Radical-mediated thiodesulfonylation of the vinyl sulfones: access to (α-fluoro)vinyl sulfides

Article abstract of DOI:10.1016/j.tetlet.2009.07.063

Radical-mediated thiodesulfonylation of the vinyl and (α-fluoro)vinyl sulfones, derived from aldehydes and ketones, with aryl thiols in organic or aqueous medium provided access to vinyl and (α-fluoro)vinyl sulfides. The vinyl sulfides were formed predominantly with E stereochemistry independent of the stereochemistry of the starting vinyl sulfones.

Full text of DOI:10.1016/j.tetlet.2009.07.063

Tetrahedron Letters 50 (2009) 5424–5427
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Tetrahedron Letters
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Radical-mediated thiodesulfonylation of the vinyl sulfones: access
to (a-fluoro)vinyl sulfides
*
Pablo R. Sacasa, Jessica Zayas, Stanislaw F. Wnuk
Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Radical-mediated thiodesulfonylation of the vinyl and (
and ketones, with aryl thiols in organic or aqueous medium provided access to vinyl and (
sulfides. The vinyl sulfides were formed predominantly with E stereochemistry independent of the ste-
reochemistry of the starting vinyl sulfones.
a
-fluoro)vinyl sulfones, derived from aldehydes
-fluoro)vinyl
Received 18 June 2009
Revised 9 July 2009
Accepted 13 July 2009
Available online 17 July 2009
a
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Fluoroalkenes
Radical reactions
Vinyl sulfides
a-Fluoro vinyl sulfides
Vinyl sulfones
Vinyl sulfides are valuable tools in organic synthesis and are
ira23 couplings, Friedel–Crafts vinylation,24 and other transfor-
mations.21,25,26
used as enolate ion equivalents,1 as components of [2+2] cyclo-
additions,2 and as substrates in transition metal-catalyzed car-
bon–carbon bond forming reactions.3 Methods for the synthesis
of 1-alkenyl sulfides include Wittig reaction,4 ionic and radical
additions of thiols to alkynes,5 coupling of 1-alkenyl halides with
thiols,6–9 and transition metal-catalyzed anti-Markovnikov hydro-
thiolation9–12 of alkynes with arenethiols and alkanethiols to pro-
duce E isomers, including hydrothiolation in water medium.13
Vinyl sulfonium ions, generated via the biological methylation
of the corresponding vinyl sulfides, act as inhibitors of thioether
S-methyltransferase14 and proteolytic enzyme papain.15 They are
highly reactive toward nucleophiles and bind covalently to DNA,
RNA, and proteins in vivo.16 Moreover, vinyl sulfonium salts are
more electrophilic than the corresponding vinyl sulfones, which
are known for their ability to inhibit cysteine proteases.17,18
Removal of the sulfonyl group from the vinylic carbon is usually
achieved by reductive methods27 or by addition–elimination pro-
cesses in which the sulfonyl group is replaced, for example, with
tributylstannyl substituent.28–30 The latter can be then conve-
niently removed by protiodestanylation or utilized in other syn-
thetic transformations.29,31 Heating of the vinyl arylsulfones with
tris(trimethylsilyl)silane or germane at reflux in benzene or tolu-
ene effected substitution of a sulfonyl group with a silyl or germyl
group to give vinyl silanes or germanes including
a-fluoro substi-
tuted analogues.32,33
Herein, we report stereoselective radical-mediated thiodesulf-
onylations of vinyl and ( -fluoro)vinyl sulfones with aryl thiols
to provide access to vinyl and ( -fluoro)vinyl sulfides. Such thio-
a
a
desulfonylation provides a flexible alternative to the hydrothiola-
The (a-halo)vinyl sulfides can be prepared by Wittig–Horner
tion of alkynes with thiols under radical or metal catalysis
conditions. It also offers convenient preparations of (a-fluoro)vinyl
sulfides—a class of interesting fluoroalkenes which remain unex-
plored.34 The thiodesulfonylation can also be viewed as reductive
deoxygenation of sulfones to the corresponding sulfides—a trans-
formation which requires harsh conditions incompatible with
most functional groups.35
reactions with diethyl chloro(phenylthio)methanephosphonate19
or by addition of the hydrogen halides (HI, HBr, and HCl) to acety-
lenic sulfides (chalcogenides).20 The regioselectivity and stereose-
lectivity of such additions were improved when equivalent
quantities of hydrogen halide, generated in situ from trimethylsilyl
halides and anhydrous methanol, were utilized21 instead of excess
aqueous HX or saturated gaseous HX in benzene. The (
a
-halo)vinyl
Treatment of the sulfonyl-stabilized enolates generated from
diethyl (phenylsulfonyl)methylphosphonate with aliphatic and aro-
matic aldehydes and ketones 1a–g gave the corresponding E-vinyl
sulfones 2a–f and vinyl sulfone 2g (72–95%, Scheme 1).32 Analogous
treatment of 1a–h with diethyl fluoro(phenylsulfonyl)methylphos-
sulfides have been employed in Stille,21,22 Negishi,22 and Sonogash-
* Corresponding author. Tel.: +1 305 348 6195; fax: +1 305 348 3772.
