Bromination-dehydrobromination/debromination of N-Methyl-N-(2- phenylethenyl)trifluoromethanesulfonamide

Full text of DOI:10.1134/S1070428013060213

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 6, pp. 924–926. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © I.V. Ushakova, B.A. Shainyan, 2013, published in Zhurnal Organicheskoi Khimii, 2013, Vol. 49, No. 6, pp. 938–939.
SHORT
COMMUNICATIONS
Bromination–Dehydrobromination/Debromination of N-Methyl-
N-(2-phenylethenyl)trifluoromethanesulfonamide
I. V. Ushakova and B. A. Shainyan
Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: bagrat@irioch.irk.ru
Received December 4, 2012
DOI: 10.1134/S1070428013060213
10.9 Hz), and a doublet at δ 6.58 ppm (J = 9.5 Hz)
with a lower intensity. The intensity of the signal at
5.15 ppm was equal to the overall intensity of the sig-
nals at δ 6.51 and 6.58 ppm; the intensity ratio of the
major and minor downfield signals was about 8:1.
Assuming that the signal at δ 5.15 ppm is a superposi-
tion of two doublets corresponding to the downfield
signals at δ 6.51 and 6.58 ppm, we can conclude that
adduct II is a mixture of two diastereoisomers at
a ratio of 8:1. These findings are consistent with the
presence in the 19F NMR spectrum of II of two signals
at δF –73.95 and –72.46 ppm with the same intensity
ratio. Insofar as initial compound I is trans isomer [1],
the major product should be stereoisomer IIa in agree-
ment with the generally preferred trans-addition pattern.
This is also supported by the results of HF/6-311G**
calculations [2], according to which the energy of
isomer IIa is lower by 1.3 kcal/mol than that of IIb.
We have recently synthesized the first N-alkenyl-
substituted trifluoromethanesulfonamide, N-methyl-N-
[(E)-2-phenylethenyl]trifluoromethanesulfonamide (I)
[1]. With a view to obtain new trifluoromethanesulfon-
amide derivatives, we tried to synthesize adducts of I
with various nucleophiles. However, compound I
failed to react with methanol, ethanol, or diethylamine
even on prolonged heating under reflux, but it readily
took up bromine molecule in dioxane, and the resulting
dibromo derivative readily underwent dehydrobromi-
nation by the action of alcoholic alkali (Scheme 1).
Scheme 1.
Br
O
O
O
O
Br2, dioxane
S
Ph
S
Ph
F3C
N
F3C
N
Me
Me
Br
I
II
O
O
Br
Br
KOH, MeOH
S
Ph
H
N
Ph
H
H
N
Br
H
F3C
N
Me
Br
III
Br
Ph
IIa
IIb
We determined stereoselectivity of the addition of
bromine to compound I and stereo- and regioselec-
tivity of the dehydrobromination of adduct II. The
first step may give rise to two diastereoisomers of II:
trans addition of bromine molecule should lead to
(R,R + SS)-diastereoisomer IIa, and cis addition, to
(R,S + S,R)-diastereoisomer IIb. The most stable con-
formations of IIa and IIb are shown below.
The 1H NMR spectrum of II contained signals from
protons in the N-methyl and phenyl groups, two dou-
blets at δ 5.15 (CHPh) and 6.51 ppm (NCH) (J =
The structure of N-(2-bromo-2-phenylvinyl)-N-
methyltrifluoromethanesulfonamide (III) was deter-
1
mined by NMR spectroscopy using H–15N HMBC
and 1H–1H NOESY correlation techniques. Splitting of
1
the vinylic proton in the H–15N HMBC spectrum
(2JHN = 6.8 Hz) indicated its position at the C1 atom,
and the presence of a cross peak between the vinylic
proton and ortho-protons in the phenyl ring was con-
sistent with their cis arrangement with respect to the
double bond.
924

