3F, q, J = 9.7), ꢀ66.44 (3F, q, J = 9.7)
Major: ꢀ64.02 (3F, q, J = 9.0), ꢀ66.82 (3F, q, J = 9.0);
minor: ꢀ62.95 (3F, q, J = 9.7), ꢀ66.35 (3F, q, J = 9.7)
Major: ꢀ63.01 (3F, q, J = 9.5), ꢀ66.45 (3F, q, J = 9.5);
minor: ꢀ64.02 (3F, q, J = 9.2), ꢀ66.72 (3F, q, J = 9.2)
Major: ꢀ64.00 (3F, q, J = 9.2), ꢀ66.98 (3F, q, J = 9.2);
minor: ꢀ62.87 (3F, q, J = 9.7), ꢀ66.21 (3F, q, J = 9.7)
Major: ꢀ58.56 (3F, q, J = 11.3) ꢀ65.42 (3F, q, J = 11.2);
minor: ꢀ60.75 (3F, q, J = 10.1), ꢀ63.69 (3F, q, J = 10.1)
ꢀ73.12 (3F, q, J = 10), ꢀ74.63 (3F, q, J = 10)
2
3bb
3cc
3dd
3ee
3ff
0.95 (t), 1.67 (m), 2.33 (m), 3.9–3.9 (m), 6.14t, J = 5.2), 6.48 (dd, J = 7.3, 4.7)
1.2–2.0, 2.81 (m), 3.78 (m), 6.26 (t, J = 5.4), 6.60 (dd, J = 7.7, 4.5)
3
4
0.93 (t), 1.39 (m), 1.63 (m), 2.32 (m), 2.82 (m), 3.69 (m), 3.94 (m), 6.13 (t, J = 5.2), 6.48
(dd, J = 7.3, 4.7)
1.33 (s), 1.34 (s), 2.27 (1H, m), 2.73 (m), 2.84 (m), 6.33 (t, J = 5.7), 6.72 (dd, J = 7.9, 4.4)
5
6
2.15 (m), 2.25 (m), 2.56 (m), 3.29 (m), 3.37 (m), 4.16–4.35 (m), 6.67 (d, J = 5.0), 7.01
(d, J = 4.0)
1.39 (3H, t, J = 7.6), 3.06 (2H, m), 3.20 (1H, dd, J = 15.0, 9.1), 3.68 (1H, ddq, 15.0, 7.6, 1.3),
4.74 (1H, t, J = 8.0)
7
3g
6.50, 28.73, 44.90, 48.03 (sept., J = 33.5),
49.50, 122.90 (q, J = 280), 126.00 (q, J = 280)
6.70, 17.53, 48.41, 56.55 (sept., J = 30),
63.20, 121.92 (q, J = 282), 122.40 (q, J = 280)
13.86, 38.08, 64.74, 87.63, 97.41 (qq, J = 34.9,
3.9), 121.00 (qd, J = 269.3, 2.9), 161.22 (d, J = 307)
10.85, 23.36, 39.14, 72.01, 99.06 (qq, J = 35.8, 4.8),
122.44 (q, J = 269.3), 162.27 (d, q, J = 307, 3.9)
8
3hg
4a
1.49 (3H, t, J = 7.4), 3.20 (4H, m), 4.80 (1H, t, J = 8.0)
ꢀ65.24 (3F, q, J = 8.2), ꢀ68.50 (3F, q, J = 8.2)
9
1.27, (3H, t, J = 7.6), 3.0 (1H, dd, J = 15.6, 5.2), 3.28 (1H, m), 3.45 (1H, qd, J = 7.6, 2.1), 3.73
(1H, qd, J = 7.6, 1.8), 5.41 (1H, t, J = 6.1)
0.96 (3H, t, J = 7.2), 1.66 (2H, sext. J = 7.6), 3.01 (1H, ddm, J = 16.3, 5.4, 1.1), 3.30 (1H, m),
3.36 (1H, dt, J = 9.0, 6.3), 5.42 (1H, dd, J = 6.8, 1.3)
1.1–2.0 (10H, m), 2.97 (1H, dd, 15.9; 5.3), 3.25 (1H, d m; J = 15.9), 3.47 (1H, m), 5.58
(1H, dt, J = 6.8, 1.5)
1.20 (3H, d, J = 6.3), 1.24 (3H, d, J = 6.3), 2.96 (1H, ddm, J = 15.9, 5.5), 3.26 (1H, dm, J = 15.9),
