Mechanism of dehydrobromination of 1,2-dibromo-1-phenylethane under conditions of phase-transfer catalysis

Article abstract of DOI:10.1023/B:RUJO.0000003184.33001.32

Selective dehydrobromination of 1,2-dibromo-1-phenylethane to α-bromostyrene was effected under conditions of phase-transfer catalysis in systems containing KOH, toluene, and tetraalkylammonium bromides. The high selectivity of the catalytic systems originates from stabilization by lipophilic cation of the phase-transfer catalyst of a E1cb-like transition state in the E2 mechanism. In the presence of a catalytic amount of lipophilic alcohols, phenylacetylene was obtained. Substrate activation by alcohol molecules is explained by enhancement of the acceptor power of halogen atoms due to solvation and by increased mobility of hydrogen atoms.

Full text of DOI:10.1023/B:RUJO.0000003184.33001.32

Russian Journal of Organic Chemistry, Vol. 39, No. 7, 2003, pp. 957 962. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 7, 2003,
pp. 1018 1023.
Original Russian Text Copyright
2003 by Suvorova, Panicheva, Mamontova, Belyatskii.
Mechanism of Dehydrobromination
of 1,2-Dibromo-1-phenylethane under Conditions
of Phase-Transfer Catalysis
V. V. Suvorova, L. P. Panicheva, Yu. V. Mamontova, and M. K. Belyatskii
Tyumen’ State University, ul. Semakova 10, Tyumen’, 625003 Russia
Received November 26, 2002
Abstract Selective dehydrobromination of 1,2-dibromo-1-phenylethane to -bromostyrene was effected
under conditions of phase-transfer catalysis in systems containing KOH, toluene, and tetraalkylammonium
bromides. The high selectivity of the catalytic systems originates from stabilization by lipophilic cation of
the phase-transfer catalyst of a E1cb-like transition state in the E2 mechanism. In the presence of a catalytic
amount of lipophilic alcohols, phenylacetylene was obtained. Substrate activation by alcohol molecules is
explained by enhancement of the acceptor power of halogen atoms due to solvation and by increased mobility
of hydrogen atoms.
Hydrogen halide elimination from alkyl halides
under conditions of phase-transfer catalysis (PTC)
attracts a keen interest, as follows from the data
summarized in [1]. Elimination reactions in two-phase
systems can be effected with various bases and
solvents. From the viewpoint of a classical version
of PTC, the most interesting are systems containing
alkali metal hydroxides and weakly polar organic
solvents which are incapable of dissolving those bases
in the absence of a phase-transfer catalyst. In order
to substantiate the mechanism of elimination it is
necessary to localize the reaction zone (whether the
reaction occurs in the organic or aqueous phase or
in the third phase formed by the catalyst itself [2])
and to determine whether does the process involve
deprotonation stage at the phase boundary [3] or it
occurs intermolecularly in a donor acceptor complex
formed by the base and the substrate [4]. There are
no unambiguous concepts on the nature of catalytic
activity of weak organic acids (HY), such as alcohols
and glycols. It is assumed that either HY gives rise
to lipophilic anion Y , which is a stronger base than
hydroxide ion and is extracted into organic phase
more readily [5] or the reaction proceeds through
a cyclic complex formed from quaternary ammonium
salts, alcohol, substrate, and base [6].
liquid solid (l/s) with finely dispersed KOH in the
presence of tetraalkylammonium bromides as phase-
transfer catalysts; toluene was used as organic phase.
Depending on the base and solvent nature, hydrogen
halide elimination from -aryl-substituted alkyl
halides follows either E1 (SN1 substitution) or E2
mechanism with an E1-like transition state (SN2 sub-
stitution). -Aryl-substituted alkyl halides undergo
elimination according to E1cb mechanism or E2
mechanism with an E1cb-like transition state (SN2
substitution) [7]. 1,2-Dibromo-1-phenylethane is
a substrate which, depending on the conditions of the
first dehydrobromination stage (elimination of one
HBr molecule), could give rise to all possible elimina-
tion mechanisms (E1, E2, and E1cb). The E1 mecha-
nism should favor formation of -bromostyrene,
while -bromostyrene should be obtained along the
E1cb path.
