Study of the mechanism of base induced dehydrobromination of trans-β-bromostyrene

Article abstract of DOI:10.1016/S0040-4020(03)00107-8

Observation that rates of dehydrobromination of trans-β-bromostyrene (1) and the Hofmann degradation of tetrabutyl ammonium cation depend on strength of base in different ways and that treatment of 1 with base results in fast abstraction of the β-proton imply the possibility that the dehydrobromination of 1 could proceed via α-elimination and Ph migration. In order to clarify this question, β-¹³C-labeled 1 was obtained and subjected to PTC dehydrobromination which proceeds without migration of Ph. The obtained results are consistent with an irreversible E1cB mechanism.

Full text of DOI:10.1016/S0040-4020(03)00107-8

Tetrahedron 59 (2003) 1995–2000
Study of the mechanism of base induced dehydrobromination
of trans-b-bromostyrene
*
Mieczysl⁄aw Ma˛kosza and Alexey A. Chesnokov
Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44, POB 58, 01-224 Warsaw 42, Poland
Received 22 October 2002; revised 13 December 2002; accepted 16 January 2003
Abstract—Observation that rates of dehydrobromination of trans-b-bromostyrene (1) and the Hofmann degradation of tetrabutyl
ammonium cation depend on strength of base in different ways and that treatment of 1 with base results in fast abstraction of the b-proton
imply the possibility that the dehydrobromination of 1 could proceed via a-elimination and Ph migration. In order to clarify this question,
b-13C-labeled 1 was obtained and subjected to PTC dehydrobromination which proceeds without migration of Ph. The obtained results are
consistent with an irreversible E1cB mechanism. q 2003 Published by Elsevier Science Ltd.
During our studies on co-catalysis in phase-transfer
catalyzed base induced dehydrobromination of trans-b-
bromostyrene (1)1 a very interesting observation was made.
The relation of the rates of two competing reactions:
dehydrobromination of 1 and the Hofmann degradation of
tetrabutylammonium (TBA) cation depends very much on
the strength of the basic agent (Table 1). Surprisingly, the
most selective was the strongest base –OH2 anion which
reacted practically exclusively with 1 not with TBA,
whereas much weaker bases such as ArO2 reacted faster
with TBA. We have shown1 that under described conditions
the degradation of the TBA cation occurs as the normal
Hofmann b-elimination process, not as a nucleophilic
substitution of Bu3N in the TBA cation, so that the
phenomenon cannot be explained in terms of nucleophili-
city of the basic agents. The observed facts prompted us to
perform some studies on the mechanism of base induced
dehydrobromination of trans-b-bromostyrene.
1. Results and Discussion
The observations mentioned above suggest that there is a
principal difference between the mechanisms of dehydro-
bromination of trans-b-bromostyrene and the Hofmann
degradation of the TBA cation. Whereas the Hofmann
degradation of the TBA cation most likely follows a one-
step E2 mechanism,3 in the case of trans-b-bromostyrene,
for which an E2-type mechanism seems to be unlikely due
to the geometry of the molecule, a two-step E1cB
Table 1. Dehydrobromination of trans-b-bromostyrene versus Hofmann degradation of TBA cation promoted by different TBA salts at 908C
Entry
RO2
pKa of ROH
(DMSO)a
Time
(min)
Ratio
(PhCuCH/n-Bu3N)b
1
2
3
4
HO2
31.2
<29.8
27.0
0.5
0.5
2
23.8
10.0
3.9
n-BuO2
PhCH2O2
ArO2c
<18.5
60
0.6
For experimental see Ref. 1.
a
According to Bordwell.2
Determined by GLC and 1H NMR using durene as an internal standard; the sum of PhCuCH and n-Bu3N was <100% in regard to the initial amount of TBA
salt.
Ar¼2,4,6-trimethylphenyl.
b
c
Keywords: trans-b-bromostyrene; b-elimination; irreversible E1cB mechanism; 13C-labeled compounds.