3.78 (1H, sept., J = 6.3), 5.52 (1H, dd, J = 5.3, 1.5)
ꢀ60.94 (3F, d, J = 17.2), ꢀ106.25 (1F, qq, J = 17.2, 5.7)
ꢀ60.99 (3F, dm, J = 17.3, 1.4), ꢀ106.20 (1F, qq, J = 17.3, 5.7)
ꢀ60.40 (3F, d, J = 16.5), ꢀ 105.58 (1 F, qq, J = 16.5; 5.5)
ꢀ60.91 (3F, d, J = 17.3), ꢀ106.12 (1F, qq, J = 17.2, 5.7)
10
11
12
4bh
4c
4d
20.09, 22.39, 38.45, 71.61, 85.49, 97.71
(qd, J = 34.9, 4.8), 121.38 (q, J = 268.4), 161.25
(dq, J = 306.0, 4.8)
13
4e
0.96 (3H, t, J = 7.5), 1.41 (2H, sext, J = 7.9), 1.61 (2H, sext, J = 7.9), 3.00 (1H, dd, J = 16.9, 5.6),
3.20 (1H, m), 3.40 (1H, dt, 16.0, 7.0), 3.63 (1H, dt, 16.0, 6.1), 5.42 (1H, ddd, J = 6.7, 5.4, 1.5)
ꢀ60.98 (3F, dt, J = 17.2, 1.1), ꢀ106.23 (1F, qqq, J = 17.2,
13.07, 19.04, 31.00, 38.00, 69.00, 87.80, 97.70
(qd, J = 35.0, 3.9), 121.20 (qd, J = 269.0, 2.9),
162.32 (dq, J = 307.0, 3.9)
9.3, 5.7)
14
15
16
17
18
4f
1.30, (9H, s), 2.96 (1H, dm, J = 17.7), 3.26 (1H, dm, J = 17.7), 5.70 (1H, m, J = 7.8)
ꢀ60.81 (3F, dt, J = 17.2, 1.1), ꢀ105.33 (1F, qm, J = 17.2)
ꢀ60.79 (3F, dt, J = 17.1, 1.4), ꢀ104.81 (1F, qm, J = 17.1)
27.63, 40.18 77.50, 81.72, 97.10 (q, J = 35.0),
121.70 (q, J = 270.0), 161.40 (dq, J = 307.0, 4.8)
14.00, 25.5, 38.52.12, 98.41 (qq, J = 34.9, 3.9),
121.00 (qd, J = 269.3, 2.9), 161.22 (d, J = 307)
5.86, 30.75, 42.76, 63.05, 99.31 (qd, J = 35.6, 3.5),
120.79 (q, J = 270), 158.90 (d, J = 318)
38.04, 41.02, 68.75, 87.24, 98.14 (qd, J = 35.7, 4.5),
121.00 (qd, J = 270, 2.7), 160.22 (dq, J = 310, 4.2)
30.85 (d, J = 2.9), 48.54, 66.40, 89.78, 99.62
(qd, J = 39.9, 2.9), 121.06 (qd, J = 269.3, 2.9),
161.06 (d, J = 310, 3.9)
4g
4h
4i
1.29, (3H, t, J = 7.5), 2.72 (2H, m), 2.91 (1H, dt, J = 15.8, 4.8), 3.35 (1H, dm, J = 15.8) 4.94
(1H, ddd, J = 7.8, 4.7, 3.1)
1.45, (3H, t, J = 1.5), 3.04 (1H, dm J = 13.5), 3.27 (1H, dm, J = 13.5), 3.46 (2H, m) 4.71 (1H,
ddd, J = 8.8, 4.2, 2.2)
3.03 (1H, dd, J = 16.3, 5.0), 3.30 (1H, mdtq, J = 16.3, 6.0, 1.9), 3.65 (3H, m), 3.90 (1H, m),
5.45 (1H, t, J = 6.0)
2.16 (2H, m), 3.28 (1H, m), 3.96 (2H, m), 4.15 (1H, m), 6.33 (1H, dd, J = 7.6, 4.6 Hz)
ꢀ61.29 (3F, dt, J = 17.1, 1.0), ꢀ103.81 (1F, qtd, J = 17.1,
5.6, 2.2)
ꢀ60.56 (3F, d, J = 17.2), ꢀ106.00 (1F, qq, J = 17.0, 5.7)
6a
ꢀ59.40 (3F, d, J = 16.6), ꢀ99.65 (1F, qt, J = 16.6, 4.6).
