Pyrolysis of α- and β-heteroatoms substituted ethyl phenyl sulfoxides

Article abstract of DOI:10.1080/10426501003773563

A study on the mechanism of the thermal decomposition of α- and β-heteroatoms substituted ethyl phenyl sulfoxides was carried out using 1-chloroethyl phenyl sulfoxide (1); two diastereomeric 1-acetoxyethyl (substituted phenyl) sulfoxides (2a) and (2b); and 2-chloroethyl phenyl, 2-bromoethyl phenyl, and 2-methoxyethyl phenyl sulfoxides (3, 4, 5). The rate of pyrolysis of 1 was 4.8 times faster at 160°C than that of ethyl phenyl sulfoxide used as a reference, while those of 2a and 2b were 107 and 155 times faster, respectively. The results indicate that the lone pair of electrons on the α-heteroatoms has a larger rate acceleration effect than the electronegativity of them. The substituent effects of the phenyl group of 2a and 2b gave positive Hammett ρ-values (ρ_a= 0.76 and ρ_b= 0.80 vs. σ). Activation parameters for 2a and 2b are as follows: 2a, ΔH?= 112 kJmol^-1, ΔS?= -20 JK^-1mol^-1; 2b, ΔH?= 107 kJmol^-1, ΔS?= -29 JK-1mol^-1. Large deuterium kinetic isotope effects for 1-acetoxyethyl-2,2,2-d₃ phenyl sulfoxides (2ad and 2bd) were observed (k_H/k_D= 3.5 ～ 4.1). These results suggest that the pyrolysis of -heteroatom substituted ethyl phenyl sulfoxides proceeds via a five-membered transition state deviated to E1-like in character. On the other hand, from the results of kinetics for the pyrolysis of 3, 4, and 5, no effect by the β-halogen atoms or some deceleration effect by the β-methoxy group was observed. Thus the reaction seems to proceed via an E1-like mechanism. Copyright Taylor & Francis Group.

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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Pyrolysis of α- and β-Heteroatoms
Substituted Ethyl Phenyl Sulfoxides
Toshiaki Yoshimura a , Hironori Sakae a , Masaki Yoshizawa a , Kiyoshi
Hasegawa a & Eiichi Tsukurimichi a
a Department of Applied Chemistry, Graduate School of Science and
Engineering (Engineering) , University of Toyama , Toyama, Japan
Published online: 27 May 2010.
To cite this article: Toshiaki Yoshimura , Hironori Sakae , Masaki Yoshizawa , Kiyoshi Hasegawa
& Eiichi Tsukurimichi (2010) Pyrolysis of α- and β-Heteroatoms Substituted Ethyl Phenyl
Sulfoxides, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:5-6, 1162-1173, DOI:
10.1080/10426501003773563
To link to this article: http://dx.doi.org/10.1080/10426501003773563
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Phosphorus, Sulfur, and Silicon, 185:1162–1173, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003773563
PYROLYSIS OF α- and β-HETEROATOMS SUBSTITUTED
ETHYL PHENYL SULFOXIDES
Toshiaki Yoshimura, Hironori Sakae, Masaki Yoshizawa,
Kiyoshi Hasegawa, and Eiichi Tsukurimichi
Department of Applied Chemistry, Graduate School of Science and Engineering
(Engineering), University of Toyama, Toyama, Japan
A study on the mechanism of the thermal decomposition of α- and β-heteroatoms substituted
ethyl phenyl sulfoxides was carried out using 1-chloroethyl phenyl sulfoxide (1); two di-
astereomeric 1-acetoxyethyl (substituted phenyl) sulfoxides (2a) and (2b); and 2-chloroethyl
phenyl, 2-bromoethyl phenyl, and 2-methoxyethyl phenyl sulfoxides (3, 4, 5). The rate of
pyrolysis of 1 was 4.8 times faster at 160◦C than that of ethyl phenyl sulfoxide used as a
reference, while those of 2a and 2b were 107 and 155 times faster, respectively. The results
indicate that the lone pair of electrons on the α-heteroatoms has a larger rate acceleration
effect than the electronegativity of them. The substituent effects of the phenyl group of 2a
and 2b gave positive Hammett ρ-values (ρa = 0.76 and ρb = 0.80 vs. σ). Activation param-
eters for 2a and 2b are as follows: 2a, ꢀH‡ = 112 kJmol−1, ꢀS‡ = −20 JK−1mol−1; 2b,
ꢀH‡ = 107 kJmol−1, ꢀS‡ = −29 JK-1mol−1. Large deuterium kinetic isotope effects for
1-acetoxyethyl-2,2,2-d3 phenyl sulfoxides (2ad and 2bd) were observed (kH/kD = 3.5 ∼ 4.1).
These results suggest that the pyrolysis of α-heteroatom substituted ethyl phenyl sulfoxides
proceeds via a five-membered transition state deviated to E1-like in character. On the other
hand, from the results of kinetics for the pyrolysis of 3, 4, and 5, no effect by the β-halogen
atoms or some deceleration effect by the β-methoxy group was observed. Thus the reaction
seems to proceed via an E1-like mechanism.
