Samarium(II) iodide-mediated deoxygenative debromination of α-bromo-β-hydroxy (acetoxy) phenyl sulfones: Synthesis of α,β-unsaturated sulfones

Article abstract of DOI:10.1016/S0040-4039(02)00224-1

Deoxygenative debromination of α-bromo-β-hydroxy (acetoxy) phenyl sulfones with samarium(II) iodide led to substituted α,β-unsaturated sulfones in good to excellent yields. The E-isomer is the major product. A possible mechanism via an α-sulfonyl radical pathway is proposed.

Full text of DOI:10.1016/S0040-4039(02)00224-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2285–2288
Samarium(II) iodide-mediated deoxygenative debromination of
a-bromo-b-hydroxy (acetoxy) phenyl sulfones: synthesis of
a,b-unsaturated sulfones
Vichai Reutrakul,* Suwatchai Jarussophon, Manat Pohmakotr, Yupa Chaiyasut,
Saengvimon U-Thet and Patoomratana Tuchinda
Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
Received 26 November 2001; revised 24 January 2002; accepted 30 January 2002
Abstract—Deoxygenative debromination of a-bromo-b-hydroxy (acetoxy) phenyl sulfones with samarium(II) iodide led to
substituted a,b-unsaturated sulfones in good to excellent yields. The E-isomer is the major product. A possible mechanism via an
a-sulfonyl radical pathway is proposed. © 2002 Elsevier Science Ltd. All rights reserved.
The reductive elimination reaction of b-oxygen substi-
tuted sulfones to olefins, known as the Julia–Lythgoe
olefination, has been recognized as a very useful syn-
thetic transformation and is commonly effected using
sodium amalgam.1 Recent modification of the reaction
employing SmI2–THF as the reducing agent has
extended the scope of this reaction considerably.2 As
part of our continuing investigation on the utilization
of the a-halomethyl phenyl sulfones as one-carbon
building blocks,3 we wish to report our studies on the
deoxygenative debromination of a-bromo-b-hydroxy
(acetoxy) phenyl sulfones with SmI2–THF with or with-
out HMPA leading to substituted a,b-unsaturated sul-
fones in good to excellent yields.4,5 These results
provide mechanistic insight and represent the first
report6 on the utilization of a-halomethyl phenyl sul-
fones for the synthesis of a,b-unsaturated sulfones
(Scheme 1).
The a-haloalkyl-b-hydroxy (acetoxy) phenyl sulfone
adducts 3 were prepared, in a straightforward manner,
by the reaction of the aldehydes and ketones 1 with
a-haloalkyl phenyl sulfones 2 employing LDA as the
base. The yields of the corresponding b-hydroxysulfone
1. 1.1 eq. LDA, THF
-78 oC, 2-3 h
R2
X
SO2Ph
R3
X
R2
R1
OR4
+
R1
R3
O
SO2Ph
2. aq.NH4Cl or Ac2O
DMAP, CH2Cl2, rt
or Ac2O in situ
2
1
3
1.5 eq. SmI2, THF, and/or
HMPA, 0 oC, 20 min
R1, R2 = -(CH2)n-, n = 3, 4, 5
R1 = H, R2 = n-heptyl, i-propyl, phenyl, alkenyl
R3 = H, CH3, n-pentyl ; R4 = H, Ac
SO2Ph
R1
R3
X = Cl, Br, I
R2
4
Scheme 1.
Keywords: a,b-unsaturated sulfones; samarium; deoxygenation; debromination; radical.
* Corresponding author. Tel.: +66(0)2245-8332; fax: +66(0)2644-5126; e-mail: scvrt@mahidol.ac.th
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00224-1

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V. Reutrakul et al. / Tetrahedron Letters 43 (2002) 2285–2288
derivatives 3 (R4=H) were in the range 64–84%. The
corresponding b-acetoxysulfone derivatives 3 (R4=Ac)
were prepared by acetylation of the hydroxy derivatives
under standard conditions. In the cases where R3=CH3
and n-pentyl, the b-acetoxysulfone derivatives could be
prepared by in situ quenching of the incipient alkoxide
adducts of compounds 1 and 2 with acetic anhydride.
The yields of compound 3 (R4=Ac) were in the range
50–98%. The key deoxygenative dehalogenation reac-
tion was carried out using 1.5 equivalents of SmI2 in
THF with or without HMPA. Initial studies to find
suitable halogen substituents, were carried out with the
adducts 3 (X=Cl, Br, I); (R1=R2=-(CH2)2-; R3=H;
R4=Ac). It was found that when X=Cl, the deoxy-
genative dechlorination was incomplete giving the cor-
responding vinyl sulfone in only 18% yield together
with 64% recovery of the starting material. When X=I
and Br, the deoxygenative dehalogenation gave the
corresponding vinyl sulfone in a comparable yield
(79%). Due to the fact that the bromo derivatives 3
(X=Br) are much easier to prepare and to handle, it
was decided to use the bromosulfonyl derivatives 3
(X=Br) in the subsequent investigations. The results
are summarized in Table 1.7
The results indicated that the deoxygenative debromi-
nation of the adducts 3 (R4=Ac) gave higher yields of
the a,b-unsaturated sulfones than the hydroxy deriva-
Table 1. Deoxygenative debromination of 3 with 1.5 equiv. SmI2/THF/0°C/20 min with and without HMPA

V. Reutrakul et al. / Tetrahedron Letters 43 (2002) 2285–2288
2287
SmI2
Br
R2
R1
R2
R1
PhO2S
PhO2S
R3
R4O
R3
OR4
Sm(III)
3
10
R3
R2
PhO2S
R1
4
Scheme 2.
H
PhO2S
Br
H
Br
PhO2S
OAc
H
H
AcO
anti-6
syn-5
SO2Ph
SO2Ph
n-C5H11
Br
H
H
n-C5H11
PhO2S
H
H
OAc
7
8 (isomers 8a and 8b)
9
tives R4=H; except in entry 7. The addition of HMPA
led to the lowering of the yields of products 4. For the
aldehyde entries 4, 5, 6 and 8, the product vinyl sul-
fones 4, R3=H and CH3 showed high E-selectivity.
When R3=n-C5H11, the product a,b-unsaturated sul-
fones 4 exhibit moderate to high E-selectivity.
for the synthesis of a,b-unsaturated sulfones and partic-
ularly sulfonyl substituted 1,3-dienes.4d Further investi-
gation into the scope of the reaction is in progress.
Acknowledgements
The deoxygenative debromination of the diastereomers
syn-5 (J=1.6 Hz) and the anti-6 (J=7.6 Hz) and the
pair of diastereomers of 8 (isomers 8a and 8b) gave
insight into the mechanism of the reaction.8
We wish to thank the Thailand Research Fund for the
award of the Senior Research Scholar to V.R. and a
Ph.D. scholarship to S.J. through the Royal Golden
Jubilee Ph.D. program. The partial supports by the
Development and Promotion of Science and Technol-
ogy Talents Project and the Higher Education Develop-
ment Project: Postgraduate Education and Research
Program in Chemistry are also gratefully acknowledged.
The reactions of syn-5 and anti-6 with SmI2–THF gave
the vinyl sulfone 7 (entry 4, R3=H) in 88% (E:Z=94:6)
and 96% (E:Z=96:4) yields, respectively, with high
E-selectivity. The deoxygenative debromination of iso-
mers 8a and 8b under the same conditions gave the
corresponding a,b-unsaturated sulfone 9 in 87% (E:Z=
87:13) and 84% (E:Z=82:18) yields, respectively. With
the currently accepted radical mechanism for the
References
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of the samarium-mediated deoxygenative debromination

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