Thus, it becomes possible to determine which
mechanism of dehydrohalogenation is favored by
phase-transfer conditions with participation of lipo-
philic cation (Q+) of the phase-transfer catalyst. For
comparison, dehydrobromination of 1,2-dibromo-1-
phenylethane was also performed in a classical homo-
geneous system with potassium tert-butoxide as base
and tert-butyl alcohol as solvent.
In the present work we studied the mechanism of
the chemical stage of dehydrobromination of 1,2-di-
bromo-1-phenylethane in two-phase systems liquid
liquid (l/l) with a 50% aqueous solution of KOH and
In all the examined systems (Table 1), the dehydro-
bromination of 1,2-dibromo-1-phenylethane follows
overall second-order kinetics (the effects of substrate
concentration and amount of base on the reaction rate
1070-4280/03/3907-0957$25.00 2003 MAIK Nauka/Interperiodica

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SUVOROVA et al.
Table 1. Efficiency and selectivity of dehydrobromination of 1,2-dibromo-1-phenylethane in homogeneous and hetero-
geneous (PTC) systems
Selectivity of elimination, %
k 102,
Selectivity of nucleophilic
substitution, %
Systema
1
l mol 1 min
-bromo-
styrene
-bromo-
styrene
phenyl-
acetylene
Homogeneous, t-BuOK/t-BuOH
3.30
32.3
96.5
94.4
3.2
3.1
4.2
Ethers
64.5 (43 and 21.5)
PTC l/l, catalyst I,
organic phase toluene
0.62b
1.50b
0.4
1.4
Traces of acetophenone
Traces of acetophenone
PTC l/s, catalyst II,
organic phase toluene
a
Phase-transfer catalysts: cetyltrimethylammonium bromide (I) and tetrabutylammonium bromide (II).
Calculated on the arbitrary concentration of KOH (M) in the organic phase.
b
will be discussed elsewhere). Therefore, the rate con-
stant for the homogeneous reaction (k) was calculated
by the second-order equation. Mathematical descrip-
tion of PTC reactions should take into account the
distribution of phase-transfer catalyst [in our case,
cetyltrimethylammonium bromide (I) or tetrabutyl-
ammonium bromide (II)] between the aqueous and
organic phases and phase boundary, as well as the ion
exchange constants for Br and OH (selectivity
constants). Therefore, for two-phase systems l/l and
l/s we calculated an arbitrary second-order rate con-
stant by dividing the pseudofirst-order rate constant
by the overall amount of KOH (mol) in the system,
reduced to the volume of the organic phase (l). The
results given in Table 1 show that the system t-BuOK/
t-BuOH is more active than heterogeneous l/l and l/s
systems containing KOH and phase-transfer catalyst;
the l/s system is more active than l/l.
Scheme 2.
It should be noted that the contribution of nucleo-
philic substitution (two ethers are formed) in the
system t-BuOK/t-BuOH exceeds the contribution of
the elimination process. In the l/l and l/s systems
containing KOH and phase-transfer catalyst, elimina-
tion occurs according to Scheme 1, yielding mainly
-bromostyrene, whereas alcohols (nucleophilic sub-
stitution products) cannot be determined quantitatively
(Table 1). Also, PTC systems give rise to formation
of small amounts of phenylacetylene (Table 1) as de-
hydrobromination product of - and -bromostyrenes
(Scheme 3).
An important aspect is the selectivity of formation
of elimination products and those resulting from
nucleophilic substitution as side process. According
to the GLC data (Table 1), elimination in the system
t-BuOK/t-BuOH gives mainly -bromostyrene and
a small amount of -bromostyrene (the latter was
identified using a mixture of the cis and trans isomers;
Scheme 1).
Scheme 3.
Thus, a considerable advantage of dehydrobromina-
tion of 1,2-dibromo-1-phenylethane under conditions
of phase-transfer catalysis in liquid liquid and liquid
solid systems against the homogeneous reaction in
the system t-BuOK/t-BuOH is high selectivity of
formation of -bromostyrene (94 96%). This result
is consistent with published data on the formation
of -bromostyrene in the system benzene 1,2-di-
bromo-1-phenylethane Aliquat 336 in 83% yield
(reaction time 3 h) [8] and in the system containing
an aqueous solution of KOH and catamine AB with
a selectivity of 85% in -bromostyrene and 5% in
Scheme 1.