Keywords Elimination; isotope effect; kinetics; mechanism; pyrolysis; sulfoxide
INTRODUCTION
Pyrolysis of sulfoxides bearing at least one β-hydrogen atom has been shown to
proceed via an intramolecular cis elimination (Ei)1 affording olefins.2–8 Kinetic investiga-
tions for the Ei reaction of the sulfoxides have revealed that both C S bond fission and
β-proton transfer are important, and thus a concerted mechanism has been suggested from
the observations of positive Hammett ρ-values8,9 for the substituent effect on the S-phenyl
group and large deuterium kinetic isotope effects on β-position.10,11 However, we recently
demonstrated that the transition state is variable depending on the substituent on the α- or
β-position of the sulfoxides. For example, introduction of a phenyl group on the α-position
of ethyl phenyl sulfoxide causes the transition state to deviate to one in which the C S
Received 1 January 2009; accepted 3 February 2009.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
Address correspondence to Toshiaki Yoshimura, Department of Applied Chemistry, Graduate School of
Science and Engineering (Engineering), University of Toyama, Gofuku, Toyama 930-8555, Japan. E-mail:
yosimura@eng.u-toyama.ac.jp
1162

ETHYL PHENYL SULFOXIDES
1163
bond cleavage is more progressed than proton transfer, that is, a partially positive charge is
developed on the α-carbon (E1-like).12 On the other hand, introduction of β-phenyl group
causes deviation to the carbanion-like, and stronger electron-withdrawing CN group on the
β-carbon results in a negative ρ-value (ρ = –0.49) for the substituent effect on the S-phenyl
group.13 Meanwhile, introduction of carbonyl group14 on the α-carbon shows that conjuga-
tion of the carbonyl group with the developing C C π-bond stabilizes the transition state
and increase the acidity of the β-proton to accelerate the reaction greatly. In a series of
systematic investigations has been undertaken to analyze the character of the Ei-reaction
and its transition state with the sulfoxides. It was of interest to investigate how electroneg-
ativity and the lone-pair electrons on the α- and β-heteroatoms affect competitively the
pyrolytic mechanism of the ethyl phenyl sulfoxide. Thus, 1-chloroethyl phenyl sulfoxide
(1); 1-acetoxyethyl (substituted phenyl) sulfoxides (2a, 2b); 1-acetoxyethyl-2,2,2-d3 phenyl
sulfoxides(2ad, 2bd); and 2-chloroethyl phenyl, 2-bromoethyl phenyl, and 2-methoxyethyl
phenyl sulfoxides (3, 4, 5) were prepared and subjected to pyrolysis. This article gives a
detailed account of the effect of the α- and β-heteroatoms of the pyrolysis of sulfoxides.
RESULTS
Synthesis of α- and β-Heteroatoms Substituted Ethyl Phenyl
Sulfoxides and Their Diastereomers
The sulfoxide 1 was prepared by treating ethyl phenyl sulfoxide with sulfuryl chlo-
ride,15 and two diastereomers 2a and 2b were obtained by oxidizing the corresponding
sulfides, which were prepared by the Pummerer reaction16 of ethyl aryl sulfoxides with
acetic anhydride, and with hydrogen peroxide. The mixture of 2a and 2b was separated by
repeated column chromatography, whereas the purification of 1 by column chromatogra-
phy gave only one isomer. All of 2a was eluted initially and displayed a quartet 1H NMR
methine signal at higher field than 2b eluted last. Though the stereochemical assignment
of 2a and 2b is not complete, the two diastereomers are distinguishable by the following
consideration. Figure 1 shows the plots of 1H NMR chemical shifts of methine protons of
2a and 2b against Hammett σ-values. Two series of the methine signals of the higher field
and the lower one showed a fairly good linear relationship. Therefore, it can be assumed
that the same diastereomers are on the same line. Thus, the chemical shifts of the methine
protons of 2 were found to be available to distinguish the diastereomers.
On the other hand, 2-chloroethy phenyl 3 and 2-bromoethyl phenyl sulfoxides 4
were prepared by the oxidation of the corresponding sulfides17,18 with hydrogen peroxide
in acetic acid. The sulfoxides 3 and 4 were then treated with sodium methoxide to give
2-methoxyethyl phenyl sulfoxide (5) (Scheme 1).19
Pyrolysis Products
The pyrolysis of 2a and 2b was carried out in CCl4 in sealed NMR tubes at 100◦C,
1
and then H NMR analysis showed the formation of vinyl acetate at δ = 4.4∼5.0 and
6.95∼7.4 ppm in agreement with those of an authentic sample. Scheme 2 shows the
pyrolytic products of 1, 2a, 2b, and 3–5. The sulfoxide 1 afforded vinyl chloride and diaryl
disulfide (6), while 2a and 2b gave vinyl acetate and 6. Although the pyrolytic products were
not actually isolated, the products were identified by comparing their HPLC and GLPC
retention times with those of the authentic samples. The disulfides 6 may be generated via
an initial formation of arenesulfenic acid followed by dimerization process.20,21