E-mail address: wnuk@fiu.edu (S.F. Wnuk).
phonate produced (a
-fluoro)vinyl sulfones 3a–h.31,32
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.063

P. R. Sacasa et al. / Tetrahedron Letters 50 (2009) 5424–5427
5425
Table 1
R1
O
R1
PhSO2CHXPO(OEt)2
HMDS/THF/-78 ºC
Synthesis of vinyl sulfides via thiodesulfonylation of vinyl sulfones
Entry
Sulfone
Methoda
Productb
(E/Z)c
Yieldd
61
O
S
O
R
R
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2a (E)
2a (E)
2a (E)
2a (E)
2a (E)
2a (E)
2a (E)
2a (Z)
2a (Z)
2a (Z)
2b (E)
2b (E)
2c (E)
2c (E)
2c (E)
2c (E)
2c (E)
2d (E)
2d (E)
2d (E)
2e (E)
2f (E)
2g
A
B
C
C
B
B
C
A
B
C
A
B
A
B
C
B
B
A
B
C
A,B,C
A,B,C
B
C
C
4a
4a
4a
4a
5a
6a
7a
4a
4a
4a
4b
4b
4c
4c
4c
5c
6c
4d
4d
4d
4e
4f
(95:5)
(95:5)
(95:5)
(95:5)
(95:5)
(98:2)
(95:5)
(95:5)
(95:5)
(95:5)
(85:15)
(85:15)
(95:5)
(95:5)
(95:5)
(95:5)
(95:5)
(75:25)
(75:25)
(85:15)
X
71e, 65e,f, 10e,g
70e
2 X = H, 3 X = F
1
R1
H
R
20e,h
Series:
a
61
55
C6H5
b
c
d
e
f
4-(CH3)C6H4
4-(CF3)C6H4
3,4-(MeO)2C6H3
C6H5CH2CH2
c-C6H11
85
H
H
H
H
H
65e
95e
70, (80e)
55e
54, (63e)
59, (70e), 5e,g
85e, 80e,f, 50g
94e, 96e,i
58
g
h
-(CH2)5-
40, (55e)
55
C6H5
CH3
64e
70e
Scheme 1. Synthesis of E-vinyl and (E/Z)-(
a
-fluoro)vinyl sulfones.
n.r.
n.r.
4g
4g
5g
55
76e
Reaction of the conjugated vinyl sulfone E-2a with benzenethiol
(2 equiv) in the presence of ACCN as a radical initiator at reflux in
toluene (12 h) produced the vinyl sulfide 4a (61%; Scheme 2;
Table 1, entry 1). The radical thiodesulfonylation was also effective
in aqueous36 medium. Thus, treatment of E-2a with benzenethiol/
ACCN or AIBN in water (100 °C) produced sulfide 4a in 71% and 65%
yields (entry 2). Replacement of water with MeOH or EtOH pro-
duced a homogenous reaction mixture and did not affect the yield
of the thiodesulfonylation reaction (entry 3). The reaction has a
general character since the presence of the alkyl (series b), elec-
tron-withdrawing (CF3, series c), or electron-donating (MeO, series
d) groups on the phenyl ring attached to the double bond had only
modest effect on rate and yield of thiodesulfonylation reactions
with benzenethiol (entries 11–15, 18–20).
