Cocatalysis in phase-transfer catalyzed base induced β-elimination. Part 2: Model studies of dehydrobromination of trans-β-bromostyrene

Article abstract of DOI:10.1016/S0040-4020(02)00762-7

Phase-transfer catalyzed β-elimination of HBr from trans-β-bromostyrene proceeds as a cocatalytic process when cocatalysts of low acidity such as n-butanol and highly concentrated aqueous NaOH at elevated temperatures are used. Without added cocatalyst the reaction is autocatalyzed because the produced phenylacetylene forms lipophilic acetylenide anion acting as a base in the organic phase. Competition between the β-elimination of HBr from trans-β-bromostyrene and the Hofmann degradation of tetrabutylammonium cation as a function of base was studied.

Full text of DOI:10.1016/S0040-4020(02)00762-7

Tetrahedron 58 (2002) 7295–7301
Cocatalysis in phase-transfer catalyzed base induced
b-elimination. Part 2: Model studies of dehydrobromination of
trans-b-bromostyrene
*
Mieczysław Ma˛kosza and Alexey A. Chesnokov
Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw 42, POB 58, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland
Received 1 May 2002; revised 31 May 2002; accepted 27 June 2002
Abstract—Phase-transfer catalyzed b-elimination of HBr from trans-b-bromostyrene proceeds as a cocatalytic process when cocatalysts of
low acidity such as n-butanol and highly concentrated aqueous NaOH at elevated temperatures are used. Without added cocatalyst the
reaction is autocatalyzed because the produced phenylacetylene forms lipophilic acetylenide anion acting as a base in the organic phase.
Competition between the b-elimination of HBr from trans-b-bromostyrene and the Hofmann degradation of tetrabutylammonium cation as a
function of base was studied. q 2002 Elsevier Science Ltd. All rights reserved.
In our earlier paper1 a general concept of cocatalysis in
phase-transfer catalyzed (PTC) base induced b-elimination
of HX from alkyl halides was formulated. It explained
numerous early observations that alcohols facilitate this
process.2 Recently we reported results of our detailed
studies of b-elimination of HBr from cyclohexyl bromide
which proceeds efficiently under phase-transfer catalysis
conditions in the presence of a proper cocatalyst Y–H
according to the general mechanism presented in Eqs. (1a)
and (1b).3
phase whereas Y2 anions still were sufficiently active basic
agents. These results can be used as guidelines for selection
of cocatalytic systems assuring efficient b-elimination from
a variety of alkyl halides.
This methodology was however tested using cyclohexyl
bromide as a model alkyl halide, which can be easily
dehydrohalogenated to cyclohexene under relatively mild
conditions. It is therefore necessary to clarify whether this
concept can be expanded for haloalkanes which undergo
dehydrohalogenation only under harsh conditions.
Y–HðorgÞ þ NaþOHð2aqÞ þ QþXð2orgÞ Y QþY2ðorgÞ þ NaþXð2aqÞ
In this paper we report results of our studies on application
of the cocatalysis methodology for PTC dehydrobromina-
tion of a model vinyl bromide—trans-b-bromostyrene 1
which is known to be rather resistant towards base induced
b-elimination. For instance, an Organic Syntheses pro-
cedure for the synthesis of phenylacetylene consists in
heating of 1 with KOH in ethylene glycol at 2008C to give
the product in 67% yield.4 Application of PTC for synthesis
of acetylenes via base induced b-elimination was reported
by Dehmlow who has shown that dehydrohalogenation of
1,2-dihalides to acetylenes proceeds satisfactorily in liquid–
solid system using KOH and highly lipophilic tetraoctyl-
ammonium bromide as PT catalyst.5 He has also shown that
in some cases 50% aq. KOH in the presence of
tetraoctylammonium chloride and some diols gave similar
results.2a It was shown that use of less lipophilic TAA salts
as PT catalysts in these reactions was not successful.6,7
þ H2OðaqÞ
ð1aÞ
ð1bÞ
We have examined a series of potential cocatalysts Y–H:
O–H and N–H acids such as alcohols, phenols, amides, etc.
and found that their cocatalytic activity is a function of two
parameters: concentration of the reacting anions Y2 in the
organic phase governed by the acid–base equilibrium (Eq.