1164
T. YOSHIMURA ET AL.
1: X = H; Y = Cl
XC6H4
C6H5
S
CH CH3
Y
2: X = p-OCH3, p-CH3, H, p-Cl, m-Cl;
O
Y = OAc
S
CH2 CH2
Z
3: Z = Cl; 4: Z = Br; 5: Z = OCH3
O
Scheme 1
C6H5
S
CH CH3
Cl
∆
∆
∆
CH2=CH-Cl
C6H5-S-S-C6H5
+
O
1
6
XC6H4
C6H5
S
CH CH3
OCOCH3
CH2=CH-OCOCH3
+
XC6H4-S-S-C6H4X
O
6
2a, 2b
S
CH2 CH2
C6H5-S-S-C6H5
CH2=CH-Z
+
O
Z
6
3 4
5
,
,
Scheme 2 Pyrolysis of sulfoxides.
5.7
5.6
5.5
5.4
5.3
5.2
p-Cl
m-Cl
m-Cl
p-OCH3
2b
2a
p-Br
p-Br
p-Cl
p-OCH3
-0.3
-0.1
0.1
0.3
0.5
σ
Figure 1 Dependence of 1H NMR methine signals of two diastereomers of 1-acetoxyethyl aryl sulfoxides
(substituted 2a and 2b) on Hammett σ-values. Solvent: CCl4.

ETHYL PHENYL SULFOXIDES
1165
Kinetics
Kinetic experiments were carried out in dry dioxane at the desired temperature for
1, 2a, and 2b (90∼130◦C) and for 3∼5 (140∼180◦C), and decreasing amounts of the
sulfoxides was followed by measuring the intensity of the UV absorption (254 nm) by
HPLC. The rates of the pyrolysis were all correlated with first-order kinetics (r = 0.999)
and have been listed in Table I. Table II shows activation parameters and relative rates
at 160◦C compared with those for the reference sulfoxide. The rate of the pyrolysis of 1
was found to be 4.8 times faster than that of the reference sulfoxide, while those of 2a
and 2b were 107 and 155 times faster, respectively. On the other hand, the effect of the
β-heteroatoms is small and rather slows the rates. Specifically, the rate of the reaction of 5 is
0.27 times faster than that for the reference. These effects will be discussed later. Activation
entropies for the reaction of 2 are in the range of –20∼−29 JK−1mol−1 and are somewhat
13
larger than that of 2-phenylethyl phenyl sulfoxide (–34.5 JK−1mol−1
that of 1-phenylethyl phenyl sulfoxide (–6 or –4 JK−1mol−1).12
)
but smaller than
Large kinetic isotope effects using 1-acetoxyethyl-2,2,2-d3 phenyl sulfoxides (2ad
and 2bd) were observed for both 2ad and 2bd at 110∼130◦C (kH/kD = 3.5∼4.1). The
values are smaller than those for the pyrolysis of 2-phenylethyl-2,2-d2 phenyl sulfoxide
(kH/kD = 4.4 at 100◦C), 13 which proceeds via a carbanion-like mechanism and for that of
1-phenylethyl-2,2,2-d3 phenyl sulfoxide (4.77∼5.15 at 100∼80◦C),12 which proceeds via
an E1-like mechanism. However, the values are larger than that for the reaction of heptyl-
2d phenyl sulfoxide (kH/kD = 2.9), which proceeds via a typical concerted five-membered
cyclic transition state.11
In order to obtain more detailed data for the effect of the α-heteroatoms on the
transition state, the effect of substituents X on the S-phenyl group in 2a and 2b was
examined. The kinetic data with respect to X are listed in Table I, and the Hammett relation
is shown in Figure 2. The Hammett plots for both substituted 2a and 2b gave separate but
Figure 2 Hammett plots for the rates of pyrolysis of two diastereomers of 1-acetoxyethyl aryl sulfoxides (sub-
stituted 2a and 2b) at 100◦C in dioxane.

1166
T. YOSHIMURA ET AL.
Table I Rate constants of pyrolysis of α- and β-heteroatoms substituted ethyl phenyl sulfoxides in dioxane
Substituents
Compd.
No.