*
Corresponding author. Tel.: þ48-22-6318788; fax: þ48-22-6326681; e-mail: icho-s@icho.edu.pl
0040–4020/03/$ - see front matter q 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4020(03)00107-8

1996
M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 59 (2003) 1995–2000
Scheme 1.
Scheme 2.
mechanism might operate. That is supported by earlier
observation that in NaOH/i-PrOH medium trans-b-bromo-
styrene was found to be 105 times less reactive than the cis-
isomer for which an E2-type mechanism is feasible.4
Moreover, the introduction of a strongly electron with-
drawing group (NO2) into the para position of the aromatic
ring resulted in a much more substantial acceleration of the
reaction rate for the trans-isomer than for the cis one.4
These observations suggest that carbanionic species are
involved in the rate-determining step of the base induced
dehydrobromination of the trans-isomer. Taking into
account the fact that the dehydrobromination of 1 according
to an E1cB mechanism would require deprotonation of a
very weak C–H acid (PhCHvCHBr) the basicity of RO2
should be a more important factor in dehydrobromination of
1 than in the Hofmann degradation of the TBA cation,
which, if it followed a concerted E2 mechanism, would not
require the generation of a carbanion. If base induced
dehydrobromination of 1 proceeds via a carbanionic
intermediate (e.g. E1cB mechanism) then one might
suppose that a deuterium exchange of acidic hydrogen(s)
in 1 should be observed during the course of the
dehydrobromination when the reaction is carried out in an
OD2/D2O medium. We performed such an experiment
under PTC conditions (Scheme 1) stopping the reaction
after 16% conversion (GLC) of 1 to phenylacetylene. To our
surprise a total H/D exchange in unreacted 1 took place at
the b-carbon atom (b-C) whereas there was no incorpor-
ation of D at a-carbon. The observed H/D ratio¼3/97 (1H
NMR) at b-C corresponded to the total ratio of exchange-
able H and D in the studied system. Absence of any
detectable H/D exchange at a-C was confirmed by 1H NMR
studies of the crude reaction mixture using characteristic
signal from two aromatic protons of phenylacetylene (d
7.55 ppm, m) as the internal standard. Thus, in trans-b-
bromostyrene b-H is much more acidic than corresponding
a-H, so that the former is rapidly abstracted by a base to
generate the a-bromocarbanion. On the other hand, it was
shown earlier that in cis- and trans- b-chlorostyrenes, b-
protons exhibit higher acidity than a-protons.5 Obviously a
bromine atom as well as a chlorine atom stablizes a vinyl
carbanion better than the phenyl group does. An example of
successful trapping of the PhCHvC2Br carbanion with
TMSCl has been reported in the literature.6 We have also
succeeded in trapping the PhCHvC2Br carbanion selec-
tively by means of rapid reaction with another active
electrophile – benzaldehyde. When a 1:1 molar mixture of 1
and PhCHO was treated at 21008C with a mixture of
1 equiv. LDA and 1.3 equiv. n-BuLi we observed the
formation of the corresponding adduct 2 as a main product
accompanied by a small amount of phenylacetylene as well
as the adduct of metallated styrene, produced by halogen
metal exchange in 1, to benzaldehyde 3 (Scheme 2). The
1
ratio of the products was established on the basis of the H
NMR spectrum and subsequently confirmed by isolation of
2 and unreacted 1 from the crude reaction mixture using
silica column chromatography. The other products found in
the crude reaction mixture were n-BuCH(OH)Ph (4, 37%),
benzyl alcohol (16%) and unreacted PhCHO (16%). It is
worth noting that use of LDA or n-BuLi as a base separately
did not give access to 2. Whereas LDA alone was proved to
be too weak a base to sufficiently deprotonate 1 under the
reaction conditions at 2100/2708C (the only isolated
products were benzyl alcohol, unreacted 1 and excess
benzaldehyde), the use of n-BuLi alone as a base mainly
gave its adduct with benzaldehyde 4. On the other hand,
Scheme 3.
Scheme 4.