19
6b
1.69 (1H, d quint., J = 9.8, 1.8), 2.06 (1H, d, J = 9.8), 2.98 (1H, s), 3.11 (1 H, s), 3.18
(1H, dd, J = 8.9, 6.1), 3.64 (1H, d quint., J = 8.9, 1.8), 6.05 (1H, dd, J = 5.8, 3.4 Hz), 6.20
(1H, dd, J = 5.8, 3.1)
ꢀ59.78 (3F, d, J = 17.2), ꢀ97.03 (1F, qdd, J = 17.2, 5.7, 2.3)
42.03, 47.51, 47.55, 49.10, 50.24, 100.30
(q, J = 33.0), 121.88 (q, J = 270), 135.49,
138.22, 163.57 (dq, J = 311, 6.8)
20
21
22
6c
1.63 (1H, dm, J = 9.6, 1.5), 1.96 (1H, dm, J = 9.5), 2.96 (1H, s), 3.14 (1H, s), 3.50 (1H, d, J = 9.6),
3.68 (1H, dd, J = 9.6, 1.6), 6.0 (1H, dd, J = 5.7, 2.7), 6.12 (1H, dd, J = 5.7, 3.0)
0.94 (3H, t, J = 7.0), 1.389 (2H, m), 1.56, (2H, m), 2.63 (1H, dm, J = 15.5), 2.93 (1H, m),
3.36 (1H, m), 3.88 (1H, m), 4.49 (1H, dt, J = 9.6, 4.4)
ꢀ58.61 (3F, q, J = 10.2) ꢀ59.24 (3F, qd, J = 10.2, 2.9)
ꢀ60.46 (3F, m), ꢀ75.13 (m), ꢀ78.81 (1F, m, J = 12.2 Hz)
7dh
7eh
1.28 (9H, s), 2.79 (2H, m), 4.78 (t, J = 6.9), 4.85, dd, J = 9.2, 3.9)
ꢀ60.008 (3F, dd, J = 20.0, 10.7), ꢀ60.14 (3F, dd, J = 20.0, 10.7),
ꢀ 74.96 (m), ꢀ77.81 (quint. q, J = 10.7, 2,4), ꢀ78.03
(quint. q, J = 10.7, 2,4)
23
8
6.75 (1H, dd, J = 6.3, 3.8) 6.87 (1H, dd, J = 6.3, 3.3)
ꢀ58.63 (3F, d, J = 12), ꢀ126.56 (1F, qt, J = 12.0, 4.0)
112.75 (qd, J = 36.8, 4.8), 113,50 (d, J = 3.0),
121.50 (q, J = 272), 121.60 (d, J = 3.0),
165.25 (dq, J = 298, 4.0)
a
d
, ppm, J, Hz; in CDCl3 unless it mentioned otherwise.
b
Mixture of two isomers, ratio ꢁ1:1.
c
d
e
f
Mixture of two isomers, ratio 58:42.
Mixture of two isomers, ratio 63:37.
Single isomer.
Mixture of two isomers, ratio 52:48 in crude; 62:38 in distilled material.
13C NMR spectrum was acquired in DMF-d6.
Mixture of diastereomers, most resonances in 1H and 19F NMR spectra are overlapped.
g
h

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783
It should be pointed out, that despite relatively high reaction
temperature and excess of oxidizing agent we could not find any
evidence for the formation of the corresponding 1-S-dioxides,
previously prepared by Smart and Middleton [10] by reaction of
(CF3)2C55SO2 with vinyl ethers. Such high selectivity is quite
unusual, since the oxidation of hydrocarbon analogs, such as
2,2,4,4-tetramethyl-3-aminothietane by MCPBA under similar
conditions leads to high yield formation of the corresponding S-
dioxides [11] and the oxidation of a close fluorinated analog 3-thia-
4,4-bis(trifluoromethyl)-tricyclo[5.2.1.02,5]non-7-ene also pro-
ceeds with complete oxidation [12]. The only reported example
of selective oxidation of fluorinated heterocycle is the reaction of
2,2,3,3-tetrakis-(trifluoromethyl)thiirane with CF3SO2OOH leading
to the corresponding -1-S-oxide [13].