Temp
(◦C)
Rate constanta
X
Y
Z
(k × 104/s−1
)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
D
D
D
D
D
D
D
D
D
140.5
150.0
155.0
160.0
160.0
90.0
0.522
0.945
1.47
2.15
10.4
0.481
0.935
1.36
2.35
3.59
5.84
8.97
15.6
21.2
33.8
4.80
5.78
6.78
10.4
11.9
12.5
19.6
16.0
21.3
0.921
1.48
2.39
3.83
6.1
9.31
9.31
14.1
21.9
0.85
1.85
4.22
0.413
0.90
2.03
0.58
1.35
3.12
0.022
0.017
0.02
0.04
0.1
0.010
0.030
0.01
0.03
0.03
0.05
0.04
0.1
0.1
0.3
0.04
0.09
0.07
0.1
1
Cl
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2a
2b
2a
2b
2ad
2bd
2ad
2bd
2ad
2bd
2ad
2ad
2bd
3
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
H
90.0
100.0
100.0
110.0
110.0
120.0
120.0
130.0
130.0
120.0
120.0
120.0
120.0
120.0
120.0
120.0
120.0
120.0
110.0
110.0
120.0
120.0
130.0
130.0
135.0
140.0
140.0
150.0
160.0
170.0
140.0
150.0
160.0
160.0
170.0
180.0
H
p-OCH3
p-OCH3
p-CH3
p-CH3
p-Cl
p-Br
p-Br
m-Cl
m-Cl
H
ρX = 0.76 for (2a)
ρX = 0.80 for (2b)
0.1
0.1
0.1
0.1
0.5
0.006
0.07
0.03
0.03
0.07
0.13
0.13
0.2
kH/kD = 3.90
kH/kD = 3.95
kH/kD = 3.75
kH/kD = 4.07
kH/kD = 3.53
kH/kD = 3.63
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.6
Cl
Cl
Cl
Br
Br
Br
OCH3
OCH3
OCH3
0.05
0.06
0.04
0.022
0.03
0.02
0.03
0.01
0.04
3
3
4
4
4
5
5
5
H
H
H
H
H
H
H
H
H
aThe rate constants were calculated by the least-squares method, and the errors are standard derivatives.

ETHYL PHENYL SULFOXIDES
1167
Table II Relative rates and activation parameters for the pyrolysis of α- and β-heteroatoms substituted ethyl
phenyl sulfoxides and related sulfoxides in dioxane
Substituents
Activation parameters (◦C)b
Compd.
No.
Relative rate
(160◦C)
Y
Z
ꢀH‡/kJ · mol−1
ꢀS‡/J · K−1 · mol−1
H
Cl
H
H
H
H
D
D
H
1
4.8
107a
155
0.68
105
7
−77 16
(160)
1
2a
2b
2ad
2bd
OAc
OAc
OAc
OAc
Ph
112
107
117
115
113
111
121
1
2
1
1
−20
−29
−20
−20
1
5
1
1
(100)
(100)
(100)
(100)
(90)
365 (Threof)a,[12]
678 (Erythreof)a,[12]
−6
−4
(90)
3
4
5
H
H
H
Cl
Br
OCH3
0.86
0.94
0.27
4
3
3
−39
9
7
6
(160)
(160)
(160)
115
136
−54
−15
aThe rate constants were calculated by Arrhenius equation from the data at other temperatures.
bThe temperature was used to calculate activation parameters from the Arrhenius plot (ꢀH‡ = Ea − RT).
parallel lines, suggesting that the present 1H NMR spectroscopic assumption (Figure 1) for
the diastereomers was valid. The plots for both substituted 2a and 2b gave similar positive
ρ-values (ρX = 0.76 for 2a and ρX = 0.80 for 2b). Both values are a little larger than those
of aryl α-methylbenzyl sulfoxides (ρ = 0.60)12 and aryl propyl sulfoxides (ρ = 0.51).9
These results for kinetic isotope effect and substituent effect reveal that both the C S bond
cleavage and the proton transfer are important in the transition state, namely the reaction
proceeds via a concerted process and the positive ρ-values suggest that the reaction of 2a
and 2b proceeds via a mechanism deviated to E1-like.
DISCUSSION
Though the Ei-reaction of sulfoxides is generally recognized as a concerted process,
there exists a variable in the amount of C S bond cleavage versus proton migration from
the β-carbon to the oxygen. This causes a deviation to an E1-like or a carbanion-like
mechanism in the five-membered cyclic transition state. Likely, a partial positive charge
develops at the α-carbon atom in the E1-like transition state, while a partial negative charge
develops at the β-carbon atom in the carbanion-like transition state, although the develop-
ment of any charge is small because of the concerted nature of the cyclic transition state.
The present positive Hammett ρ-values for X-substituents are consistent with an E1-like
mechanism and are larger than that for the pyrolysis of substituted phenyl propyl sulfoxides
(ρ = 0.51),9 threo-1-phenylethyl substituted phenyl sulfoxides (ρ = 0.60∼0.64),12 and
t-butyl substituted phenyl sulfoxides (ρ = 0.695).8 These pyrolyses proceed via an E1-like
mechanism. The magnitude of the ρ-values may be related to the degree of stretch of the
C S bond in the transition state. The “looseness” of the transition state reflects an increase