M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 59 (2003) 1995–2000
1997
Scheme 5.
when the reaction was carried out at 2788C we observed
formation of 2 and phenylacetylene in practically equal
proportions.
halogenoethylenes and involves a-elimination of hydrogen
halide and migration of an aryl group to the neighboring
carbon atom to give diarylacetylenes.10 Experiments with
labeled cis–trans-isomeric starting compounds have shown
that migration of the aryl group trans to the vinyl halogen
always predominates indicating that departure of the leaving
group is assisted by the migrating group.10,11 For direct
determination of which mechanistic way, namely the
normal b-elimination or a-elimination followed by 1,2-
shift rearrangement of the formed carbene, operates in the
case of base induced dehydrobromination of 1 it is
necessary to use an isotopically labeled sample of 1. Since
the formed product (phenylacetylene) is much more acidic
than the substrate, D/H isotope labeling is of no use, because
the obtained phenylacetylene would always contain the
equilibrium D/H isotope ratio. As was observed by Curtin et
al.11b dehydrobromination of cis- and trans-b-bromo-
styrene-a-14C by butyllithium in ether at 2358C — the
typical conditions for the execution of the Fritsch–
Buttenberg–Wiechell rearrangement of a,a-diaryl-b-
bromoethylenes to diarylacetylenes, proceeds without
migration of the phenyl group during the course of the
reaction. However, under the conditions used by Curtin
et al.11b (very strong base such as BuLi and low
temperatures) a so-called E2cB mechanism (Scheme 5)
might operate as was suggested and experimentally
evidenced by Schlosser5,10 in the case of the organo-
lithium-induced elimination of hydrogen chloride from cis-
and trans-b-chlorostyrene. It should be stressed that the
particular mechanism of dehydrobromination of 1 obviously
depends very much on the kind of base used and the applied
conditions. For instance, Cristol et al.12 reported that the rate
of the reaction of trans-b-bromostyrene with phenyllithium
in ether or Bu2O at þ258C was 2–6 times faster than that of
the cis-b-bromostyrene in contrast to their behavior toward
alkali in isopropyl alcohol.4 Taking into account the
Taking into account the observations mentioned above we
supposed that base induced dehydrobromination of 1 might
follow the irreversible E1cB mechanism (Scheme 3) or
might involve a migration of phenyl group or hydrogen to
the electron deficient alkylidene carbene centre produced
via elimination of Br2 from PhCHvC2Br carbanion (a-
elimination process, Scheme 4). Alkylidene carbenes are
known species and intermediates in organic reactions,
which can easily undergo 1,2-shift rearrangements to give
alkynes.7 It has already been observed that a-methyl-b-
halostyrenes in the presence of strong bases gave methyl-
phenylacetylenes.8 For instance, treatment of diethyl ether
solution of (E)- or (Z)-a-methyl-b-bromostyrene
(PhC(Me)vCHBr) with equimolar amount of n-BuLi at
230 to 08C gave rise to methylphenylacetylene
(PhCuCMe) in a high yield.8c The nature of the products
and trapping experiments provided compelling evidence for
the intermediacy of alkylidene carbenes in these reactions
(Scheme 4).8a However, all our attempts to trap eventual
carbene species in the studied PTC dehydrobromination of 1
have failed. When the reaction was carried out in the
presence of 3 equiv. of styrene as a ‘carbene trap’ no
corresponding cyclopropane derivatives formed from the
alkylidene carbene and styrene were detected by 1H NMR in
the crude reaction mixture. On the other hand, the migration
of phenyl or hydrogen to the carbene center (Scheme 4)
should be very rapid or even simultaneous with the
departure of bromide.9,10 Thus, the life time of the carbene
might be too short to make possible its trapping. The process
presented in Scheme 4 is very close to the known Fritsch–
Buttenberg–Wiechell rearrangement which is generally
observed during the action of bases on a,a-diaryl-b-
Scheme 6. Reagents and conditions: (i)13 (a) 90% aq. EtOH, 258C, 24 h, then 508C, 5 h; (b) 30% aq. NaOH, reflux, 24 h. (ii)14 (a) Me2CHCMe2BHCl·Me2S
(2.2 equiv.), CH2Cl2, Ar, 0–258C, 30 min; (b) aq. NaHSO3, CH2Cl2/THF, 258C, 12 h; (c) 37% aq. CH2O (1.0 equiv.), aq. MgSO4, pentane, 258C, 1 h. (iii)15 o-
C6H4O2PBr3 (1.1 equiv.), CH2Cl2, Ar, 258C, 2 h. (iv)1,16 50% aq. NaOH, TBAB (0.05 equiv.), mesitol (0.05 equiv.), 258C, 24 h. (v)1 50% aq. NaOH, TBAB
(0.05 equiv.), chlorobenzene, 908C, 24 h (80% conversion).

