Highly chemoselective synthesis of aryl allylic sulfoxides through calcium hypobromite oxidation of aryl allylic sulfides

Article abstract of DOI:10.1016/j.tetlet.2011.12.046

A highly chemoselective oxidation of widely substituted aryl allylic sulfides, prepared by allylation of arylthioethers with KF-Celite, to the corresponding aryl allylic sulfoxide was achieved by employing calcium hypobromite. Neither over-oxidation to sulfones nor halogenation of the aromatic rings was observed. The protocol may be successfully applied for the oxidation of substituted allylic systems (i.e., 2-haloallyl) that per se could interact with the oxidizing agent.

Full text of DOI:10.1016/j.tetlet.2011.12.046

Tetrahedron Letters 53 (2012) 967–972
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Tetrahedron Letters
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Highly chemoselective synthesis of aryl allylic sulfoxides through calcium
hypobromite oxidation of aryl allylic sulfides
Vittorio Pace a,b, , Laura Castoldi c, Wolfgang Holzer b
⇑
a Department of Organic and Pharmaceutical Chemistry, Complutense University, Pza. Ramón y Cajal s/n, Madrid 28040, Spain
b Department of Drug and Natural Product Synthesis, University of Vienna, Althanstrasse 14, Vienna 1090, Austria
c Department of Pharmaceutical Chemistry, University of Pavia, Via Taramelli 12, 27100 Pavia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly chemoselective oxidation of widely substituted aryl allylic sulfides, prepared by allylation of
arylthioethers with KF-Celite, to the corresponding aryl allylic sulfoxide was achieved by employing cal-
cium hypobromite. Neither over-oxidation to sulfones nor halogenation of the aromatic rings was
observed. The protocol may be successfully applied for the oxidation of substituted allylic systems (i.e.,
2-haloallyl) that per se could interact with the oxidizing agent.
Received 15 November 2011
Revised 8 December 2011
Accepted 12 December 2011
Available online 17 December 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Sulfoxides
Allylic compounds
Chemoselectivity
Hypohalites
KF-Celite
Allylic sulfoxides constitute useful intermediates in organic syn-
thesis, because of their employment in the preparation of widely
functionalized structures including natural products (e.g., (ꢀ)-Agel-
astin A,1 (+)-pyrenolide D,2 rac-14-deoxyisoamijiol,3 deoxymalay-
amicin A4). In particular, their use as valuable scaffolds has been
exploited since the publication of the pioneering studies by Mis-
low5 and Evans6 concerning their conversion into allylic alcohols,
often with a well defined (E) configuration at the C–C double bond,7
through a [2,3]-sigmatropic rearrangement, which involves the for-
mation of an intermediate allylic sulfenate ester that, in presence of
a thiophilic reagent (e.g., phosphines) affords the allylic alcohols.8,9
Recently, Poli and co-workers found that such allylic sulfenate
esters undergo an oxidative addition to Pd(0), thus generating sulf-
enate anions, that in the presence of a haloarene are smoothly con-
verted into aryl sulfoxides.10 Furthermore, allylic sulfoxides
represent starting materials for the formation of stabilized allyl
anions11 under basic conditions, the latter susceptible to regioselec-
tive alkylations with a broad range of electrophiles including
olefin catalyzed by ethylaluminum chloride17 or zinc chloride,18
Michael-type additions of
-sulfinyl carbanions to acetylenes19 or
a
additions of alkylcopper reagents to allenyl sulfoxides.20 Moreover,
recently the synthesis of the title compounds has been achieved by
the Pd-catalyzed allylic alkylation of sulfenate anions21 or through
the Yb(OTf)3/TMSCl-catalyzed coupling of olefins with sulfona-
mides22 or via the nucleophilic addition of lithium sulfinyl carba-
nions to group 6 Fischer carbene complexes.23 However, such
protocols often require the use of highly moisture sensitive sub-
strates (e.g., sulfinyl halides or organometallic carbanions) that
per se circumscribe the application only at a laboratory scale.
Thus, despite a number of alternative methods available for
the preparation of allylic sulfoxides, the oxidation of the allylic
sulfides to the corresponding sulfoxides represents the most
simple approach to these compounds and, nowadays in the case
of simple sulfides not densely functionalized is a well established
procedure for the preparation of such sulfoxides by means of a
plethora of oxidizing agents.24,25 Effectively, the application of a
standard oxidation protocol to organic sulfides bearing an allylic
functionality is affected by, at least, two chemoselective draw-
backs: (1) the potential over-oxidation of the initially formed sulf-
oxide (II) to the corresponding sulfone (III) (via a) and, (2) the
reactivity of the C–C double bond toward the oxidant (via b)
(Scheme 1). In principle, the over-oxidation maybe controlled by
the careful optimization of the reaction conditions because of the
fact that the less nucleophilic sulfoxide shows lower tendency
(compared to sulfides) to react with oxidants. On the other hand,
the reactivity of the olefinic moiety is much more difficult to
a
,b-conjugated systems,12–14 or precursors of 2,3,4-trisubstituted
furans via a [3+2] annulation with aldehydes.15 In addition, allyl
sulfoxides are suitable precursors for allyloxy radicals in the two-
carbon ring expansion of cyclobutanones.16 The increasing interest
and applications of this motif have stimulated investigations on
new methodologies for their preparation. Among these procedures
are ene-type reactions between an arylsulfinyl chloride and an
⇑
Corresponding author. Tel.: +43 1 4277 55623; fax: +43 1 4277 9556.
E-mail address: vpace@farm.ucm.es (V. Pace).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.046