2g
2g
62
a
Method A: thiol/ACCN/toluene/110 °C/12 h; Method B: thiol/ACCN/H2O/100 °C/
10 h; Method C: thiol/ACCN/MeOH/65 °C/10 h.
b
Reactions were performed on 0.1–1.0 mmol scale of sulfones (0.05 mM) with
1.25–2.0 equiv of thiols and 0.1–0.5 equiv of ACCN or AIBN.
c
Determined by GC–MS and/or 1H NMR.
Isolated yield.
Based on GC–MS.
AIBN instead of ACCN.
Without ACCN.
Disulfide instead of thiol, 16 h.
In EtOH.
d
e
f
g
h
i
Treatment of the vinyl sulfones derived from the aliphatic alde-
hydes (series e and f) with benzenethiol/AIBN or ACCN under prot-
ic and aprotic conditions did not yield the corresponding vinyl
sulfides (entries 21 and 22). However, sulfone 2g, derived from
cyclohexanone, underwent thiodesulfonylation reaction affording
sulfide 4g (entries 23 and 24), demonstrating that the thiodesulf-
onylation can serve as a convenient method for the synthesis of
the trisubstituted vinyl sulfides.
replacement of benzenethiol with phenyl disulfide also effected
conversion of the sulfone 2a to the sulfide 4a in low yield, although
reaction required longer time (entry 4). However, reaction of 2a
with phenyl disulfide without radical initiator failed to afford 4a.
Treatment of E-2a with 4-methylbenzenethiol or 4-aminoben-
zenethiol in H2O/ACCN produced the corresponding vinyl sulfides
5a (61%) and 6a (55%) (entries 5 and 6). Analogously, E-2c was con-
verted to 5c and 6c (entries 16 and 17). Thiodesulfonylation of E-2a
with 4-mercaptobenzoic acid (R2 = CO2H) in MeOH produced the
methyl ester 7a (85%; entry 7). Thus, thiodesulfonylation protocol
is compatible with amino and carboxylate functional groups
vulnerable to the oxidative and reductive procedures. Attempted
thiodesulfonylations of 2a or 2c, as well as 3a, with alkanethiols
(2-mercaptoethanol, 1-propanethiol) or thioacetic acid under rad-
ical conditions did not produce the corresponding vinyl sulfides.
Radical-mediated thiodesulfonylation of the vinyl sulfones 2 oc-
curred basically with retention of the E stereochemistry although
small amounts of the Z isomers were detectable by GC–MS and
1H NMR of the crude reaction mixtures (Table 1). In order to study
stereochemical outcome of the thiodesulfonylation reactions Z-vi-
nyl sulfone 2a was prepared by anti-Markovnikov addition of
PhSH/NaOH to phenylacetylene followed by the oxidation of the
resulting (Z)-2-phenyl-1-phenylthioethene.37 Treatment of Z-2a
with PhSH in aqueous or organic medium produced sulfide 4a in
very good yields with inversion of stereochemistry (E/Z, 95:5; en-
tries 8–10). Thus, the vinyl sulfides are formed predominantly with
E stereochemistry independent of the stereochemistry of the start-
ing vinyl sulfones.
Thermal reaction of E-2a with benzenethiol without radical ini-
tiators produced 4a but in lower yield (entry 2). Likewise, the
R2
R2
Method A, B or C
SH
R1
R1
O
S
S
R
R
O
4 R2 = H
2
R1
H
R
5 R2 = CH3
6 R2 = NH2
7 R2 = CO2Me
Series:
a
b
c
d
e
f
C6H5
4-(CH3)C6H4
4-(CF3)C6H4
3,4-(MeO)2C6H3
C6H5CH2CH2
c-C6H11
H
H
H
H
H
g
-(CH2)5-
Radical thiodesulfonylation permitted synthesis of the sparsely
Scheme 2. Thiodesulfonylation of vinyl sulfones. Synthesis of vinyl sulfides.
developed34
(a-fluoro)vinyl sulfides 8–11 in high yields (Scheme

5426
P. R. Sacasa et al. / Tetrahedron Letters 50 (2009) 5424–5427
Table 2
Synthesis of
sulfones
3). Thus, treatment of 3a (E/Z, 96:4) with benzenethiol in organic or
protic medium in the presence of ACCN gave E/Z-8a in good to
excellent yields with the ‘overall’ retention of stereochemistry
(Table 2, entries 1 and 2).
