, 2005, 15(2), 84–86
no published data on the use of α-methylstyrene in these
reactions.
[n(C=C)], 1417.8 [d(planar)(>C=CH2)] and 897.8 cm–1 [d(nonplanar)
(>C=CH2)] in the IR spectra of the products and the absence of
bands in the region 980–960 cm–1 (–C=C–) suggests that each
of these compounds contains a terminal double bond.
The final products were obtained in the yields of 88.3% (3a)
and 91.3% (3b); their melting points are 117–118 and 123–125 °C,
respectively.
We studied the addition of arylsulfonyl chlorides to α-methyl-
styrene 1 under standard conditions commonly used to obtain
adducts of sulfonyl chlorides with alkenes, in particular, with
substituted styrenes.3,4 The reaction was carried out with an
equimolar ratio of the reagents in acetonitrile in the presence of
copper(II) chloride and triethylamine hydrochloride. (The role
of the latter is to convert copper salts to more soluble complex
compounds).† We encountered some unexpected results in this
reaction (Scheme 1).
By shortening the synthesis time of sulfone 3a from 3 to 0.5 h
and reducing the reaction temperature from 85 to 60 °C, we
1
obtained a product which, according to its H NMR spectrum,
consisted of sulfone 3a and adduct 4a in the ~2:1 ratio. The
methyl protons of adduct 4a manifest themselves in the spectrum
as a singlet signal at d 1.63 ppm, while the methylene protons
form two doublets at d 3.75 and 3.87 ppm. The multiplicity and
the positions of proton signals of compounds 3a and 4a are in
good agreement with simulated spectra. The presence of com-
pound 4a in the product of the uncompleted reaction suggests
that it actually occurs via an intermediate adduct of sulfonyl
chloride with α-methylstyrene. The spectrum of the product
does not contain the signals of protons of sulfone 2a.
R1
SO2Cl
C
H2C
Me
1
R2
R1
CuCl2, Et3N·HCl MeCN
SO2CH2
Cl
C
The spontaneous abstraction of hydrogen chloride from adducts
of arylsulfonyl chlorides with alkenes during their synthesis has
previously been observed for 1,1-diphenylethene, 1-phenyl-
3,4-dihydronaphthalene, 1-phenylcyclohexene, and acenaph-
thylene.11 Apparently, in all of these cases including this study,
the structures of the starting alkenes were such that, upon addi-
tion of sulfonyl chlorides to them, the chlorine atom was bound
to the tertiary carbon atom. Probably, the resulting tertiary
chloride then readily undergoes thermal dehydrochlorination by
the E1 mechanism, which does not require an additional base
added. (Apparently, the solvent, i.e., acetonitrile, serves as a
base). Incidentally, when the addition of sulfonyl chlorides to
α-methylstyrene was carried out in the absence of triethylamine
hydrochloride, this affected only slightly the yields of the pro-
ducts but not its direction. Sulfones 3a and 3b were obtained in
the yields of 85.6 and 88.2%, respectively.
Me
R2
4
R1
SO2CH
C
Me
R2
2
R1
SO2CH2
C
CH2
a R1 = Cl, R2 = H
b R1 = R2 = Cl
R2
3
Scheme 1
As regards the violation of the Zaitsev rule in the dehydro-
chlorination of adducts 4a,b, we apparently encounter the situa-
tion where the formation of a less-substituted alkene is more
favourable, since steric repulsion of cis substituents in the more
substituted alkene is too high and makes the latter less stable. In
fact, it is known12 that the dehydrobromination of 2-bromo-
2,4,4-trimethylpentane predominantly results in 2,4,4-trimethyl-
pent-1-ene rather than 2,4,4-trimethylpent-2-ene as could be
expected in accordance with the Zaitsev rule. This is due to the
considerable steric repulsion of the methyl and tert-butyl groups
that are cis-arranged in the latter isomer. In this work, an even
higher steric hindrance should be expected for isomers 2a,b
due to the presence of more bulky substituents at the double
bond. Because of this, the reaction gives only isomers 3a,b.