Also, nucleophilic substitution products are formed
in the system according to Scheme 2.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003

MECHANISM OF DEHYDROBROMINATION OF 1,2-DIBROMO-1-PHENYLETHANE
959
-bromostyrene [3]. The high selectivity of formation
of -bromostyrene under conditions of phase-transfer
catalysis is explained by replacement of t-BuOK as
a base by KOH and by the presence of an interphase
carrier having a lipophilic cation Q+. In order to
elucidate the role of lipophilic cation Q+ in the
mechanism of elimination (chemical stage) in two-
phase systems (l/l and l/s) we used various organic
solvents and cetyltrimethylammonium bromide as
phase-transfer catalyst (Table 2).
The data given in Table 2 show that toluene is
incapable of dissolving KOH (almost no reaction is
observed in the absence of phase-transfer catalyst).
Therefore, phase-transfer catalyst ensures not only
selective formation of -bromostyrene but also
hydroxide ion transfer from the aqueous or solid phase
to the phase boundary and partially to the bulk
organic phase. Such solvents as dimethyl sulfoxide
and pyridine dissolve KOH, and the l/l and l/s systems
are active in the absence of phase-transfer catalyst, but
the yield of -bromostyrene (56 73%) insignificantly
exceeds the yield of -bromostyrene (22 38%). Addi-
tion of phase-transfer catalyst to the above systems
considerably reduces the yield of -bromostyrene
(to 2 6%) and increases the yield of -bromostyrene
(to 91 98%). These data unambiguously indicate that
lipophilic cation Q+ not only acts as interphase carrier
in two-phase systems but also directly participates in
the mechanism of chemical elimination stage. Insofar
as lipophilic cation Q+ could favor stabilization of
essentially carbanionic transition state (E1cb) of the
E2 mechanism rather than of carbocationic transition
state (E1), the major product of 1,2-dibromo-1-phenyl-
ethane dehydrobromination under PTC conditions is
-bromostyrene.
Table 2. Yields of 1,2-dibromo-1-phenylethane dehydro-
bromination products (%) in the presence of cetyltri-
methylammonium bromide (I) as phase-transfer catalyst
in various solvents
-Bromo-
styrene
-Bromo-
styrene
Phenyl-
acetylene
Solvent
l/l
0.6
l/s
l/l
l/s
l/l
l/s
Toluenea
Toluene
DMSOa
DMSO
0.8
0.1
2.1
67.8 73.9
3.5
0.4
0.8
56.7 73.6 34.2 21.7
96.7 97.8 2.9 2.0 Traces Traces
Pyridinea 56.3 72.9 37.3 24.3
Pyridine
91.0 93.1
5.3
5.9
0.1
0.7
a
In the absence of phase-transfer catalyst.
Probably, in systems containing an aprotic dipolar
solvent and a strong organic base, substrates having
a highly labile hydrogen atom are capable of giving
rise to pure E1cb mechanism. In this case, inter-
mediate carbanion and lipophilic cation of the phase-
transfer catalyst form an ion pair. However, aprotic
dipolar solvents are extremely rarely used in phase-
transfer reactions, for they are expensive, toxic, and
difficult to regenerate. There are no reasons to assume
pure E1cb mechanism in classical PTC systems based
on alkali metal hydroxides and weakly polar organic
solvents like toluene. The role of lipophilic cation Q+
in the stabilization and evolution of a carbanionic
E1cb-like transition state may be illustrated by
Scheme 4.
We can conclude that the most appropriate sub-
strates for elimination in two-phase systems with
a lipophilic cation Q+ are those having a hydrogen
atom activated by electron-acceptor substituent or
located in the benzylic position. This conclusion is
consistent with published data [1].
Scheme 4.
Replacement of 1,2-dibromo-1-phenylethane by
1,2-dichloro-1-phenylethane in l/l and l/s two-phase
systems containing KOH, toluene, and phase-transfer
catalyst leads to increased yield of -chlorostyrene as
compared to -bromostyrene (Table 3). Presumably,
the -hydrogen atom in the chlorine-containing sub-
strate is more labile due to stronger I effect of
chlorine as compared to bromine. This leads to in-
creased carbanionic character of E1cb-like transition
state in the E2 mechanism. Although chloride ion is
inferior to Br in nucleofugality, the rate of elimina-
tion increases and the yield of -chlorostyrene is
lower than the yield of -bromostyrene (Table 3).
The general scheme of stabilization of carbanionic
transition state by lipophilic cation is shown below:
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003

960
SUVOROVA et al.