(1a)) and activity of Y2 as basic reagents affording b-
elimination (Eq. (1b)). The optimal cocatalytic activity was
observed for Y–H of moderate acidity so there was
relatively high concentration of Y2Qþ in the organic
It should be mentioned that dehydrobromination of 1 proceeds
efficiently in the presence of concentrated aqueous NaOH and
an equimolar amount of tetrabutylammonium hydrogen-
sulfate—the so called ion-pair extraction technique.8
Keywords: phase-transfer catalysis; cocatalysis; b-elimination; Hofmann
degradation.
*
Corresponding author. Tel.: þ48-22-631-87-88; fax: þ48-22-632-66-8;
e-mail: icho-s@icho.edu.pl
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00762-7

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Table 1. b-Elimination of HBr from trans-b-bromostyrene: the cocatalytic and simple PTC systems
Entry
QþBr2
None
Aq. NaOH (wt%)
Y–H (pKa)a
None
None
n-BuOH (<29.8)
PhCH2OH (27.0)
Mesitol (<18.5)
None
None
None
None
None
n-BuOH
Conversion of 1 (%)b
Degradation of Qþ (%)b
1
2
3
4
5
6
7
8
50
50
50
50
50
50
50
50
50
70
70
0
–
n-Bu4NþBr2
n-Bu4NþBr2
n-Bu4NþBr2
n-Bu4NþBr2
n-Pr4NþBr2
Et4NþBr2
64
70
61
10
60
6
0
<10
100
100
c
–
–
–
–
c
Me4NþBr2
7
c
c
9
10
11
n-Oct3NþMeBr2
n-Bu4NþBr2
n-Bu4NþBr2
80
40
75
0
<10
a
The pKa values (in DMSO) according to Bordwell9 are given in brackets unless noted otherwise. Sign ‘<’ denotes that presented pKa value refers to a close
analogue of the corresponding Y–H.
Amount of tributylamine was determined by GLC using chlorobenzene as an internal standard.
Not determined.
b
c
1. Results and discussion
BuO2Qþ. At the high temperatures used in our experiments
solubility of NaOH in water is much higher so some
experiments were repeated using 70% aq. NaOH. Under
such conditions basic activity of OH2 anions are much
higher so deprotonation of n-BuOH is more efficient and
cocatalytic reaction in the presence of n-BuOH proceeded
substantially faster than simple PTC: 40 and 75%,
respectively, conversion after 1 h (Table 1, Entries 10 and
11). Thus under properly selected conditions, a cocatalytic
process operates also for dehydrobromination of trans-b-
bromostyrene.
The conditions elaborated for efficient cocatalytic b-
elimination of HBr from cyclohexyl bromide (excess of
50% aq. NaOH, 5 mol% of n-Bu4NþBr2, 5 mol% of Y–H,
408C) were too mild for the b-elimination of HBr from 1.
The process was very slow, even after 24 h conversion did
not exceed 5%. Since under the conditions employed for the
cocatalytic b-elimination of HBr from cyclohexyl bromide,
1 was practically unreactive all further experiments were
conducted at 908C using chlorobenzene both as a solvent
and as an internal standard for GLC measurements. Results
of the experiments are given in Table 1.
On the basis of these results, the following general picture
can be proposed: in spite of the very unfavorable ion-
exchange equilibrium (Eq. (2a)) classical PTC operates at
high temperatures. It appears that the rate constant of the
elimination reaction with very active OH2 anions (Eq. (2b))
is so high that the b-elimination proceeds with reasonable
observed rate in spite of negligible concentration of QþOH2
in the organic phase.