1998
M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 59 (2003) 1995–2000
Table 2. 1H and 13C NMR data on the synthesized 13C-labeled compounds
Compound
1H NMR
13C NMR
PhCH123COOH
d 3.7 (d, 2H, H2, 2JH2C1¼7.7 Hz); 7.4 (m, 5H, Ph);
d 41.1 (d, C2, 1JC2C1¼55.4 Hz); 127.3 (s, C6); 128.6 (s, C5);
129.3 (d, C4, 3JC4C1¼1.8 Hz); 133.2 (d, C3, 2JC3C1¼2.7 Hz);
178.1 (intensive s, C1)
10.9 (bs, 1H, COOH)
PhCH123CHO
d 3.7 (dd, 2H, H2, 2JH2C1¼7.3 Hz, JH2H1¼2.4 Hz);
1JH1C1¼175.6 Hz, 3JH1H2¼2.4 Hz)
d 50.5 (d, C2, 1JC2C1¼37.5 Hz); 127.4 (s, C6); 128.9 (s, C5);
129.6 (d, C4, 3JC4C1¼1.8 Hz); 131.8 (d, C3, 2JC3C1¼2.3 Hz);
199.3 (intensive s, C1)
3
7.3–7.4 (m, 5H, Ph); 9.4, 10.2 (dt, 1H, H1,
PhCH123CHBr2
Ph13CHBrCH2Br
trans-PhCHv13CHBr
cis-PhCHv13CHBr
Ph13CBrvCH2
PhCu13CH
d 3.8 (t (unresolved dd), 2H, H2, 3JH2H1¼6.8 Hz,
d 45.1 (intensive s, C1); 51.4 (d, C2, JC2C1¼35.7 Hz);
1
3
2JH2C1 < 6.2 Hz); 5.4, 6.3 (dt, 1H, H1,
127.6 (s, C6); 128.6 (s, C5); 129.4 (d, C4, JC4C1¼2.2 Hz);
1JH1C1¼178.7 Hz, 3JH1H2¼6.8 Hz); 7.3–7.5 (m, 5H, Ph)
d 4.1 (m, 2H, H1); 4.8, 5.6 (ddd, 1H, H2,
136.7 (d, C3, 2JC3C1¼2.2 Hz)
1
JH2C2¼157.0 Hz, 3JH2H1¼10.1 Hz; 3JH2H1 ¼6.0 Hz);
0
7.4 (m, 5H, Ph)
d 6.3, 7.3 (dd, 1H, H1, 1JH1C1¼194.2 Hz,
3JH1H2¼14.0 Hz); 7.1–7.2 (dd, 1H, H2, 3JH2H1¼14.0 Hz,
2JH2C1¼9.2 Hz); 7.3–7.4 (m, 5H, Ph)
d 106.5 (intensive s, C1); 126.0 (d, C4, 3JC4C1¼5.9 Hz);
2
128.2 (s, C6); 128.7 (s, C5); 135.9 (d, C3, JC3C1¼1.8 Hz);
137.1 (d, C2, 1JC2C1¼79.6 Hz)
d 6.0, 7.0 (dd, 1H, H1, 1JH1C1¼198.6 Hz, 3JH1H2¼8.2 Hz);
7.1 (t (unresolved dd), 1H, H2, 2JH2C1<8.6 Hz,
3JH2H1¼8.2 Hz); 7.4 (m, 5H, Ph)
2
d 5.8 (dd, 1H, H1, 2JH1C2¼6.5 Hz, JH1H1 ¼2.0 Hz);
0
2
6.2 (dd, 1H, H10, JH1 C2¼5.3 Hz, 2JH1 H1¼2.0 Hz);
0
0
7.4 (m, 5H, Ph)
1
d 2.5, 3.8 (d, 1H, H1, 1JH1C1¼251.1 Hz);
d 77.2 (intensive s, C1); 84.0 (d, C2, JC2C1¼181.5 Hz);
7.4 (m, 3H, H5, H50, H6); 7.6 (m, 2H, H4, H40)
122.0 (d, C3, 2JC3C1¼12.8 Hz); 128.3 (s, C5); 128.7 (s, C6);
132.1 (d, C4, 3JC4C1¼2.5 Hz)
Ph13CuCH
d 3.1 (d, 1H, H1, 2JH1C2¼49.6 Hz);
7.4 (m, 3H, H5, H50, H6); 7.6 (m, 2H, H4, H40)
considerably higher temperature under the PTC conditions
applied by us (50% aq. NaOH, cat. QþBr2, 908C) we
supposed that in this case the mechanism of base induced
dehydrobromination of 1 might involve a migration of the
phenyl group. We have synthesized a sample of b-13C-
labeled trans-b-bromostyrene in a 25% overall yield on the
basis of known procedures13–16 starting from K13CN (1.0 g
scale) and benzyl bromide (Scheme 6, i–iv; notice a