968
V. Pace et al. / Tetrahedron Letters 53 (2012) 967–972
R
O
O
S
R2
O
S
R
R1
III
R2
X
R1
V
via a
X + based
oxidant
via d
O
S
R
O
S
R
R
OH
R1
R2
R2
X
S
R2
oxidant
X+ based
oxidant
via c
IVa
R1
R1
II
I
via b
O
S
R
O
R2
R1
IV
Scheme 1. Potential reaction products derived by the oxidation of an aryl allylic sulfide with an electrophilic oxidizing agent.
control, due to the inherent tendency of the nucleophilic C–C dou-
ble bond to react with electrophilic oxidants, and thus the forma-
tion of side-products like epoxides (IV) or halohydrins (IVa) from
which the former maybe obtained, remains a challenge to pursue.
In addition to such drawbacks, halonium based oxidations may
further interfere with the chemoselectivity of the process, as a con-
sequence of the formation of products (V) derived by halogenating
the aromatic ring (via d).
Significantly, as noticed by Koo and co-workers the use of con-
ventional oxidants (MCPBA, Oxone) does not allow to observe a
chemoselective control of the oxidation,26 and thus, the use of less
electrophilic oxidizing agents may constitute the key to achieve a
chemoselective process: for this purpose, these authors, by using
the trirutile-type solid oxide catalyst LiNbMoO6 and H2O2, were
able to oxidize sulfides to the corresponding sulfoxides in which
the allylic function remained untouched.26 However, this method
is associated to the impossibility to avoid the formation of sulfones
as side-products of the oxidation and, to the best of our knowledge
there is only a method available to promote such oxidation under
chemoselective control based on a H2O2 oxidation catalyzed by
flavin.27
2-methyltetrahydrofuran32,36–38 did not give similar excellent con-
versions (entry 2) thus, recovering noticeable amount of the start-
ing thioether. It is worth mentioning how this procedure allows a
clean conversion into the desired allylic arylthioethers, regardless
of the nature of the substituents present on both the aromatic ring
and the olefin. Interestingly, KF-Celite shows a better performance
compared to CsF-Celite during the preparation of sulfide 1c, which
was previously obtained in only a 75% yield upon refluxing in ace-
tonitrile for 48 h.39
Thus, with the series of sulfides in hand, we turned our atten-
tion toward the selective oxidation of the sulfur atom of the allylic
aryl sulfide to the corresponding sulfoxide in which no modifica-
tion occurred at the olefinic moiety.
As a model reaction we selected the oxidation of the sulfide 1a
that in principle, upon treatment with an oxidizing agent could af-
ford the different products shown above (Scheme 1).
Table 2 shows the experimental results obtained under differ-
ent oxidation conditions. Sodium hypochlorite is able to efficiently
promote the oxidation of the sulfur atom to sulfoxide but the para-
chlorinated aryl compound 3a (31%) was formed jointly with the
desired allylic sulfoxide 2a (entry 1). On the other hand, by switch-
ing to calcium hypochlorite,28 a valuable alternative to the corre-
sponding sodium hypochlorite, the formation of the side-product
3a was not suppressed at all, although it was obtained in less
amount (15%) (entry 2); this preliminary result seems to confirm
our previous work concerning the less tendency of Ca(OCl)2 to
chlorinate aromatic rings. Effectively, the formation of such side
product maybe explained on the basis of the moderate activating
effect toward the electrophilic substitution played by the allylic
sulfide on the aromatic nucleus. It is interesting to note that the
underlying chlorination of the aromatic ring is suppressed once
the sulfide is oxidized to the sulfoxide, which is certainly a deacti-
vating group for such electrophilic substitutions. In fact, in a sepa-
rate experiment we treated the so obtained sulfoxide 2a with
Ca(OCl)2 (1.05 equiv) under the same reaction conditions and,
effectively no modification was detected even at longer reaction
times (24 h). As a consequence, we hypothesize that the chlorina-
tion of the aromatic ring takes place in the early stages of the reac-
tion and is efficiently suppressed once the sulfoxide (a deactivating
group) has been obtained. In fact, also when a 0.5 equiv of Ca(OCl)2
were employed such concomitant ring chlorination is not elimi-
nated (entry 3). A careful check of the reaction conditions could
Hence, efficient and convenient methods for the chemoselective
preparation of such aryl allylic sulfoxides are still in demand and
represent an interesting challenge to pursue. Recently, in our group
an efficient preparation of not previously described a -
-arylamino-a0
haloacetones28–30 through a selective oxidation of the correspond-
ing vinyl halides28 has been achieved by means of a chemoselective
oxidation of vinyl halides to the corresponding
a-haloketones: in
such study, we showed the crucial role played by the correct choice
of the oxidant in determining the chemoselectivity of the process
and, therefore in this Letter we report on the highly chemoselective
oxidation, achieved under mild reaction conditions, of aryl allylic
sulfides mediated by calcium hypobromite.
First, we prepared the starting aryl allylic sulfides under heter-
ogeneous conditions by employing a base potassium fluoride that
previously enabled us to functionalize with very good results
carbon,31 nitrogen,32,33 and oxygen34 atoms under mild conditions:
analogously to the allylation of arylamines, the use of the
coated KF-Celite in refluxing acetonitrile35,33 allowed the high
yielding preparation of the corresponding sulfides in very short
reaction times (Table 1). Remarkably, an attempt to replace
acetonitrile with the more environmentally benign solvent