The S-oxides 3a–f were purified by short-path distillation under
reduced pressure and were fully characterized by NMR, IR and
mass- spectroscopy. The 1H and 19F NMR spectra oxides 3a–f
suggest that these compounds form as a mixture of isomers with
trans- and cis- orientation of oxygen and alkoxy groups. While the
oxidation of thietanes 3a,b,d,f results in the formation of
approximately equal amount of two isomers, in the case of
thietanes carrying bulky cyclohexyl-O- and t-BuO- groups (3c and
e, respectively), a noticeable difference in the ratio of two isomers
was observed, in favor of the less hindered trans-isomer (ratios
58:46 and 69:31, respectively). A reasonable correlation of the
ratio of isomers with an increase in the value of the Tafts steric
parameters (Es: n-Bu ꢀ 0.39; c-Hex ꢀ 0.79; t-Bu ꢀ 1.54 [14]) of the
substituent connected to oxygen is another argument in favor of
steric influence of the substituent on oxidation (see also discussion
below on oxidation of compound 2g).
Fig. 1. Space-filled drawing of thietane 3g.
19F NMR spectra of sulfoxides 3a–f typically contain four
resonances around ꢀ63 to ꢀ69 ppm, which are significantly
shifted (by ꢁ10 ppm) upfield compared to the corresponding
resonances of CF3– groups in starting thietanes. It should be
pointed out, that similar trend was previously reported for 4-
Fig. 2. Space-filled drawing of 3h.
The structure of 3g was firmly established by X-ray diffraction
(see Fig. 1).
alkoxy-2,2-bis(trifluoromethyl)-1-thietane-1,1-dioxides,
[10]
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithiet/ane -1- oxide and -
1,3-dioxide [13,15].
The presence of two bands in IR spectra of 3a–f in 1070–
1090 cmꢀ1 region is consistent with the presence of two isomeric
S-oxides and these values agree well with values reported for
This compound seems to be the first representative of
fluorinated thietanes containing a sulfone group of established
structure. In 1965, Middleton has reported the oxidation of 4-t-
butyl-2,2-bis(trifluoromethyl)thietane by peroxyacetic acid and
isolated the material containing two oxygens, but the structure of
the product was not reported [1]. It should be pointed out, that the
selective formation of compound 3g in this reaction is a clear
indication of a significant difference in the reactivity of the sulfur
atom in the C2H5S– group and thietane ring.
The oxidation of a sulfur atom of thietane ring in compound 3g
can be achieved, when the reaction with MCPBA is carried out at
ambient temperature. It leads to exclusive formation of S-oxide 3h.
In sharp contrast to thietanes 3a–f, the oxidation of 3g has
surprisingly high stereoselectivity resulting in the formation of a
single isomer of S-oxide 3h (NMR) with trans-relationship of
oxygen and C2H5SO2– group (single crystal X-ray diffraction, see
Fig. 2).
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithiethane-1-oxide,
-1,3-
dioxide [13,15] and S-oxide of 3-thia-4,4-bis(trifluoromethyl)tri-
cyclo[5.2.1.02,5]non-7-ene [12].
The outcome of the reaction of thietane 2g (bearing a C2H5S-
substituent) with MCPBA can vary depending on reaction
conditions. The ‘‘electron-rich’’ sulfur of C2H5S– group is oxidized
first and this process leads to the formation of sulfone 3g.
It is believed that the stereoselectivity of oxidation of 3g by
MCPBA is controlled by a steric bulk of C2H5SO2- and CF3-
substituents, which are effectively blocking one side of the
thietane ring (for space-filled drawing of thietane 3g see Fig. 1)
and forcing the attack of MCPBA on sulfur of the ring from less
hindered side of the molecule, leading to selective formation of
trans-isomer 3h.
