Solvolytic stereoselective debromination of vic-dibromides with HMPA

Article abstract of DOI:10.1246/bcsj.74.1089

A simple and efficient procedure for the debromination of vic-dibromides has been reported with hexamethylphosphoric triamide at 155-160°C under a nitrogen atmosphere without the aid of any reagent.

Full text of DOI:10.1246/bcsj.74.1089

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1089—1091 (2001) 1089
e Chemical
ciety of Ja-
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Solvolytic Stereoselective Debromination of vic-Dibromides with HMPA1
e Chemical
ciety of Ja-
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Debromination of vic-Dibromides with HMPA
J. M. Khurana
Jitender Mohan Khurana,* Geeti Bansal, and Sushma Chauhan
Department of Chemistry, University of Delhi, Delhi-110007, India
(Received October 10, 2000)
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A simple and efficient procedure for the debromination of vic-dibromides has been reported with hexamethylphos-
phoric triamide at 155–160 ˚C under a nitrogen atmosphere without the aid of any reagent.
tober
00
The debromination of vic-dibromides to the corresponding
alkenes is an important transformation in organic synthesis.
Debrominations have been carried out with diverse reducing
(2)
(E)-stilbene under these conditions has been ruled out by (i) an
01
0
agents.2 Recent reports on debrominations include reactions
with sodium dithionite,3 selenium,4 tellurides,5 copper, and
copper(II) perchlorate,6 samarium,7 indium,8 and iron.9 Both
stereospecific and stereoselective eliminations have been re-
ported under different conditions. The role of a solvent in
these reactions has not been thoroughly examined. Methanol
has been reported to bring about the methanolysis of meso-stil-
bene dibromide.10 While working on elimination reactions, we
observed that debrominations could be achieved by N,N-dime-
thylformamide without the aid of any reagents.11 In view of
the importance of debromination in organic synthesis, reports
on debromination with newer and expensive reagents keep
pouring in.
independent reaction of (Z)-stilbene with HMPA, when (Z)-
stilbene was recovered unchanged quantitatively, and (ii) a re-
action of (Z)-stilbene with bromine at 100 ˚C. The reaction
mixture showed the distinct formation of dl-stilbene dibro-
mide, the debromination of which was the aim of this paper.
Therefore, the possibility of an E2 elimination or elimination
by an SN2 attack from the N/O end of HMPA, followed by E2
elimination, is unlikely because both of these pathways would
lead to a stereospecific elimination, contrary to our observa-
tions. Also, the anti-alignment, favored by orbital symmetry
rules12 in meso- and dl-stilbene dibromides, is opposed in the
latter case conformationally by a free-energy diffrence of 4.4
kcal molꢀ1 13
Stereoselective elimination via free radicals by
.
Results and Discussion
electron transfer from HMPA or pyrolysis has been ruled out
by a reaction of meso-stilbene dibromide with HMPA in the
presence of cumene (10 molar excess), which yielded stilbenes
(E- and Z-) exclusively.
These debrominations are proposed to proceed by the E1
pathway by initial ionization to a stable onium species, i.e. a
bromide/carbonium ion pair or isolated carbocation, which
collapses to (E)-alkenes. HMPA has been reported to promote
even the ionization of hydrocarbons.14 The carbocation,
formed initially from dl-stilbene dibromide, undergoes rapid
C–C bond rotation owing to a conformational instability, to a
more stable onium species (Scheme 1).10,11,15
Rate retardation by sodium bromide is not applicable to all
vic-dibromides, which could be due to the reduced ability of
HMPA to stabilize the carbocation or due to the participation
of sodium bromide, itself, in debromination.16 The observed
order of reactivity, Br > Cl, in our reactions, since only 10%
and 27% of dechlorination was observed in the reaction of
meso-stilbene dichloride with HMPA after 60 and 120 min, re-
spectively, is in agreement with the order of reactivity of E1 re-
actions, I > Br > Cl > F.17 The formation of an intermediate
onium species is further corroborated by the formation of α-
bromochalcone derivatives in the reactions of chalcone dibro-
mides (Scheme 2). The absence of any elimination in the reac-
tion of diphenylacetylene dibromide is due to its inability to
form a stable onium species. The formation of a mixture of
We now report on the quantitative debromination of vic-di-
bromides to the corresponding olefins with dry hexameth-
ylphosphoric triamide (HMPA) at 155–160 ˚C under a nitro-
gen atmosphere without the aid of any reagent (Eq. 1). HMPA
(1)
acts not only as a solvent, but also as a reagent, since no debro-
minations have been observed in o-dichlorobenzene and xy-
lene under similar reaction conditions. Debrominations have
been carried out on a variety of vic-dibromides. The elimina-
tions are complete in 15–60 min under the aforesaid condi-
tions, and the olefins are obtained in high yields by a simple
work up. Small amounts of α-bromochalcones were also ob-
tained in the reactions of chalcone dibromides. The results are
listed in Table 1. The reactions are much slower at 100 ˚C as
meso-stilbene dibromide and dl-stilbene dibromide underwent
complete debromination in 4 h and 7 h (15 min at 155–160
˚C), respectively. No debrominations were observed at room
temperature as starting materials were recovered unchanged
after 24 h. Debrominations are stereoselective as both meso-
and dl-stilbene dibromides yielded (E)-stilbene predominantly
(Eq. 2). The possibility for the isomerization of (Z)-stilbene to