V. Pace et al. / Tetrahedron Letters 53 (2012) 967–972
969
Table 1
Preparation of allylic arylsulfides by the allylation of arylthioethers mediated by KF-Celitea
R
X
R2
R
S
R2
SH
R1
KF-Celite
Z
Z
R1
Product
Entry
1
Arylthiol (Ar)
Ph-
Allyl halide
Solvent/reaction time (h)
Yield (%)
95
S
Allyl bromide
Allyl bromide
Allyl bromide
CH3CN 2 h
1a
S
2
3
Ph-
2-MeTHF
43
92
1a
S
4-Me–C6H4–
CH3CN 2 h
1b
S
4
5
4-NO2–C6H4–
Allyl bromide
Allyl bromide
CH3CN 6 h
CH3CN 5 h
90
87
O2N
1c
S
4-CO2Me–C6H4–
MeO2C
Cl
1d
S
6
7
3-Cl–C6H4–
2-Br–C6H4–
Allyl bromide
Allyl bromide
CH3CN 3 h
CH3CN 4 h
93
90
1e
S
Br
1f
Cl
Br
S
8
9
Ph-
Ph-
3-Iodo-1-chloropropene
2,3-Dibromopropene
CH3CN 4 h
CH3CN 3 h
94
91
1g
S
1h
Br
S
10
11
4-NO2–C6H4–
4-Me–C6H4–
2,3-Dibromopropene
CH3CN 6 h
CH3CN 3 h
86
90
O2N
1k
Cl
S
3-Iodo-1-chloropropene
1j
Ph
S
12
13
Ph-
Ph-
3-Bromo-2-phenylpropene
3-Bromo-2-methylpropene
CH3CN 6 h
CH3CN 3 h
83
88
1l
S
1m
(continued on next page)