Under the conditions specified in Table 1, PTC dehydroha-
logenation of 1 proceeded with reasonable rate. As can be
seen from these results, without PT catalyst no reaction was
observed during 12 h of stirring at 908C (Table 1, Entry 1)
whereas in the presence of tetrabutylammonium bromide
(TBAB) under the same conditions conversion of 1 attained
64% (Table 1, Entry 2). Addition of n-BuOH as a cocatalyst
resulted in a small increase of the conversion (Table 1, Entry
3), whereas more acidic cocatalysts PhCH2OH and mesitol
exert small or substantial inhibitory action (Table 1, Entries
4 and 5). It should be stressed that these compounds were
efficient cocatalysts in PTC dehydrobromination of cyclo-
hexyl bromide, more efficient than n-BuOH. Simple PTC b-
elimination (without cocatalysts), in spite of the rather harsh
conditions and long reaction time, was not accompanied
with the Hofmann degradation of n-Bu4Nþ cation—no n-
Bu3N was detected in the reaction mixture (Table 1, Entry
2). Such degradation proceeded to a small degree when n-
BuOH was used as the cocatalyst, whereas in the presence
of more acidic cocatalysts PhCH2OH and mesitol there was
complete degradation of n-Bu4Nþ as indicated by the
amount of n-Bu3N determined in the reaction mixture.
QþBr2ðorgÞ þ NaþOHð2aqÞCQþOHð2orgÞ þ NaþBrð2aqÞ
ð2aÞ
PhCHvCHBrðorgÞ þ QþOHð2orgÞ ! PhCuCHðorgÞ þ QþBrð2orgÞ
þ H2O
ð2bÞ
It should be stressed that the simple PTC as well as
cocatalytic PTC dehydrobromination of trans-b-bromostyr-
ene proceeds under very mild conditions in comparison with
those recommended in Organic Syntheses.4 It is a rather
peculiar observation that ‘simple’ PTC b-elimination
proceeds efficiently in spite of the large concentration of
bromide anions in the system generated during the reaction,
which practically preclude extraction of QþOH2 into the
organic phase. Thus there is an important question in which
way PTC operates in this reaction. There is no doubt that the
b-elimination is PT catalyzed since in the absence of TBAB
there are not even traces of phenylacetylene in the organic
The weak cocatalytic action of n-BuOH is connected with
its low acidity hence in the presence of 50% aq. NaOH only
a small part of n-BuOH is converted into reactive n-

M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 58 (2002) 7295–7301
7297
Table 2. b-Elimination of HBr from trans-b-bromostyrene and cyclohexyl bromide under simple PTC conditions at 908C
Entry
Educts
Aq. NaOH (wt%)
Time (h)
Conversion of 1 (%)a
Conversion of 2 (%)a
1
2
1
2
3
4
5b
6b
7b
8b
9c
þ
2
þ
2
þ
þ
þ
þ
2
2
þ
2
þ
þ
þ
þ
þ
þ
50
50
70
70
50
50
70
70
70
12
12
1
1
3
12
1
3
64
–
40
–
23
36
28
50
–
–
52
–
16
25
55
32
65
27
1
a
Determined by GLC using chlorobenzene as an internal standard.
1:1 Molar mixture of the halides 1 and 2 was used.
PhCuCH (5 mol%) was added to the reaction mixture before the experiment.
b
c
phase whereas with this TAA salt conversion is up to 100%
for a certain time of the reaction. Since in this system
concentration of OH2 anions in the organic phase is really
negligible the catalytic effect of TBAB could eventually be
rationalized in two ways:
which is known to be much more susceptible to b-
elimination than 1. When cyclohexyl bromide 2 was
subjected to PTC b-elimination under analogous conditions
as 1 the elimination took place to a similar extent (Table 2,
Entries 1 and 2). Surprisingly under these conditions the
reaction with 2 proceeded somewhat slower than with 1.