1168
T. YOSHIMURA ET AL.
in the activation entropy. As discussed a the previous article,12 the looseness of the tran-
sition state in the Ei-reaction of sulfoxides is attributed to the extension of the C S bond
and the progress of the proton transfer, although the latter contribution is small due to the
cancellation of the contribution of looseness of the C H bond by that of development of the
O H bond formation. Therefore, the large activation entropy may be related to looseness
of the C S bond in the transition state and thus to the E1-like mechanism to give a large
positive Hammett ρ-value. In fact, the pyrolysis of threo-1-phenylethyl aryl sulfoxides 12
shows a larger activation entropy (ꢀS‡ = −6 JK−1mol−1) and a larger positive ρ-value
(ρ = 0.60∼0.64) than aryl propyl sulfoxide (ꢀS‡ = −63 JK−1mol−1, ρ = 0.51).9 Pyrolyses
of aryl t-butyl and aryl 1-methylcyclohexyl sulfoxides also reveal a large ρ-value (ρ =
0.695,8 ρ = 0.86522). Though there is no activation data for the pyrolysis of aryl t-butyl
sulfoxides, the reaction of the sulfoxides can be assumed to have a large activation entropy
from the fact that the data for di-t-butyl sulfoxide and 1-methylcyclohexyl phenyl sulfoxide
give a large activation entropy (ꢀS‡ = –21∼+34 JK−1mol−1 22
,
ꢀS‡ = –1.7 JK−1mol−1
22). However, in spite of the large ρ-values for the pyrolysis of 2a (ρ = 0.76) and 2b (ρ =
0.80), the activation entropy is not very large, i.e., the value is between that of 1-phenylethyl
phenyl sulfoxide and propyl phenyl sulfoxide or ethyl phenyl sulfoxide. This must be the
result of an α-oxygen heteroatom effect.
Table II indicates that the enhancement effect of α-chloro atom is smaller than that of
α-phenyl or α-acetoxy group. There are two factors concerning the effect of the heteroatom
on the sulfoxides. One is the effect of electronegativity, which resists the heterolytic cleavage
of the Cα S bond in the transition state. The other is the effect of a lone pair of electrons,
which deviates the transition state to the E1-like one by a resonance stabilization with
the partially positive α-carbon or with the developing C C π-bond involving lowering
Cα S σ∗ orbital, thereby assisting the Cα S bond cleavage but resisting the C H bond
scission. The present results suggest that the electronegativity of α-heteroatoms diminishes
the rate enhancement effect. The data indicate that the acceleration of the pyrolysis by
α-heteroatoms is due mainly to the conjugation of partially positive α-carbon atom with
the lone-pair electrons on the α-heteroatom in the transition state, changing the reaction
mechanism of the sulfoxides into a nearly E1-like one as shown in Figure 3. In general,
conjugation is more effective than electronegativity, as can be seen from the fact that the
methoxy group substituted on the α-carbon atom accelerates solvolysis23,24 very much. In
the present system using α-acetoxy group, the rate-enhancing effect by the conjugation
is also more effective than the electronegativity effect, though it is not so much with the
methoxy group. As a result, the lone pair of electrons on the oxygen atom operates to
loosen the C S bond in the transition state, but the electronegativity of the oxygen atom
resists the development of the positive charge on the α-carbon atom to suppresses the
stretch of the C S bond, giving the middle extent of the activation entropy. Why were
+
Y
δ
Y
-
+
C
-
δ
S
δ
δ
S
C
O
C
O
C
H
H
Y = Cl, OAc
Figure 3 α-Heteroatom effect on the transition state of the pyrolysis of sulfoxides.

ETHYL PHENYL SULFOXIDES
1169
the large ρ-values obtained for the reaction of 2? The substituent effect relates to an
increase in electron density of the S-phenyl group, which is a result of balance of positive
contribution by C S bond stretch and negative contribution by O H bond formation. The
conjugation of partially positive α-carbon with the lone-electron pair of the α-heteroatom
in turn diminishes the acidity of the β-CH protons to resist the C H bond cleavage and
thus O H bond formation unlike that with the α-phenyl group,12 which involves both
electron-attracting and -releasing conjugation with the developing π-bond. Therefore, the
increasing electron density on the S-phenyl group is larger than other sulfoxides such as
propyl aryl sulfoxides9 or 1-phenylethyl phenyl sulfoxides12 in the transition state and thus
gives the larger ρ-values.
On the other hand, as shown in Table II, the pyrolysis rates of β-halogen substituted
ethyl phenyl sulfoxides are very similar to that of ethyl phenyl sulfoxide, but that of
β-methoxyethyl phenyl sulfoxide is considerably slower. Both activation enthalpies and
entropies are larger than those of ethyl phenyl sulfoxide. The electronegative β-heteroatoms
acidify the β-proton, but the lone pair of electrons repulses the developing C C π-electrons
to reduce the rates. Therefore, it is necessary to stretch the C S bond using high energy in
the transition state deviating to an E1-like mechanism.