(4)
2.2. Reductive ring expansion of 4-substituted 2,2-
bis(trifluoromethyl)thietanes
Another very interesting feature of thietanes 2a–h and 3g is an
ability to undergo unusual ring expansion under the action of

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V.A. Petrov, W.J. Marshall / Journal of Fluorine Chemistry 130 (2009) 780–787
certain reducing agents. For example, the treatment of fluorinated
thietanes 2a–h and 3g by aluminum powder activated by a small
amount of MCl2 (M = Hg or Pb, 10–20 mol%) in DMF solvent results
in an exothermic reaction and
a high yield formation of
dihydrothiophenes 4a–i (Eq. (5))
(5)
Polycyclic thietanes 5a–c were also converted into the
corresponding fused dihydrothiophenes 6a–c under action of
aluminum/MCl2 system (Eq. (6)).
Scheme 1.
sometimes can be observed at early stages of the reaction by NMR.
Disulfides 7d,e (R = n-Bu and t-Bu) were actually isolated in
reaction of thietanes 4e,f. Due to the presence of two asymmetrical
centers, both compounds 7d,e exist as a mixture of diastereomers
(see Table 1, entries 19, 20). Rather complicated 19F NMR spectra of
disulfides 7d,e contain multiplets around ꢀ60, ꢀ74 and ꢀ79 ppm
(ratio 3:1:1), attributed to CF255C(CF3)– group [3,9] and IR spectra
exhibit a strong band at ꢁ1747 cmꢀ1 which is also consistent with
the presence in the molecule of a fluorinated terminal double bond
[29].
(6)
On the contrary, well resolved 19F NMR spectra of dihydrothio-
phenes 4a–i, 6a,b exhibit only two resonances around ꢀ60 and
between ꢀ105 and ꢀ110 ppm (relative intensity 3:1) assigned to
CF3– and –F substituents, respectively. 13C NMR spectra of 4a–i,
6a,b along with other signals, exhibit a doublet, at ꢁ160 ppm
(J ꢁ 310 Hz) and a quartet at ꢁ100 ppm) (J ꢁ 35 Hz) assigned to
55CF and55CCF3, respectively. The IR spectra of dihydrothiophenes
4a–i, 6a,b contain an intense band ꢁ1675–1700 cmꢀ1 typical for a
trisubstituted double bond in fluorinated olefins [29].
Lead dichloride was found to be more efficient promoter for the
ring expansion of fluorinated thietanes, compared to HgCl2. In
general, reactions in the presence of PbCl2 are more exothermic,
faster and produce less disulfide by-products. The ring expansion
proceeds well in amide-like solvents such as DMF, dimethylace-
tamide or N-methylpyrrolidone, but no reaction between fluori-
nated thietanes and Al/MCl2 (M = Hg or Pb) was observed in ether
type solvents, such as THF, monoglyme or diglyme. Among other
reducing agents which were tried in this study, P[N(C2H5)3]3 also
was found to be effective for the conversion of bicyclic thietane 5a
and b into dihydrothiophene 6a and b (>95% yield after 1–2 h at
25 8C in DMF-d6 solvent, NMR). It should be pointed out, that
monocyclic thietanes 4a–e, i in the reaction with phosphines
behave differently and this transformation will be reported in
forthcoming publication. Although Mg/(CH3)3SiCl system was
widely employed by the Uneyama group for defluorination
We believe that the reductive ring expansion is a multistep
process which is initiated by single electron transfer (SET) from a
metal to the sulfur atom of the thietane ring (Scheme 1), leading to
the formation of the radical-anion 7a, which further undergoes
ring opening isomerization into a radical-anion 7b. This inter-
mediate forms unsaturated radical 7c after fluoride anion
elimination, which further undergoes either dimerization (forming
disulfides 7d,e) or
a reduction into anion 7f. Nucleophilic
intramolecular attack of the sulfide anion on the positively
charged carbon of the CF2– group of 7f results in the formation
of dihydrothiophenes 4a–i, 6a–c.
Indeed, reducing agents such as Al/PbX2 [16–20] or
P[N(C2H5)3]3 [21,22] are known to activate C-Hal bonds in CCl4,
FCCl3, CF3CCl3 through SET mechanism. For example, recently
Zeifman and Postovoii employed an Al/HgCl2 system for alkylation
of polyfluorinated ketones [20,23–25] and esters [26] by CCl4,
FCCl3 and CF3CCl3 and also reported a very interesting reductive
defluorination process under action of Al/HgCl2 on fluorinated
ketones leading to the formation of polyfluorinated enols after
acidic hydrolysis [27,28].