(
a
-fluoro)vinyl sulfides via thiodesulfonylation of
(a
-fluoro)vinyl
Yieldd
Entry
Sulfone
Methoda
Productb
(E/Z)c
Thiodesulfonylation appears fairly general since sulfones 3b, 3c,
and 3d with the alkyl (Me), electron-withdrawing (CF3), or elec-
tron-donating (MeO) substituents on the phenyl ring attached to
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
3a (96:4)
3a (96:4)
3a (94:4)
3b (86:14)
3b (86:14)
3b (86:14)
3b (86:14)
3b (86:14)
3c (90:10)
3d (97:3)
3e (88:12)
3f (84:16)
3g
3h (57:43)
3h (57:43)
3h (100:0)
3h (0:100)
3h (100:0)
3h (57:43)
3h (57:43)
A
B
A
A
B
C
C
C
B
B
B
C
B
B
C
C
C
C
B
C
8a
8a
(92:8)
(94:6)
92
65e
50
10a
8b
8b
8b
9b
11b
8c
8d
8e
8f
(83:17)
(93:7)
(92:8)
(92:8)
(92:8)
(94:6)
(93:7)
(93:7)
(70:30)
(70:30)
82, 62f
56
the double bond also produced (
4–6, 9, and 10).38 Thiodesulfonylation of
84:16; 0.5 equiv) with PhSH (1.0 equiv; Method C) in the presence
of the parent -H sulfone E-2b (0.5 equiv) showed that product 8b
a
-fluoro)vinyl sulfides (entries
92f
90
a-fluoro sulfone 3b (E/Z,
69f
60f, 72
73
a
[30 min (70%; E/Z, 93:7; with all Z-3b being consumed); 1 h (88%;
E/Z, 93:7); 2 h (95%; E/Z, 93:7)] is formed faster than 4b [30 min
(35%, E/Z, 90:10), 1 h (45%, E/Z, 88:12); 2 h (48%; E/Z, 88:12)].
Treatment of the unconjugated sulfone 3e or 3f with benzene-
thiol produced the vinyl sulfide 8e or 8f in low yields (entries 11
and 12). Careful analysis of the crude reaction mixture indicated
that Z-3e or Z-3f isomer was consumed during reactions to pro-
duce primarily E-sulfides, while the E-sulfones remained mostly
unreacted. These results are in agreement with the lack of reactiv-
ity of E-2e and E-2f vinyl sulfones. Sulfone 3g produced tetrasub-
stituted sulfide 8g (entry 13).
Reaction of 3h (E/Z, 57:43) with benzenethiol also afforded tet-
rasubstituted (a-fluoro)vinyl sulfide 8h (E/Z, 50:50; entries 14 and
15). Thiodesulfonylation was not stereospecific since reactions of
pure E-3h or Z-3h with benzenethiol also gave 8h as mixture of
E/Z-isomers (entries 16 and 17). Thiodesulfonylation occurred with
other aromatic thiols to yield various vinyl sulfides (entries 3, 7,
and 8, 18–20). It is noteworthy that hydrothiolation approaches
10g
12h
65
58
68
85
70
96
59
8g
8h
8h
8h
8h
9h
9h
11h
(50:50)
(50:50)
(50:50)
(33:67)
(64:36)
(50:50)
(55:45)
42
a
Method A: thiol/ACCN/toluene/110 °C/6 h; Method B: thiol/ACCN/H2O/100 °C/
6 h; Method C: thiol/ACCN/MeOH/65 °C/5 h.
b
Reactions were performed on 0.1–0.5 mmol scale of sulfones (0.05 mM) with
1.25–2.0 equiv of thiols and 0.25–0.50 equiv of ACCN or AIBN.
c
Determined by GC–MS and 1H or 19F NMR.
Isolated yield.
Reaction with phenyl disulfide gave 8a (55%, Method B or C).
AIBN instead of ACCN.
83% based on Z-3e.