formation of considerable amounts of Ph13CHBrCH2Br
from PhCH123CHBr2 as a result of migration of the phenyl
group to the positively charged carbon atom under acidic
conditions, see Scheme 6, iii). 1H and 13C NMR data for the
synthesized 13C-labeled compounds are summarized in
Table 2. Under the PTC conditions the obtained 13C-labeled
1 gave phenylacetylene without any migration of the phenyl
group (Scheme 6, v), i.e. path ’a’ in Scheme 4 can be
excluded. As was mentioned above, eventual migration of
hydrogen during dehydrobromination of 1 (path ‘b’ in
Scheme 4) cannot be determined directly by use of
deuterium labeling in 1. There is no literature data on the
relative migratory aptitudes of hydrogen and phenyl for the
1,2-shift to the carbene centre. However, taking into account
that both these processes are very fast7–10 and migration of
the phenyl group in 1 would be rather preferred to the
migration of the a-H since Ph is in the trans position to the
halogen,11 the absence of any detectable migration of the
phenyl group during the studied dehydrobromination of 1
suggests that a-elimination/1,2-H migration pathway ‘b’ is
probably not operating in this case either.
On the basis of these data we could suggest that base
induced dehydrobromination of trans-b-bromostyrene car-
ried out under PTC conditions17 or in alcoholic alkali
medium most likely follows an irreversible E1cB mechan-
ism (Scheme 3). Moreover, taking into account the fact that
acidity of a hydrogen in the a-position to the leaving group
is much higher than in the b-position, the extremely low
reactivity of the trans-b-bromostyrene towards base
induced b-elimination of HBr by the action of bases of
moderate strength (OH2, RO2, etc.) becomes more
understandable.
2. Experimental
Isomerically pure trans-b-bromostyrene 1 (trans/cis .99/1
1
by H NMR and GLC) was obtained from commercially
available b-bromostyrene (Fluka, trans/cis <85/15) by
selective conversion of the more reactive cis isomer into
phenylacetylene under mild cocatalytic PTC conditions,
excess of 50% aq. NaOH, 1 mol% of TBAB, 1 mol% of
mesitol, stirring for 24 h at room temperature,16 followed by
distillation of the crude organic product (1/PhCuCH <85/
15) under reduced pressure. It was stored in a refrigerator at

M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 59 (2003) 1995–2000
1999
0–48C. 50 wt% NaOD/D2O was prepared by careful
treatment of Na2O (Aldrich) with calculated amount of
D2O under Ar and external cooling with an ice/water bath.