EXPERIMENTAL
Measurement
The IR spectra were determined on a Jasco 810 spectrometer, and the 1H NMR spectra
were obtained on a Hitachi R-24B spectrometer in CDCl3 or CCl4 using TMS as an internal
standard. The HPLC analyses were carried out with a Jasco Familic-100N apparatus with
UV monitor UVIDEC-100II using silica gel (SS-10) and styrene-divinylbenzene copolymer
(HP-01) columns (CH3CN/H2O, hexane/THF as eluents). The peak area was measured by
a Takeda-Riken TR-2217 integrator. The mass spectra were taken with JEOL JMS-D300
mass spectrometer.
Procedures and Materials
All the reactions were monitored by TLC (Merck, silica gel 60 GF), and the reaction
mixtures were separated by column chromatography using Merck silica gel 60 or Wako ac-
tivated alumina (200 mesh). The products obtained were identified by IR, 1H NMR, GLPC,
MS, and elemental analyses. All the melting points are uncorrected. Elemental analyses
were carried out by Chemical Analysis Center in Toyama Medical and Pharmaceutical Uni-
versity. All reagents were obtained from Wako Pure Chemical Industries Ltd., Tokyo Kasei
Co. Ltd., or Aldrich Chemical Company, Inc., and were further purified by usual methods.
Preparation of 1-Chloroethyl Phenyl Sulfoxide (1)
1-Chloroethyl phenyl sulfoxide was prepared by treating ethyl phenyl sulfoxide
(2.8 g, 15 mmol) with sulfuryl chloride (2.8 g, 2.1 mmol)15 in dichloromethane (100 mL)
at 0◦C for 3 h. The reaction mixture was neutralized with sodium bicarbonate solution
and extracted with dichloromethane. After removal of the solvent, the crude sulfoxide
was purified through silica gel column chromatography using chloroform as an eluent;
1
62% yield. No stereoisomer was formed in the preparation of (1). H NMR (CDCl3):

1170
T. YOSHIMURA ET AL.
δ = 7.35−7.70 (5 H, m, C6H5), 4.50−4.82 (1 H, q, J = 6 Hz, CH), 1.55 (3 H, d, J = 6 Hz,
CH3). IR (neat) : 1045 cm−1 (S O).
Preparation of 1-Acetoxyethyl Aryl Sulfoxides (2a) and (2b)
Sulfoxides (2) were prepared by oxidation of the corresponding sulfides, which were
prepared by the Pummerer reaction.16 A typical example is as follows:
Preparation of 1-acetoxyethyl phenyl sulfides. Ethyl phenyl sulfoxide
(19.8 g, 0.13 mol) dissolved in acetic anhydride was heated under reflux for 3 h.
The reaction mixture was then poured into ice water and neutralized with sodium
bicarbonate. The organic layer was extracted with chloroform and dried over anhydrous
magnesium sulfate and filtered. The solvent was evaporated, and the residue was distilled
under reduced pressure to give 1-acetoxyethyl phenyl sulfide as colorless oil in 7.7%
yield (bp 92−100◦C/1 mmHg) together with phenyl vinyl sulfide as a major product.
The 1-acetoxyethyl phenyl sulfide obtained was further purified by silica gel column
chromatography using benzene as an eluent. Substituent: p-OCH3; yield 28.6%, bp
103−113◦C/2 mm Hg. p-CH3; yield 1.2%, bp 102–103◦C/2 mm Hg. p-Cl; yield 2.5%, bp
108−122◦C/1 mm Hg. p-Br; yield 11%, bp 130−135◦C/1 mm Hg. m-Cl; yield 9.1%.
Oxidation of 1-acetoxyethyl phenyl sulfide. To a stirred solution of
1-acetoxyethyl phenyl sulfide (2.1 g, 10 mmol) in dichloromethane (5 mL), m-
chloroperbenzoic acid (m-CPBA) (2 g, 12 mmol) in dichloromethane (10 mL) was added
dropwise with cooling. After the mixture was stirred at 0◦C for 2 h, dimethyl sulfide
was added to decompose the unreacted m-CPBA, and the solution was poured into cold
aqueous sodium bicarbonate. The organic layer was extracted with chloroform, washed
with water, and dried over anhydrous magnesium sulfate. After evaporation of the solvent,
the residue was separated by silica gel column chromatography using ether:hexane (1:1) as
an eluent. The sulfoxides thus obtained have the following physical properties. Elemental
analyses of the oily sulfoxides (X: p-OCH3, p-CH3, H) were made after oxidation to the
corresponding sulfones with m-CPBA.
p-OCH3, Isomer (a): oil. 1H NMR (CCl4); δ = 7.41 and 6.89 (4 H, AB, JAB = 8 Hz,
C6H4), 5.26 (1 H, q, J = 6 Hz, CH), 3.76 (3 H, s, OCH3), 2.08 (3 H, s, OAc), 1.24 (3 H, d,
J = 6 Hz, CH3). IR (neat); 1745 (C O), 1250 ( O CH3), 1210 (C O C), 1045 cm−1
(S O). Sulfone, mp 51−53◦C. Anal. Found; C, 50.93, H, 5.27%. Calcd for C11H14O5S;
C, 51.15, H, 5.46%.