BROMINATION–DEHYDROBROMINATION/DEBROMINATION OF N-METHYL-...
925
Compound III is formed via cis elimination of
hydrogen bromide molecule from diastereoisomer IIa.
This mechanism dominates provided that the base
(a ~0.5 M solution of KOH in methanol) exists mainly
as ion pairs [3] (Scheme 2). The only alternative
version, trans elimination of HBr from the cis adduct
with antiperiplanar orientation of the phenyl and
sulfonamide groups, seems to be much less probable
since the antiperiplanar conformer is less favorable
than IIa by 3.2 kcal/mol and its formation from IIb
requires even higher energy to overcome the barrier to
rotation through the eclipsed (B···Br)-conformation.
residue was kept in a vacuum. Yield 1.8 g (72%),
mp 81°C. IR spectrum, ν, cm–1: 3029, 2955, 2927,
1496, 1404, 1384, 1228, 1117, 946, 788, 695. 1H NMR
spectrum (CDCl3), δ, ppm: 3.20 s (3H, NCH3), 5.15 d
(1H, PhCHBr, J = 10.9 Hz), 6.51 d (0.89H, NCHBr in
IIa, J = 10.9 Hz), 6.58 d (0.11H, NCHBr in IIb, J =
13
9.5 Hz), 7.40 s (5H, Ph). C NMR spectrum (CDCl3),
δC, ppm: 30.80 (NCH3), 52.59 (PhCHBr), 68.05
(NCHBr), 119.56 q (CF3, JCF = 323.2 Hz), 128.06 (Co),
19
129.06 (Cm), 129.65 (Cp), 136.97 (C1). F NMR spec-
trum (CDCl3): δF –73.95 ppm. Found, %: C 32.69;
H 2.92; Br 38.40; F 11.77; N 3.62; S 7.81.
C10H10Br2F3NO2S. Calculated, %: C 28.26; H 3.37;
Br 37.6; F 13.41; N 3.30; S 7.54.
Scheme 2.
K
N-(2-Bromo-2-phenylvinyl)-N-(methyl)trifluoro-
methanesulfonamide (III). A solution of 0.84 g
(15 mmol) of potassium hydroxide in 10 ml of metha-
nol was added dropwise to a solution of 4.2 g
(0.01 mol) of compound II in 10 ml of carbon tetra-
chloride, and the mixture was stirred for 3 h. The
precipitate was filtered off and washed with a small
amount of chloroform, and the filtrate was evaporated
to dryness under reduced pressure (10 mm). Yield
3.3 g (75%), mp 48–52°C. 1H NMR spectrum (CDCl3),
δ, ppm: 3.35 s (3H, NCH3), 7.02 s (1H, NCH=),
7.41 m (5H, Ph). 13C NMR spectrum (CDCl3), δC,
ppm: 37.97 (NCH3), 115.82 (=CBrPh), 119.62 q (CF3,
JCF = 324.6 Hz), 128.19 (Co), 129.06 (Cm), 129.57
(Cp), 133.17 (Ci), 138.60 (NCH=). 15N NMR spectrum
(CDCl3): δN –277.4 ppm (2JNH = 6.8 Hz). 19F NMR
spectrum (CDCl3): δF –72. 48 ppm. Found:
m/z 342.9477 [M]+. C10H9BrF3NO2S. Calculated:
M 342.9489.
MeO
Br
MeOK
H
Ph
H
IIa
III
–MeOH, –KBr
N
Br
Unlike dehydrobromination with alcoholic alkali,
prolonged treatment of adduct II in triethylamine
resulted in debromination with formation of initial
N-methyl-N-[(E)-2-phenylethenyl]trifluoromethanesul-
fonamide (I) (Scheme 3). This transformation is anal-
ogous to the debromination of (R,R)- and (R,S)-dia-
stereoisomers of methyl 2,3-diaryl-2,3-dibromopro-
panoate ArCHBrCBr(Ar)COOMe by the action of tri-
ethylamine [4]. According to [4], the (R,R)-diastereo-
isomer reacted with KOH to give the corresponding
dehydrobromination product ArC(Br)=C(Ar)COOMe,
whereas the (R,S)-diastereoisomer underwent debromi-
nation to ArCH=C(Ar)COOMe [5].
Reaction of compound II with triethylamine.
Freshly distilled triethylamine, 14.5 ml (0.1 mol), was
added to a solution of 4.2 g (0.01 mol) of compound II
in 10 ml of carbon tetrachloride, the mixture was kept
for 5 days, triethylamine was distilled off under
reduced pressure (10 mm), and the residue was
distilled in a vacuum. Yield of I 2.25 g (85%), bp 80–
82°C (0.5 mm). The melting point and IR and NMR
spectra of the product coincided with those reported
previously [1].
Scheme 3.
Br
O
O
O
O
Et3N
–Br2
S
Ph
S
Ph
F3C
N
F3C
N
Me
Br
Me
II
I
N-Methyl-N-[(E)-2-phenylethenyl]trifluorometh-
anesulfonamide (I) was synthesized according to the
procedure described in [1].
The IR spectra were recorded in KBr on a Bruker
Vertex 70 spectrometer. The NMR spectra were meas-
ured on a Bruker DPX 400 instrument at 400 (1H), 100
(13C), 40 (15N), and 386 MHz (19F); the chemical shifts
are given relative to tetramethylsilane (1H, 13C),
CH3NO2 (15N), or CCl3F (19F). The 15N chemical shifts
N-(1,2-Dibromo-2-phenylethyl)-N-(methyl)tri-
fluoromethanesulfonamide (II). Compound I, 2.65 g
(0.01 mol), was dissolved in 10 ml of dioxane, and
an equimolar amount of bromine was added dropwise
until persistent yellow color. The solvent was distilled
off under reduced pressure (10 mm), and the solid
1
were determined from the two-dimensional H–15N
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

USHAKOVA, SHAINYAN
926
spectra using a gradient probe (hmbcgp mode). The
high-resolution mass spectrum of III (electrospray
ionization, positive ion detection) was obtained on
a Micromass Q-TOFmicro instrument.
REFERENCES
1. Shainyan, B.A. and Ushakova, I.V., Russ. J. Org. Chem.,
2012, vol. 48, p. 141.
2. Frisch, M.J., et al., Gaussian 03, revision C.02,
This study was performed under financial support
by the Russian Foundation for Basic Research (project
nos. 13-03-00055, 12-03-31295). The authors thank
L.I. Larina (Irkutsk Institute of Chemistry, Siberian
Branch, Russian Academy of Sciences) for recording
the 15N NMR spectra and I. Starke (Universität
Potsdam) for measuring the precise molecular weight
of compound III.
Wallingford, CT: Gaussian, 2004.
3. Ingold, C.K., Structure and Mechanism in Organic
Chemistry, Ithaca: Cornell Univ., 1969, 2nd ed.
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5. Badajoz, M.A., Montani, R.S., and Cabaleiro, M.C.,
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