It should be pointed out that in the reaction of thietanes 4a–i
with activated Al the formation of disulfide similar to 7d,e

V.A. Petrov, W.J. Marshall / Journal of Fluorine Chemistry 130 (2009) 780–787
785
reactions of hexafluoroacetone and its derivatives [30], we found
that it has a relatively low activity for the conversion of 5b into 6b
(conversion <10%, after 3d at 25 8C, in THF or DMF solvents).
Some interesting examples of this new reductive defluorina-
tion process are reaction of thietanes 2g or 3g, bearing C2H5S– and
C2H5SO2– substituents, respectively (Eq. (5)). The reaction of 2g
with Al/PbCl2 system resulted in clean formation of 4g (73%
isolated yield, Eq. (5)). In sharp contrast to reaction with MCPBA,
which starts with oxidation of C2H5Sꢀ group (see Eq. (4)), a highly
selective ring expansion process involves exclusively a sulfur of
the thietane ring, while the C2H5S– group remains intact. The
reaction of thietane 3g, containing in the same position an
electron deficient C2H5SO2– group with Al/PbCl2 system is
another striking example. In general RSO2 group is sensitive
towards action of reducing agents and can be readily replaced by
hydrogen. For example, Na/Hg or activated Mg metal is widely
employed for the reduction of a PhSO2– group in fluorinated
organic molecules [31–33]). Interestingly, in the reaction of 3g
with the Al/PbCl2 system only the S(II) atom of the fluorinated
thietane ring, is involved in the process, leading to a selective
formation of dihydrothiophene 4h, containing C2H5SO2– group.
The structure of this compound was firmly established by single
crystal X-ray diffraction (Fig. 3).
Fig. 3. ORTEP drawing of 4h with thermal ellipsoids drawn to the 50% probability
level.
1,3-dithietane or thietanes 2a,b,d,e includes three consecutive
steps (see Eq. (7)):
(a) the generation of 2,2,4,4-tetrrakis(trifluoromethyl)-1,3-dithie-
tane by KF catalyzed reaction of sulfur with HFP in DMF
solvent;
(b) the synthesis of the corresponding thietane by the reaction of
the corresponding vinyl ether with 2,2,4,4-tetrrakis(trifluor-
omethyl)-1,3-dithietane;
(c) the ring expansion step, which is carried out by slow addition of
the crude reaction mixture containing thietane to a suspension
of Al/PbCl2 in DMF solvent.
2.3. ‘‘One-pot’’ type synthesis of 2-alkoxy-5-fluoro-4-
(trifluoromethyl)-2,3-dihydrothiophenes
Recently, we reported that 3-thia-4,4-bis(trifluoromethyl)-
tricyclo[5.2.1.02,5]non-7-ene [5] and other cycloadducts of HFTA
can be prepared by
a
direct reaction of sulfur/HFP and
Using these protocol compounds 4a,b,d,e were prepared in a
50–85% yield (see Section 3.3). Overall, this procedure significantly
simplifies the preparation of 5-fluoro-4-(trifluoromethyl)-2,3-
dihydro-2-alkoxythiophenes, by avoiding the isolation of either
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane and or 4-alkoxy-
2,2-bis(trifluoromethyl)thietane intermediates.
hydrocarbon substrate without isolation of HFTA dimer inter-
mediate [34]. In the present work this ‘‘one-pot’’ methodology
was extended for the synthesis of fluorinated dihydrothiophenes
4a,b,d,e (Eq. (7)).
Due to the presence of the highly electrophilic double bond,
dihydrothiophenes 4a–h are a valuable synthetic intermediates.
The synthetic utility of this group of materials was widely explored
by the Burger group [35–37]. In this work it was demonstrated that
compounds 4d,e can serve as precursors for the synthesis of 2-
fluoro-3-trifluoromethylthiophene. Using the procedure devel-
oped by the Sizov group for conversion of fluorinated sulfides into
thiols [38], dihydrothiophenes 4d,e were converted into previously
unknown 2-fluoro-3-trifluoromethylthiophene
yield (Eq. (8)).