Isomer (b): oil. 1H NMR (CCl4); δ = 7.39 and 6.91 (4 H, AB, JAB = 9 Hz, C6H4),
5.55 (1 H, q, J = 6 Hz, CH), 3.77 (3 H, s, OCH3), 1.96 (3 H, s, OAc), 1.28 (3 H, d, J = 6 Hz,
CH3). IR (neat); 1750 (C O), 1250 ( O CH3), 1205 (C O C), 1045 cm−1 (S O).
p-CH3, Isomer (a): oil. 1H NMR (CDCl3); δ = 7.45 and 7.25 (4 H, AB, JAB = 8 Hz,
C6H4), 5.47 (1 H, q, J = 6 Hz, CH), 2.39 (3 H, s, CH3), 2.10 (3 H, s, OAc), 1.34 (3 H,
d, J = 6 Hz, CH3). IR (neat); 1745 (C O), 1210 (C O C), 1050 cm−1 (S O). Sulfone,
mp 67.5−69.0◦C. Anal. Found; C, 54.81, H, 5.87%. Calcd for C11H14O4S; C, 54.53, H,
5.82%.
Isomer (b): oil. 1H NMR (CDCl3); δ = 7.44 and 7.26 (4 H, AB, JAB = 8 Hz, C6H4),
5.68 (1 H, q, J = 6 Hz, CH), 2.39 (3 H, s, CH3), 1.99 (3 H, s, OAc), 1.40 (3 H, d, J = 6 Hz,
CH3), IR (neat); 1750 (C O), 1205 (C O C), 1045 cm−1 (S O).
H, Isomer (a): oil. 1H NMR (CDCl3); δ = 7.25−7.70 (5 H, m, C6H5), 5.45 (1 H, q,
J = 6 Hz, CH), 2.10 (3 H, s, OAc), 1.35 (3 H, d, J = 6 Hz, CH3). (CCl4); δ = 7.1−7.6 (5 H,
m, C6H5), 5.30 (1 H, q, J = 6 Hz, CH), 2.11 (3 H, s, OAc), 1.24 (3 H, d, J = 6 Hz, CH3).

ETHYL PHENYL SULFOXIDES
1171
IR (neat); 1745 (C O), 1210 (C O C), 1045 cm−1 (S O). Sulfone, mp 52.5−54.0◦C.
Anal. Found; C, 52.85, H, 5.44%. Calcd for C10H12O4S; C, 52.62, H, 5.30%.
1
Isomer (b): mp 66.5−67.5◦C. H NMR (CDCl3); δ = 7.20−7.84 (5 H, m, C6H5),
5.68 (1 H, q, J = 6 Hz, CH), 1.97 (3 H, s, OAc), 1.40 (3 H, d, J = 6 Hz, CH3). (CCl4); δ =
7.3−7.5 (5 H, m, C6H5), 5.55 (1 H, q, J = 6 Hz, CH), 1.90 (3 H, s, OAc), 1.35 (3 H, d,
J = 6 Hz, CH3). IR (KBr); 1750 (C O), 1205 (C O C), 1045 cm−1 (S O). Sulfone, mp
52.5−54.0◦C.
p-Cl, Isomer (a): mp 87.5−89.0◦C. 1H NMR (CCl4); δ = 7.42 (4 H, m, C6H4), 5.31
(1 H, q, J = 6 Hz, CH), 2.11 (3 H, s, OAc), 1.24 (3 H, d, J = 6 Hz, CH3). IR (KBr); 1745
(C O), 1205 (C O C), 1045 cm−1 (S O). Anal. Found; C, 48.53, H, 4.45%. Calcd for
C10H11ClO3S; C, 48.68, H, 4.49%.
Isomer (b): oil. 1H NMR (CCl4); δ = 7.41 (4 H, m, C6H4), 5.60 (1 H, q, J = 7 Hz,
CH), 1.97 (3 H, s, OAc), 1.35 (3 H, d, J = 7 Hz, CH3). IR (neat); 1745 (C O), 1205
(C O C), 1045 cm−1 (S O).
m-Cl, Isomer (a): oil. 1H NMR (CCl4); δ = 7.1−7.7 (4 H, m, C6H4), 5.36 (1 H, q,
J = 6 Hz, CH), 2.13 (3 H, s, OAc), 1.28 (3 H, d, J = 6 Hz, CH3). IR (neat); 1750 (C O),
1210 (C O C), 1050 cm−1 (S O).
1
Isomer (b): oil. H NMR (CCl4); δ = 7.2−7.6 (4 H, m, C6H4), 5.60 (1 H, q, J =
6 Hz, CH), 1.92 (3 H, s, OAc), 1.37 (3 H, d, J = 6 Hz, CH3). IR (neat); 1755 (C O), 1210
(C O C), 1050 cm−1 (S O).