Comparison of the conversion degree of 1 and 2 when 70%
aq. NaOH was used revealed that under such conditions 1
reacted much faster than 2—conversion 40 and 16%
correspondingly (Table 2, Entries 3 and 4). These surprising
and peculiar results, namely faster reaction of otherwise
much less active compound should be rationalized, because
one can expect that explanation of this phenomenon should
help understanding how the PTC reaction operates in these
situations. For direct comparison of rates of PTC b-
elimination of 1 and 2 we performed competitive exper-
iments in which mixtures of equimolar quantities of these
halides were subjected to the PTC b-elimination in the
presence of 50 and 70% aq. NaOH (Table 2, Entries 5, 6 and
7, 8). In all these experiments cyclohexyl bromide 2 reacted
somewhat faster than 1. The apparent discrepancy between
the results shown in Table 2 (Entries 1, 2 and 3, 4) in which
the single halides were subjected to the reaction, with those
competitive in which equimolar mixtures of 1 and 2 were
used, can be reasonably rationalized taking into account that
the product of the b-elimination from 1—phenylacetylene is
a moderately acidic C–H acid, pKa¼28.7 (DMSO).9 When
produced in the system it undergoes deprotonation at the
interface and the generated carbanion of phenylacetylene
forms, with the TAA cation, a lipophilic ion pair entering
the organic phase. The carbanion derived from weak C–H
acid exhibits high basicity and affect fast b-elimination of
HBr from 1 and 2. So in fact the reactions with 1 proceed as
cocatalytic PTC processes. On the other hand, during PTC
b-elimination of HBr from 2 there is no cocatalyst to
increase the rate of the elimination, thus it proceeds slower
than analogous process with 1 for which the cocatalysis,
being actually autocatalysis, operates. This reasoning is
supported by the observation that this cocatalytic effect is
much stronger when more concentrated NaOH is used,
thus weak C–H acids such as phenylacetylene can be
1. In spite of very low concentration of QþOH2 in the
organic phase, thanks to a very high rate constant of the
reaction with highly active OH2 anions, particularly at
elevated temperature, the process proceeds with reason-
able observed rate.
2. The reaction proceeds at the interfacial region, in which
there is a gradient of concentration and concentration of
OH2 in the form of QþOH2 in this region substantially
exceeds those governed by the ion-exchange equilibrium
(Eq. (2a)).
In order to verify the latter hypothesis less lipophilic TAA
salts were used as the catalysts in supposition that the
interfacial processes should be less sensitive to the
lipophilicity of TAA cations. The experiments in which n-
Pr4NþBr2, Et4NþBr2 and Me4NþBr2 were used as PT
catalysts indicated, that the first salt was almost as efficient
as TBAB whereas the two latter salts did not show marked
activity (Table 1, Entries 6–8). Perhaps low effectiveness of
the latter is due to their insufficient solubility in the organic
phase. On the other hand use of the much more lipophilic
salt n-Oct3NþMeBr2 gave substantial improvement over
TBAB (Table 1, Entry 9). It shall be therefore accepted for
PT catalyzed b-elimination of HBr from trans-b-bromo-
styrene in the presence of concentrated aqueous NaOH and
TAA salts at elevated temperatures that the reaction
proceeds via transfer of QþOH2 into the organic phase,
albeit to a very low extent. It should be noted that in the
vigorously stirred system the concentration of QþOH2 in
the organic phase can exceed those measured in the
equilibrated static systems.10 In spite of very low concen-
tration, high activity of these anions at elevated tempera-
tures assures a reasonable reaction rate. This conclusion is
supported by control experiments with cyclohexyl bromide,

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M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 58 (2002) 7295–7301
Table 3. Competitive b-elimination of HBr from 1 and 2 promoted by different TBA salts
Entry
Y2
Time
Conversion of 1 (%)a
Conversion of 2 (%)a
(a) Two-phase system (chlorobenzene/50% aq. NaOH)
1
2
3
OH2
Br2
1 minb
48 h
0.5 h
24
27
0
27
29
86
ArO2c
(b) Homogeneous system (chlorobenzene solution)
4
5
6
7
8
OH2
OH2
OH2
OH2
OH2
0.5 minb
2 minb
10 minb
3 hb
5
5
6
7
7
8
14
19
32
42
.12 hb
a
b
c
Determined by GLC using durene as an internal standard; no significant degradation of TBA cation was observed in all these experiments.