Table 3. Yields (%) of -halostyrenes, -halostyrenes, and phenylacetylene from 1,2-dibromo-1-phenylethane and
1,2-dichloro-1-phenylethane
-Halostyrene
-Halostyrene
Phenylacetylene
Substrate
Catalysta
l/l
l/s
l/l
l/s
l/l
l/s
1,2-Dibromo-1-phenylethane
1,2-Dichloro-1-phenylethane
I
II
I
67.8
58.8
93.0
92.0
73.9
83.3
93.4
96.9
2.1
2.9
0.9
1.0
3.5
3.7
1.3
1.3
0.4
0.4
0.5
0.5
0.8
1.2
1.8
1.5
II
a
Phase-transfer catalysts: cetyltrimethylammonium bromide (I) and tetrabutylammonium bromide (II).
According to this scheme, electron density transfer
of such additives, e.g., alcohols, favor stabilization
of E1cb-like transition state in the E2 mechanism.
Interaction of alcohol molecule with bromine atoms in
1,2-dibromo-1-phenylethane enhances their acceptor
properties and I effect without essential heterolysis
of the C Br bond in a nonpolar solvent (toluene).
This leads to increased mobility of the -hydrogen
atom and favors formation of a carbanionic E1cb-like
transition state. Moreover, activation of -bromo-
styrene with alcohol facilitates its dehydrobromination
to phenylacetylene. The data in Table 4 characterize
the effect of some lipophilic alcohols, namely
2-phenyl-2-propanol (III), 2-methyl-2-propanol (IV),
and 2-methyl-2-butanol (V), on the yield of elimina-
tion products from 1,2-dibromo-1-phenylethane in l/l
and l/s systems containing KOH, cetyltrimethylam-
monium bromide (I), and toluene as solvent. It is seen
that in the absence of phase-transfer catalyst alcohols
exhibit no catalytic effect and that in the presence
of phase-transfer catalyst all the examined alcohols
accelerate transformation of the substrate into
-bromostyrene and its further dehydrobromination to
phenylacetylene.
in the transition state is accompanied by movement of
the cation Q+ toward the site with maximal localiza-
tion of the negative charge. Thus, in the initial inter-
action between the ion pair Q+ OH with substrate
the cation Q+ acts as a carrier which transfers the base
to the substrate; its further movement toward the
halogen atom X facilitates nucleofuge departure as
Q+ X ion pair.
Elimination of hydrogen under phase-transfer
catalysis can be facilitated by addition of phenols,
alcohols, or carboxylic acid salts [9]. If the departing
group X is an anion, the promoting additive favors
heterolytic dissociation of the C X bond via hydro-
gen bonding and transition from the E2 mechanism
to E1 [7]. However, we believe that catalytic amounts
Figure shows typical kinetic curves for accumula-
tion of elimination products in l/l and l/s systems
containing tetrabutylammonium bromide (II) and
2-methyl-2-butanol (V). Selective transformation of
the substrate into -bromostyrene occurs at a very
high rate (within 1 min in l/s system), and -bromo-
styrene is then converted into phenylacetylene. The
l/s system is more active, and the yield of phenyl-
acetylene reaches 85% in 1 h.
Thus, study of the dehydrobromination of 1,2-di-
bromo-1-phenylethane under conditions of phase-
transfer catalysis showed that lipophilic cation Q+
of the catalyst stabilizes E1cb-like transition state
in the E2 mechanism. Elimination of hydrogen atom
is favored by addition of catalytic amounts of lipo-
philic alcohols which enhance acceptor power of the
halogen atoms via solvation.
Kinetic curves for formation of (1, 1 ) -bromostyrene,
(2, 2 ) -bromostyrene, and (3, 3 ) phenylacetylene in two-
phase systems (1 3) liquid liquid and (1 3 ) liquid solid.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003

MECHANISM OF DEHYDROBROMINATION OF 1,2-DIBROMO-1-PHENYLETHANE
961
Table 4. Yields (%) of 1,2-dibromo-1-phenylethane dehydrobromination products in the presence of alcohols
-Bromostyrene
-Bromostyrene
Phenylacetylene
Phase-transfer
catalyst
Alcohol
l/l
l/s
l/l
l/s
l/l
l/s
2-Phenyl-2-propanol (III)
3.9
67.9
95.6
94.9
81.3
2.4
73.9
82.7
77.0
77.9
2.1
3.5
21.7
3.6
I
I
I
I
2.1
34.2
3.2
0.4
0.8
2-Phenyl-2-propanol (III)
2-Methyl-2-propanol (IV)
2-Methyl-2-butanol (V)
0.5
0.5
11.8
13.6
2.9
2.8
EXPERIMENTAL
alcohol and 10 ml of a 50% aqueous solution of
KOH. The reaction time was 8 min.