1
p-Br, Isomer (a): mp 115−116◦C. H NMR (CCl4); δ = 7.53 and 7.43 (4 H, AB,
JAB = 8 Hz, C6H4), 5.33 (1 H, q, J = 6 Hz, CH), 2.14 (3 H, s, OAc), 1.25 (3 H, d, J =
6 Hz, CH3). IR (KBr); 1740 (C O), 1210 (C O C), 1040 cm−1 (S O). Anal. Found; C,
41.13, H, 3.78%. Calcd for C10H11BrO3S; C, 41.25, H, 3.81%.
Isomer (b): mp 59.5−64.5◦C. 1H NMR (CCl4); δ = 7.50 and 7.33 (4 H, AB, JAB
=
8 Hz, C6H4), 5.58 (1 H, q, J = 6 Hz, CH), 1.92 (3 H, s, OAc), 1.32 (3 H, d, J = 6 Hz, CH3).
IR (KBr); 1755 (C O), 1200 (C O C), 1050 cm−1 (S O). Anal. Found; C, 41.38, H,
3.96%. Calcd for C10H11BrO3S; C, 41.25, H, 3.81%.
Preparation of 1-Acetoxyethyl-2,2,2-d3 Phenyl Sulfoxide (2ad) and (2bd)
The deuterated acetic acid-2,2,2-d3 (5.0 g, 79 mmol) was reduced with lithium
aluminum hydride (7.0 g, 180 mmol) in dry ether by the usual method to give ethanol-2,2,2-
d3, yield 60%, bp 78−79◦C. The deuterated alcohol was converted into the corresponding
1
bromide in 48% yield by treatment with phosphorus tribromide. The H NMR spectrum
showed no signal due to methyl proton. Sulfoxides 2ad and 2bd were obtained by the
method similar to that described in the preparation of 2a and 2b.
Ethyl-2,2,2-d3 phenyl sulfide: bp 95−101◦C/28 mmHg, yield 76%.
Ethyl-2,2,2-d3 phenyl sulfoxide: bp 120−123◦C/5 mmHg, yield 87%.
1-Acetoxyethyl-2,2,2-d3 phenyl sulfide: Purified by silica gel column chromatography
using benzene as an eluent, yield 17%.
1-Acetoxyethyl-2,2,2-d3 phenyl sulfoxides (2ad) and (2bd): Purified by silica gel column
chromatography using ether-hexane as an eluent, yield 55%.
Isomer (2ad): 1H NMR (CDCl3) δ = 7.2−7.7 (5 H, m, C6H5), 5.52 (1 H, s, CH), 2.15 (3 H,
s, OAc). IR (neat): 1748 (C O), 1045 cm−1 (S O).
Isomer (2bd): 1H NMR (CDCl3) δ = 7.36−7.80 (5 H, m, C6H5), 5.75 (1 H, s, CH), 2.00
(3 H, s, OAc). IR (neat): 1748 (C O), 1045 cm−1 (S O).

1172
T. YOSHIMURA ET AL.
Preparation of (2-Substituted Ethyl) Phenyl Sulfoxides (3, 4, 5)
25
2-Chloroethyl phenyl sulfoxide (3) and 2-bromoethyl phenyl sulfoxide (4) were
prepared by oxidation of the corresponding sulfides 17, 18 with hydrogen peroxide in acetic
acid, and then treated with sodium methoxide to give 2-methoxyethyl phenyl sulfoxide
1
(5).19 3: mp 31−32◦C. H NMR (CDCl3); δ = 7.50 (5 H, s, C6H5), 3.00−4.04 (4 H, m,
1
CH2CH2). IR (KBr); 1045 cm−1 (S O). 4: H NMR (CDCl3); δ = 7.50 (5 H, s, C6H5),
3.10−4.14 (4 H, m, CH2CH2). IR (KBr); 1045 cm−1 (S O). 5: oil. 1H NMR (CDCl3); δ =
7.3−7.7 (5 H, s, C6H5), 3.4−4.0 (2 H, m, CH2O), 3.30 (3 H, s,OCH3), 2.8−3.1 (2 H, m,
CH2CH2O). IR (neat); 1040 cm−1 (S O).
Kinetics
A pre-cooled solution of sulfoxide (7.0 × 10−2 mmol) and 0.10 mmol of an internal
standard (diphenyl ether or diphenyl sulfide) in anhydrous dioxane was prepared in 10 µL
of sealed capillary tubes, and the tubes were immersed in a constant temperature silicon oil
bath ( 0.1◦C) at a desired temperature. At appropriate time intervals, the tubes were taken
out and the pyrolysis was stopped in an ice bath. The reaction rate was then determined
by following the decreasing peak area of the sulfoxide at 254 nm in HPLC measurement
(column: HP−01, eluent: CH3CN:H2O = 5:2). The rate of the reaction was found to fit a
first-order kinetics nicely, and the rate constant was calculated by the least-squares method.
Activation parameters (ꢀH‡ and ꢀS‡) were computer calculated by the least-squares
method from the Arrhenius equation. The Hammett ρ-value was computed by the least-
squares method using σ-values and logarithms of the rate constants.
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