GLC analyses were performed using ‘Shimadzu GC-14A’
gas chromatograph; injection port temperature 1208C;
injection time 1 min. 1H and 13C NMR spectra were
recorded on Varian Gemini spectrometer (200 MHz for
1H). Chemical shifts are indirectly referenced to TMS via
the solvent signal (chloroform-d1 7.26 and 77.0 ppm).
191 (74), 131 (72), 103 (67), 77 (100). C15H13BrO
calculated: C 62.30, H 4.53, Br 27.63; found: C 62.06, H
4.34, Br 27.52.
trans-1,3-Diphenylallyl alcohol (3): 1H NMR d 2.5 (broad
s, 1H, OH), 5.4 (d, 1H, H1, J¼6.4 Hz), 6.4 (dd, 1H, H2,
J1¼6.4 Hz, J2¼15.9 Hz), 6.7 (d, 1H, H3, J¼15.9 Hz), 7.3–
7.5 (m, 11H, 2Ph) (consistent with the literature data18).
1-Phenyl-1-pentanol (4): 1H NMR d 0.9 (t (unresolved dd),
3H, H5, J<6.9 Hz), 1.4 (m, 4H, H4þH3), 1.8 (m, 2H, H2),
2.3 (broad s, 1H, OH), 4.7 (dd, 1H, H1, J1¼6.0 Hz,
J2¼7.2 Hz), 7.3–7.4 (m, 5H, Ph). 13C NMR: d 14.0, 22.6,
28.0, 38.8, 74.5, 125.8, 127.2, 128.2, 144.8 (consistent with
the literature data19).
2.1. Studies of H/D exchange during base induced
dehydrobromination of trans-b-bromostyrene (1)
A 10 ml flask was charged with a mixture of 1 (0.366 g,
2 mmol), chlorobenzene (0.20 g), TBA bromide (0.032 g,
0.1 mmol) and 50 wt% NaOD/D2O (1.7 g, <1 ml,
<90 mmol D). The flask was tightly closed with a stopper
using a metallic clip and placed on the magnetic stirrer
equipped with an oil bath thermostated at 908C. The mixture
was stirred at 908C for 1 h and cooled with cold water. Small
samples of the organic phase were taken and analyzed by
Acknowledgements
This work was partially supported by The Foundation for
Polish Science (Fundacja na Rzecz Nauki Polskiej).
1
GLC (diluted with CH2Cl2) and H NMR techniques.
2.1.1. trans-b-Bromostyrene (1). 1H NMR (see also Table
3
2) d 6.8 (d, 1H, H1, JH1H2¼14.0 Hz); 7.1 (d, 1H, H2,
3JH2H1¼14.0 Hz); 7.3–7.4 (m, 5H, Ph). 13C NMR (see also
Table 2): d 106.5 (C1); 126.0 (C4); 128.2 (C6); 128.7 (C5);
135.9 (C3); 137.1 (C2).
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A 25 ml oven-dried three-neck round-bottom flask equipped
with a septum, an inlet of dry Ar and an oil pump was
degassed in vacuum and filled with Ar (3 times). The flask
was cooled to about 2808C (an ether–liquid nitrogen bath)
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cally pure sample was obtained by recrystallization from n-
pentane of the crude material after SiO2 column. Mp 79–
808C (uncorrected). 1H NMR d 2.7 (broad s, 1H, OH), 5.9 (s,
1H, H1), 7.3–7.5 (m, 11H, H3þ2Ph). 13C NMR: d 71.2,
125.8, 127.7, 128.0, 128.3, 128.6, 131.6, 134.9, 135.3,
140.3. MS (EI, 70 eV) m/z (%): 290 (36), 288 (37), 209 (86),
¨
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17. For discussion of how PTC proceeds in this case see our
previous paper.1
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CH2O during decomposition of the bisulfite adduct of
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