8 in moderate
(7)
(8)
3. Experimental
1H and 19F NMR spectra were recorded on Bruker DRX-500
(499.87 MHz) and DRX-400 (376.8485 MHz) instruments, respec-
tively, using CFCl3 or TMS as an internal standards and CDCl3 as a
lock solvent. IR spectra were recorded on a PerkinElmer 1600 FT
spectrometer (KCl plates, liquid film or in KBr pellet for solids).
Moisture sensitive materials were handled in a glove box. The
purity of isolated materials was established using NMR and GC. GC
and GC/MS analysis were carried out on a HP-6890 instrument,
The synthesis of dihydrothiophenes 4a,b,d,e, which does not
require the isolation of either 2,2,4,4-tetrrakis(trifluoromethyl)-

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V.A. Petrov, W.J. Marshall / Journal of Fluorine Chemistry 130 (2009) 780–787
using HP FFAP capillary column and either TCD (GC) or mass
selective (GS/MS) detector. MCPBA (Aldrich, 65%), DMF (Aldrich,
anhydrous), CH2Cl2, Al powder (Aldrich, 99.5%, ꢁ200 mesh), HgCl2
and PbCl2 (Aldrich), sulfur (sublimed, 99.5%, ꢁ100 mesh, Alfa
Aesar), hexafluoropropene (DuPont) were obtained from com-
mercial sources and used without further purification. KF (Aldrich,
99%) was dried under dynamic vacuum at 100–130 8C for 3–4 h
and was stored and handled inside of dry box.
Caution: Dryness of DMF solvent and especially KF is crucial for
successful preparation of thietanes and dihydrothiophenes, since
the presence of insignificant amount of water severely effects the
activity of the catalyst.
3.4. Preparation of dihydrothiophenes 4a–i, 6a–c
3.4.1. From thietanes 2a–h, 3g, 5a–c
In a dry 100 mL flask equipped with thermocouple, reflux
condenser and addition funnel 1–3 g of aluminum powder and 50–
70 mL of dry DMF was placed under nitrogen atmosphere. To a
vigorously agitated reaction mixture 0.2–0.6 g of PbCl2 was added
and after ꢁ1–5 min induction period an exothermic reaction
started and the temperature rose to 27–30 8C. The solution of ꢁ10–
12 g of the corresponding thietane in 20 mL of dry DMF was slowly
added to reaction mixture over ꢁ1–1.5 h period with the rate
which was sufficient to keep internal temperature of the reaction
mixture <35 8C. An agitated reaction mixture was left overnight.
Next morning the reaction mixture was filtered, filter cake was
washed with hexane (20 mL) and filtrate was diluted with 300 mL
of 10% hydrochloric acid, extracted with hexane (3ꢂ 50 mL).
Combined organic solution was washed by 10% hydrochloric acid
(3ꢂ 300 mL), dried over MgSO4, a solvent was removed under
vacuum and the residue was distilled under reduced pressure to
afford dihydrothiophenes 4a–i, 6a–c of 98–99.8% purity (GC,
NMR). Yields, boiling and melting points, IR, mass and NMR
spectroscopy data of 4a–i, 6a–c are given in Tables 1 and 2.
Compounds 2a–i, 5b,
c were prepared using literature
procedures [3,5]. Due to a high ratio of sulfur to fluorine, elemental
analyses were not attempted for new materials. The purity of all
isolated compounds was >98%, with exception of compounds 6c
and 7d,e (>95%; GC and/or NMR spectroscopy).
3.1. Crystallography
X-ray data for 3g,h and 4h were collected at ꢀ100 8C using a
Bruker 1K CCD system equipped with a sealed tube molybdenum
source and a graphite monochromator.
The structures were solved and refined using the Shelxtl [39]
software package, refinement by full-matrix least squares on F2,
scattering factors from Int. Tab. Vol C Tables 4.2.6.8 and 6.1.1.4.
Crystallographic data (excluding structure factors) for the struc-
tures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC #721667, #721668 and #721669. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk).