970
V. Pace et al. / Tetrahedron Letters 53 (2012) 967–972
Table 1 (continued)
Entry
Arylthiol (Ar)
Allyl halide
Solvent/reaction time (h)
Product
Yield (%)
86
Cl
S
14
Naphthyl-
Ph-
3-Iodo-1-chloropropene
CH3CN 4 h
CH3CN 2 h
1n
S
15
16
1-Bromo-3-methylbut-2-ene
95
87
1o
Cl
S
4-CHO–C6H4–
3-Iodo-1-chloropropene
CH3CN 6 h
OHC
1p
a
Conditions: Arylthiol (1.0 equiv), allylic halide (1.10 equiv), KF-Celite (1.50 equiv), 83 °C.
Table 2
Optimization of the hypochlorite-based oxidation
R
O
S
R
O
S
R
S
R2
oxidant
R2
R2
+
R1
R1
R1
X
1a
2a
3a
X = Cl, Br
Entry
Oxidanta
Solventb
Temp. (°C)
Reaction Time (h)
Yield of 2a (%)
Yield of 3a (%)
1
NaOCl
Acetone
Acetone
Acetone
Acetone
CH3CN
0
0
0
1
1
1
1
1
24
1
0.5
24
60
69
33
49
62
19
94
95
23
31
15
9
10
17
8
0
0
0
2
Ca(OCl)2
Ca(OCl)2
Ca(OCl)2
Ca(OCl)2
Ca(OCl)2
Ca(OBr)2
Ca(OBr)2
Ca(OBr)2
3c
4
ꢀ15
5
6
0
CH2Cl2
0
0
0
0
7
Acetone
Acetone
Acetone
8d
9e
Conditions:
a
1.05 equiv of oxidant.
b
c
All reactions were run in the presence of 1p v/v of water–AcOH.
0.50 equiv of oxidant were employed.
3.00 equiv of oxidant were employed.
No AcOH was added.
d
e
not improve chemoselectivity, with compound 3a being recovered
at the end of the reactions. In fact, attempts to lower the temper-
ature (entry 4) resulted in a slight reduction of the amount of com-
pound 3a detected, but simultaneously dropped the yield of the
desired sulfoxide. It is worth considering the importance of the sol-
vent in determining a good conversion: the best solvent system
was a mixture of acetone and water, while switching to a biphasic
system dramatically dropped the yield as a consequence of the
insolubility of hypochlorites in such media (entry 6).
With the aim of eliminating the concomitant ring halogenation,
we decided to turn our attention toward the use of an alternative
agent, as calcium hypobromite: this choice was not casual and is
motivated by the fact that, as stressed above, calcium hypochlorite
shows a lower tendency to halogenate the ring28 and thus, the
possible modification of the hypohalite could be beneficial for
the outcome of the reaction. Since it is not commercially available,
we prepared it by the slow addition of molecular bromine to an
aqueous suspension of calcium hydroxide at 0 °C (see Supplemen-
tary data for full details). Effectively, by using such an oxidizing
agent we observed the high yielding (94%) and clean conversion of
sulfide 1a into the desired sulfoxide 2a without any modification
neither to the aromatic ring, nor to the allylic exocyclic functionality
(entry 7). The best result was obtained by using only a slight excess
of the hypobromite (1.05 equiv) and remarkably the reaction
reached to completion within 1 h. Further, the use of higher load-
ings of Ca(OBr)2 did not affect the chemoselectivity of the reaction
to any significant extent and the yield of the desired product was
found to be almost the same (entry 8). Moreover, the beneficial role
played by the addition of acetic acid to the reaction mixture should
be stressed: as emphasized in our previous related studies,28 it plays
a double role: first, it protonates the hypohalite anion, thus afford-
ing a molecule of hypohalic acid, secondly it reacts with the latter
giving the acetyl hypohalite, which is a better electrophile than
HXO and thus reasonably it acts as the effective oxidant species.
When the reaction was run in the absence of AcOH even after a long-
er time (24 h) the conversion into the desired sulfoxide was mini-
mal (entry 9). It is noteworthy that by using such protocol for this
sensitive chemoselective oxidation better results compared to other