1090 Bull. Chem. Soc. Jpn., 74, No. 6 (2001)
Debromination of vic-Dibromides with HMPA
Table 1.
Run
Reactions of vic-Dibromide with Dry HMPA under N2 Atmosphere at 155–160 ˚C
Substrate (vic-dibromide)
meso-Stilbene dibromide
Reaction time/min
Isolated yield/% (olefin)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
15
15
30
30
60
60
30
15
60
120
60
20
30
30
30
60
30
30
240
93 (97)a)
83 (96.4)b)
99
dl-Stilbene dibromide
meso-p,p′-Dichlorostilbene dibromide
meso-o,p′-Dinitrostilbene dibromide
meso-p,p′-Dinitrostilbene dibromide
meso-p,p′-Dimethylstilbene dibromide
meso-p-Chloro-p′-nitrostilbene dibromide
p-Nitrostilbene dibromide
99
98
96
98
94
p-Chlorostilbene dibromide
9,9′-Dibromobifluorenyl
Cinnamic acid dibromide
82
87
78
Methyl cinnamate dibromide
Chalcone dibromide
p-Nitrochalcone dibromide
p-Bromochalcone dibromide
p-Chlorochalcone dibromide
p′-Chlorochalcone dibromide
p′-Methylchalcone dibromide
Diphenylacetylene dibromide
59c)
80d)
68e)
62e)
65e)
67e)
70e)
f)
—
a) Yield in brackets is HPLC yield. HPLC also showed the presence of 3.0% of (Z)-stilbene.
b) Yield in brackets is HPLC yield. HPLC also showed the presence of 3.6% of (Z)-stilbene.
c) Other unidentified products were also obtained.
d) 6% of α-bromochalcone was also isolated.
e) Products believed to be corresponding α-bromochalcones were also obtained in 7–15% yield.
f) The starting material was recovered unchanged.
Scheme 1.
Scheme 2.
points were determined on a Tropical Labequip apparatus, and are
products in the reactions of aliphatic vic-dibromides could be
due to a rapid rearrangement of carbocations and other com-
peting elimination reactions.
uncorrected. IR and NMR spectra were recorded on a Perkin–
Elmer FT-IR and Hitachi FT-NMR 60 MHz, respectively. HPLC
analyses were carried out on a Shimadzu LC-4A using SIL (Zor-
bax 150ꢁ4.6 mm) column and hexane as the eluent.
General Procedure. In a dried, N2-filled round-bottomed
flask mounted over a magnetic stirrer and fitted with a reflux con-
denser, was placed a mixture of vic-dibromide (1 g) and dry
HMPA (4 mL). The system was de-aerated by flushing with N2
for 15 min. The contents of the flask were heated in an oil bath
Experimental
vic-Dibromides were prepared by the addition of bromine to the
corresponding olefins. Hexamethylphosphoric triamide (Aldrich)
was dried by allowing it to stand over calcium oxide for 24 h, fol-
lowed by filtration and distillation under a vacuum. The melting

J. M. Khurana
Bull. Chem. Soc. Jpn., 74, No. 6 (2001) 1091
maintained at 155–160 ˚C. The progress of the reaction was mon-
itored by TLC (eluent: petroleum ether/benzene). After the start-
ing material disappeared, the flask was cooled to room tempera-
ture and the mixture was poured into ice cold water (25 mL), at
which time a solid was obtained. The solid was filtered at the
pump, dried, and analyzed by mp, mixed mp, and spectra. The re-
actions of meso- and dl-stilbene dibromides were repeated. The
reaction mixture was extracted with diethyl ether (2ꢁ15 mL) and
subjected to an HPLC analyses. The crude-product mixture from
reactions 13–18 was extracted with diethyl ether (2ꢁ15 mL) and
concentrated on a rotary evaporator. The concentrate was chro-
matographed on a silica-gel column (100–200 mesh) using petro-
leum ether/benzene as the eluent. The products were isolated and
identified.
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