75% based on Z-3f.
d
e
f
g
h
are inapplicable for the synthesis of (a-fluoro)vinyl sulfides since
the 1-fluoroalkynes are unstable and virtually unknown.39
Desulfonylation occurred probably via b-elimination of the sul-
fonyl radical from the radical intermediates formed after addition
of PhSÅ to vinyl sulfones (presumably via a radical addition–elimi-
nation mechanism).30,32 Lack of stereochemistry is probably the re-
sult of cis–trans isomerization of a radical intermediate leading
predominantly to the formation of the more stable E isomers under
thermal conditions.5
offers for the first time a general and bench-friendly procedure for
the synthesis of (a-fluoro)vinyl sulfides.
Acknowledgments
This investigation was supported by award SC1CA138176 from
NIGMS and NCI. P.R.S. and J.Z. were sponsored by the MBRS RISE
program (NIGMS; R25 GM61347).
In summary, we have developed radical-mediated thiodesulf-
onylations of the vinyl and (a-fluoro)vinyl sulfones with aryl thiols
to provide access to vinyl and (
a
-fluoro)vinyl sulfides. This method
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Series:
a
b
c
d
e
f
C6H5
H
H
H
H
H
H
4-(CH3)C6H4
4-(CF3)C6H4
3,4-(MeO)2C6H3
C6H5CH2CH2
c-C6H11
g
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-(CH2)5-
C6H5
CH3
Scheme 3. Thiodesulfonylation of (E/Z)-(
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a-fluoro)vinyl sulfones. Synthesis of (E/Z)-
(a

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and the resulting mixture was degassed for 30 min at rt. PhSH (18 mg, 17 lL,
0.2 mmol) and ACCN (7 mg, 0.03 mmol) were added and the heterogenous
reaction mixture was heated at 105 °C (oil bath) for 6 h. The resulting brownish
residue was partitioned (CHCl3/H2O/brine), and the organic layer was dried
(MgSO4), evaporated, and purified by column chromatography (hexane/EtOAc,
9:1) to give 8d (21 mg, 73%: E/Z, 93:7): 19F NMR (CDCl3) d ꢀ81.5 (d, J = 16.3 Hz,
0.07F), ꢀ88.2 (d, J = 32.4 Hz, 0.93F); HRMS calcd for C16H15FNaO2S [M+Na]+:
313.0675; found: 313.0687; GC–MS (EI) m/z 290 (M+; tR = 29.32 min, E,
tR = 30.04 min, Z). E-8d had: 1H NMR (CDCl3) d 3.86 (s, 3H), 3.88 (s, 3H), 6.24
(d, J = 32.4 Hz, 1H), 6.82 (d, J = 8.4 Hz, 1H), 7.04 (dd, J = 2.1, 8.4 Hz, 1H), 7.15 (d,
J = 1.7 Hz, 1H), 7.24–7.35 (m, 3H), 7.42–7.46 (m, 2H); 13C NMR (CDCl3) d 55.8
and 55.9 (CH3), 110.1 (C5), 111.6 (d, J = 10.1 Hz, C2), 118.3 (d, J = 13.2 Hz, Cb),
122.2 (d, J = 7.5 Hz, C6), 125.9 (d, J = 6.0 Hz, C1), 127.5 (C40), 129.3 (C30), 129.5
(C20), 132.6 (d, J = 3.7 Hz, C10), 148.8 (C4), 149.1 (d, J = 3.1 Hz, C3), 150.6 (d,
J = 301.7 Hz, C ); Z-8a had: 1H NMR d 6.74 (d, J = 16.4 Hz).
a
34. 2,2-Difluorovinyl ketones react with thiols to give (
a
-fluoro)-vinyl sulfides:
39. Viehe, H. G.; Merényi, R.; Oth, J. F. M.; Valange, J. F. M. Angew. Chem., Int. Ed.
1964, 3, 746.
40. Inbasekaran, M.; Peet, N. P.; McCarthy, J. R.; Le Tourneau, M. E. J. Chem. Soc.,
Chem. Commun. 1985, 678–679.
Ichikawa, J.; Kobayashi, M.; Yokota, N.; Noda, Y.; Minami, T. Tetrahedron 1994,
50, 11637–11646.
35. Simpkins, N. S. Sulphones in Organic Synthesis; Pergamon Press: Oxford, 1993.