Isomers 2a,b were not detected.
Note that unsaturated sulfones 3a,b are considerably less
reactive, e.g., in the nucleophilic addition of thiols, than vinyl-
sulfones obtained by the dehydrochlorination of adducts of
sulfonyl chlorides with styrene. These vinylsulfones undergo
thiylation in DMF in a few minutes at room temperature in the
presence of catalytic amounts of triethylamine; the yield was
virtually quantitative. Under these conditions, sulfones 3a,b do
not react with thiols. The reason for the lower reactivity of the
double bond in allyl sulfones 3a,b in comparison with vinyl
sulfones is apparently that in this case the strong electronegative
sulfonyl group cannot activate the double bond directly.
First, the addition of arylsulfonyl chlorides to α-methylstyrene
is accompanied by the simultaneous dehydrochlorination of
the β-chlorosulfone formed. Hydrogen chloride is liberated
throughout the entire reaction, giving an unsaturated sulfone as
the only end product. In almost all the studies reported, the
dehydrochlorination stage is carried out as a separate process,
e.g., by treatment with triethylamine in benzene at room tem-
perature.1–4
Second, the dehydrohalogenation of arylsulfonyl chloride
adducts with α-methylstyrene does not obey the Zaitsev rule.
Less substituted allyl sulfones 3a,b are formed instead of expected
vinyl sulfones 2a,b with a higher degree of substitution. This is
confirmed by 1H NMR and IR spectroscopic data. The
1H NMR spectra of products 3a,b contain singlets of two
protons of the terminal double bond at d 5.62 and 5.21 (3a) and
at 5.57 and 5.30 (3b). The protons of the CH2SO2 group
manifest themselves in the spectrum as singlet signals at d 4.65
(3a) and 4.54 (3b). The presence of absorption bands at 1623.6
†
Synthesis of sulfones 3a and 3b.
A mixture of an arylsulfonyl chloride (0.01 mol), α-methylstyrene 1
(1.18 g, 0.01 mol), copper chloride (0.0135 g, 0.1 mmol), triethylamine
hydrochloride (0.0207 g, 0.15 mmol) and acetonitrile (3.69 g, 0.09 mol)
was refluxed for 3 h, cooled and poured into 4 ml of cold methanol. The
resulting white precipitate was filtered off, dried, and recrystallised from
95% ethanol.
1H NMR spectra of a 5% solution of samples in [2H6]DMSO were
recorded on a Bruker AC-250 instrument; TMS was used as an internal
standard. IR spectra were measured on a Specord 75 R in the range
300–4000 cm–1 using suspensions in Vaseline oil.
References
1 M. Asscher and D. Vofsi, J. Chem. Soc., 1964, 4962.
4 A. Orochov, M. Asscher and D. Vofsi, J. Chem. Soc., Perkin Trans. 2,
1973, 1000.
5 K. Inomata, S. Sasaoka and T. Kobayashi, Bull. Chem. Soc. Jpn., 1987,
60, 1767.
3a: mp 117–118 °C. 1H NMR, d: 7.67 (q, 4H, Ph), 7.41–7.24 (m, 5H,
Ph), 5.62 (s, 1H, C=CH2), 5.20 (s, 1H, CH2), 4.65 (s, 2H, CH2). Found
(%): C, 63.14; H, 4.34; S, 10.44. Calc. for C16H13O2SCl (%): C, 63.05;
H, 4.27; S, 10.51.
1
3b: mp 123–125 °C. H NMR, d: 7.25–7.45 (m, 5H, Ph), 7.50–7.95
(m, 3H, Ph), 5.57 (s, 1H, C=CH2), 5.80 (s, 1H, C=CH2), 4.54 (s, 2H, CH2).
Found (%): C, 55.13; H, 3.76; S, 9.62. Calc. for C15H12O2SCl2 (%): C,
55.05; H, 3.67; S, 9.79.
Mendeleev Commun. 2005 85