V. Pace et al. / Tetrahedron Letters 53 (2012) 967–972
971
Table 3
Expansion of the Ca(OBr)2 oxidation methodology to functionalized aryl allylic sulfides
R
O
S
R
S
R2
Ca(OBr)2 (1.05 equiv.)
R2
Z
Z
R1
water:AcOH:acetone (1:1:1)
0 ºC
R1
1b-q
2b-q
Entry
Aryl-allylic sulfide
Reaction time (h)
Product/yield (%)
1
2
3
4
5
6
7
8
9
11
12
13
14
15
1b (Z = 4-Me, R = R1 = R2 = H)
1c (Z = 4-NO2, R = R1 = R2 = H)
1d (Z = 4-CO2Me, R = R1 = R2 = H)
1e (Z = 3-Cl, R = R1 = R2 = H)
1
6
5
1
3
1
1
5
1
1
1
4
1
5
2b (91)
2c (85)
2d (78)
2e (90)
2f (91)
2g (97)
2h (99)
2k (89)
2j (92)
2l (95)
2m (86)
2n (79)
2o (83)
2p (88)
1f (Z = 2-Br, R = R1 = R2 = H)
1g (Z = H, R = Cl, R1 = R2 = H)
1h (Z = H, R = Br, R1 = R2 = H)
1k (Z = 4-NO2, R = Br, R1 = R2 = H)
1j (Z = 4-Me, R = Cl, R1 = R2 = H)
1l (Z = H, R = Ph, R1 = R2 = H)
1m (Z = H, R = Me, R1 = R2 = H)
1n (Z = 3,4-C4H4, R = Cl, R1 = R2 = H)
1o (Z = H, R = H, R1 = R2 = Me)
1p (Z = 4-CHO, R = Cl, R1 = R2 = H)
procedures were obtained: in fact, for example, based on a thiourea
analog/TBHP system, Russo and Lattanzi obtained 2a in a 79% yield
after 21 h.40
also be observed for such substituents on the aromatic ring like
methyl that in principle may suffer benzylic bromination (entries
1, 9). Subsequently, we focused our attention on the effect played
by the substitution at the olefin moiety: evidently, the presence of
an electron-releasing group (entries 12, 14) enhances its reactivity
with an electrophile such as an oxidant like hypobromite. Pleas-
ingly, our methodology did not affect such alkenes at all and the
desired sulfoxides were formed in high yields.
In order to study the generality of this procedure, the series of
allylic sulfides, prepared as shown above, containing different
groups attached to both the aryl moiety and allylic function, was
reacted according to optimized conditions, and the results are pre-
sented in Table 3. As shown, regardless of the nature of the substit-
uents all substrates were oxygenated exclusively at the sulfur
atom. All the reactions occurred with excellent chemoselectivity
for sulfoxide formation without any detectable overoxidation to
sulfones as well as ring bromination, neither hydrobromination/
epoxidation of the allylic function. Among the various substrates
studied, sulfides bearing electron withdrawing groups on the aro-
matic ring were found to react slower and required longer reaction
times to observe a full conversion into the desired sulfoxide: such
observation was particularly evident for those sulfides bearing
strong deactivating groups in the para position of the aromatic ring
(entries 2, 3, 8, 15) which were converted into the corresponding
sulfoxides within 6 h. However, for the oxidation of the nitro bear-
ing substrates the use of our protocol allows to maximize the
yields: a similar oxidation method based on the Cu-catalyzed oxi-
dation promoted by H2O2 afforded only a 71% yield of sulfoxide 2c
after 22 h.41 The chemical inertness under our conditions should
Of particular interest is the oxidation of aryl sulfides bearing a
2-haloallylic moiety (entries 6–9, 13, 15 and Scheme 2): as we pre-
viously reported such 2-haloallylic system represents a useful pre-
cursor of
a-halocarbonyls that maybe conveniently prepared by
oxidative hydrolysis in the presence of a hypohalite in water/ace-
tone/acetic acid.28 To our surprise, by using Ca(OBr)2 in such sol-
vent system we could only observe the oxidation of the sulfur
atom to the sulfoxide without any modification to the halo-substi-
tuted alkene. Evidently, due to the its nucleophilicity, the sulfur
atom reacts immediately with the oxidizing agent and thus, there
is no possibility for the haloallylic moiety to interact with it. As a
consequence, the protocol described herein represents a versatile
procedure for the high yielding preparation of such 2-haloallylic
sulfoxides that, to the best of our knowledge, have been prepared
before only by the hydrohalogenation of not easily obtainable
1,2-allenic sulfoxides with AlX3 and water.42 In fact, by employing
O
X
S
Z
unique product formed
Ca(OBr)2 (1.05 equiv.)
X
water:AcOH:acetone (1:1:1)
S
0 ºC, 1 h
O
Z
O
S
O
S
X
X
and
or
X = Cl, Br
Z
Z
not detected products
Scheme 2. Chemoselective sulfur oxidation of 2-haloallyl-arylsulfides with Ca(OBr)2.

972
V. Pace et al. / Tetrahedron Letters 53 (2012) 967–972
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our method a substantial improvement for their preparation was
achieved: for example, compound 2h was obtained in a 99% yield,
while the former technique was able to afford it in an 81% yield.42
In conclusion, we have shown the high yielding chemoselective
Ca(OBr)2 mediated oxidation of easily accessible aryl allylic sul-
fides to the corresponding sulfoxides. Chemoselectivity is not af-
fected by the electronic nature of the substituents and is thus
applicable to a series of widely functionalized systems both on
the aromatic ring and the allylic system.
Acknowledgment
One of the Authors (V.P.) is grateful to the Federal Austrian Min-
istry of Education for a postdoctoral Ernst Mach fellowship.
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1953.
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Koo, S. J. Org. Chem. 2001, 66, 8192–8198.
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Synthesis 2010, 3545–3555.
30. Pace, V.; Verniest, G.; Sinisterra, J.-V.; Alcántara, A. R.; De Kimpe, N. J. Org. Chem.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.046.
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