3.4.2. ‘‘One-pot’’ procedure for the preparation of 5-fluoro-4-
(trifluoromethyl)-2,3-dihydro-2-alkoxythiophenes 2a,b,d,e
In a dry 250 mL dry flask equipped with thermocouple, dry ice
condenser and gas inlet tube under nitrogen blanket it was placed
2–3 g of dry KF, 100–150 mL of dry DMF and of 3.2–6.4 g (0.1–
0.2 mol) of sublimed sulfur. The addition of sulfur resulted in the
appearance of green or blue color of the solution. Vigorously
agitated reaction mixture was brought up to ꢁ30–35 8C and the
addition gaseous hexafluoropropene (HFP) was started with the
rate, which allows to maintain internal temperature at 35–45 8C,
but at the same time to avoid the condensation of excessive
amount of HFP on dry ice condenser. After required amount of HFP
was added and all sulfur went into solution, the reaction mixture
was agitated for an hour without additional heating. One
equivalent of vinyl ether 2a,b,d,e was added to the reaction
mixture and it was agitated at ambient temperature for 24–48 h,
till the conversion of starting materials would exceed 90% (GC). The
agitation was stopped and the reaction mixture was decanted into
addition funnel (reaction mixture A). In a separate 500 mL flask
equipped with thermocouple, reflux condenser and addition
funnel aluminum powder (1–3 g) and dry DMF (50–100 mL) were
added under nitrogen blanket. To a vigorously agitated reaction
mixture it was added 0.2–0.6 g of PbCl2, after ꢁ1–5 min induction
period an exothermic reaction started and the internal tempera-
ture rose to 27–30 8C. At this point the addition of the reaction
mixture A was started with the rate, sufficient to maintain internal
temperature at 30–35 8C. After addition was finished (1–2 h), the
reaction mixture was agitated for 12–16 h at ambient temperature
and worked up using hexane extraction, as it was described above.
The isolated yields of dihydrothiophenes 4a,b,d,e were 50–85%.
3.2. Oxidation of thietanes 2a–c,e–f,i
To a vigorously agitated suspension of 10–12 g of MCPBA in 75–
100 mL of CH2Cl2, placed in 250 mL round-bottomed flask,
equipped with thermocouple, dry-ice condenser and addition
funnel, it was slowly added over ꢁ1 h period a solution of 5–7 g of
2a–c,e–f,i in 10–20 mL of CH2Cl2 with the rate, which allow to
maintain the internal temperature at 30–35 8C. The reaction
mixture was agitated overnight and next morning was filtered. The
filter cake was washed with ꢁ 10 mL of cold CH2Cl2 and combined
filtrate was treated in a separatory funnel with 1 M solution of
sodium thiosulfate (2ꢂ 200 mL). Organic layer was separated,
checked for the presence of peroxides using wet KI/starch paper,
washed with 300 mL of saturated solution of NaHCO3 to bring pH
of to ꢁ7–8. Organic layer was separated, dried over MgSO4, filtered,
solvent was removed under reduced pressure at 25–40 8C and the
residue was distilled under vacuum using micro distillation
column with a short deflegmator to give 4–6 g of clear colorless
liquid. In case of sulfoxides 3c,e,g distilled material crystallized
upon standing at ambient temperature. Yields, boiling and melting
points, IR, mass and NMR spectroscopy data of sulfoxides 3a–e are
given in Tables 1 and 2.
3.5. Preparation of thiophene 8
A mixture of 10–15 g P2O5 and 6–8 g of 4d or 4f was slowly
heated to ꢁ200 8C in the flask equipped with short-path distillation
column and the product was distilled out. It was collected 3.5–4.0 g
of clear liquid, b.p. 99–100 8C. Data of IR, mass and NMR
spectroscopy are given in Tables 1 and 2.
3.3. Preparation of sulfone 3g
To a vigorously agitated suspension of 12 g of MCPBA in
100 mL of CH2Cl2 a solution of 6 g of 2g in 20 mL of CH2Cl2 was
slowly added at 2–4 8C. The reaction mixture was agitated at
this temperature for 2 h and was worked up as it was described
in the previous experiment. It was isolated 4.5 g (60%) of
3g. Data of IR, mass and NMR spectroscopy are given in Tables 1
and 2.
Acknowledgements
Authors thank Dr. Victor Cherstkov for valuable discussions and
Timothy Bentz for technical assistance.

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787
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