The reaction mixtures were analyzed by GLC on
a Kristall-2000M chromatograph equipped with
a flame-ionization detector (SE-30 quartz capillary
column, 22 m 0.32 mm; isothermal mode, oven
temperature 170 C; injector and detector temperature
200 C; split ratio 1:20; time 7 min; carrier gas argon,
inlet pressure 5 atm).
In the study of the kinetics of dehydrobromination,
6
6
8 10 mol of catalyst II and 8 10 mol of alcohol
V were added to a solution (4 ml) of 1,2-dibromo-
1-phenylethane in toluene (0.8 M), and 4 ml of
a 50% aqueous solution of KOH was added.
In all experiments, the reaction temperature was
80 C, stirring speed 2100 rpm.
The dehydrohalogenation was performed in a glass
reactor equipped with a jacket (to maintain a constant
temperature) and a reflux condenser. The reaction
mixture was vigorously stirred with a magnetic stirrer.
Homogeneous dehydrobromination of 1,2-di-
bromo-1-phenylethane. Metallic potassium,
0.005 mol (0.195 g), was dissolved in 10 ml of tert-
butyl alcohol at room temperature. The resulting solu-
tion of potassium tert-butoxide in tert-butyl alcohol
was added to a solution of 0.005 mol (1.32 g) of
1,2-dibromo-1-phenylethane in 10 ml of tert-butyl
alcohol, heated to 80 C. The mixture was heated for
8 min at 80 C at a stirring speed of 2100 rpm.
b. A solution (4 ml) of 1,2-dibromo-1-phenyl-
ethane or 1,2-dichloro-1-phenylethane (c = 0.8 M) in
appropriate solvent (toluene, DMSO, or pyridine) was
6
mixed with 4 10 mol of catalyst I or II, and 2.5
2
10 mol (1.4256 g) of finely powdered (and screened
through a sieve; grain size 400 m) solid KOH and
3.9 10 4 mol (7 10 3 ml) of water were added.
While studying the effect of catalytic amounts of
alcohols, the substrate (1,2-dibromo-1-phenylethane)
solution in toluene (0.8 M) was adjusted to a volume
of 10 ml, and the amount of catalyst I was increased
5
to 1 10 mol. To the resulting mixture we added
5
2
1 10 mol of appropriate alcohol, 6.4 10 mol
3
2
Catalytic dehydrohalogenation of 1,2-dihalo-
1-phenylethanes in two-phase systems. a. Liquid
liquid. To 4 ml of a solution of 1,2-dibromo-1-
phenylethane or 1,2-dichloro-1-phenylethane at a con-
centration of 0.8 M in appropriate solvent (toluene,
(3.654 g) of KOH, and 1 10 mol (1.8 10 ml)
of water. The reaction time was 8 min, and the
process was complete in 10 min.
In the study of the kinetics of dehydrobromination,
4 ml of a 0.8 M solution of 1,2-dibromo-1-phenyl-
6
6
DMSO, or pyridine) we added 4 10 mol of phase-
ethane in toluene was mixed with 8 10 mol of
6
transfer catalyst, cetyltrimethylammonium bromide (I)
catalyst II and 8 10 mol of alcohol V. Finely
3
(1.5 10 g) or tetrabutylammonium bromide (II)
powdered (and screened through a sieve; grain size
3
2
(1.3 10 g); 4 ml of a 50% aqueous solution of
400 m) solid KOH, 2.5 10 mol (1.4256 g), and
3.9 10 4 mol (7 10 3 ml) of water were added.
KOH (13.45 M) was then added (the volume ratio of
the aqueous and organic phases was 1:1).
In all experiments, the reaction temperature was
70 C, and the speed of stirring, 1500 rpm.
While studying the effect of addition of alcohols,
the volume of the substrate (1,2-dibromo-1-phenyl-
ethane) solution in toluene (0.8 M) was increased to
10 ml, and the amount of cetyltrimethylammonium
bromide, to 1 10 5 mol (3.6 10 3 g); to the resulting
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