At 258C.
Ar¼2,4,6-trimethylphenyl.
deprotonated efficiently. Moreover, in separate experiments
using 70% aq. NaOH we were able to observe a
considerable cocatalytic action of PhuCH in the b-
elimination of HBr from cyclohexyl bromide (Table 2,
Entries 4 and 9).
various conditions (Table 3). These results indicate that the
reaction progress is limited by availability of the highly
active basic agent in the organic phase because there is no
discrimination between 1 and 2 (Table 3, Entries 1 and 2),
although the reaction progress in the presence of TBAB is
relatively slow due to the very low concentration of OH2 in
the organic phase. On the other hand rates of b-elimination
of these halides with a mild basic agent (e.g. ArO2) differ
tremendously, the reaction of 2 is much faster. The
experiments (Table 3, Entries 1–3) were carried out in the
presence of an excess of 50% aq. NaOH in order to remove
water or ArOH generated during the reaction. The same
competitive experiments carried out with QþOH2 in a
chlorobenzene solution in the absence of 50% aq. NaOH
(Table 3, Entries 4–8) revealed that the competition
depends on the degree of conversion. Obviously OH2
anions hydrated by water generated in the reaction are much
weaker bases11 so the reaction with 2 is preferred.
An almost identical degree of b-elimination of HBr from 1
and 2 under simple PTC conditions in spite of much higher
sensitivity of 2 to the action of base (see e.g. Table 2, Entry
5) indicates that in both these cases the reaction is promoted
by the highly active reagent being in very low concen-
tration, apparently QþOH2 and PhCuC2Qþ and the
conversion is limited not by the rate constant of the
chemical reaction but by availability of OH2 or other
strongly basic anions in the organic phase, thus in fact
diffusion controlled. This supposition is supported by the
results of competitive experiments in which equimolar
quantities of 1 and 2 were subjected to b-elimination under
Figure 1. Conversion of trans-b-bromostyrene 1 as a function of time for simple PT system (5 mol% TBAB) and cocatalytic PT system (5 mol%
TBABþ5 mol% PhCH2OH).

M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 58 (2002) 7295–7301
7299
Scheme 1.
Another interesting issue is that during PTC dehydrohalo-
genation of 1 at high temperature, which proceeds
apparently via transfer of OH2 and PhCuC2 into the
organic phase, the Hofmann degradation of the TBA cation
is not observed. On the other hand in the presence of
cocatalysts n-BuOH, PhCH2OH and mesitol the degradation
proceeded to a small extent (n-BuOH) or completely
(PhCH2OH and mesitol). In fact, the absence of a
cocatalytic effect of PhCH2OH (Table 1, Entry 4) in the
b-elimination is an artifact. This alcohol acts as an efficient
cocatalyst for the b-elimination which indicates high
conversion after a short time whereas after 5 h the PTC
process is arrested due to total degradation of TBAB. This
can be seen in Fig. 1, where conversion of 1 is shown as a
function of time. Formation of phenylacetylene is acceler-
ated in the presence of PhCH2OH at the beginning of the
process, so there is an efficient cocatalytic b-elimination
however accompanied by Hofmann degradation, after that it
stops because all the TBA cation is degraded, whereas
without this cocatalyst the PTC reaction, although slower at
the beginning, is not arrested even after a long reaction time.
Deceleration of the process with time is due to consumption
of the educts and accumulation of Br2 in the aqueous phase
so concentration of OH2 and PhCuC2 anions in the
organic phase decreases continuously.
PhCH2O2 in chlorobenzene was heated at 908C for 5 h in a
tightly closed flask H NMR analysis of the crude reaction
1
mixture indicated that it contained n-Bu3N ((PrCH2)3N d
2.48, t), PhCH2OH (d 4.76, s) and –CH2–CHvCH2 (d
2.15, 2H, m; 5.95, 1H, m; 5.05, 2H, m) in an approximate
ratio of 1:1:0.5 and traces of unreacted n-Bu4Nþ, whereas
characteristic signals for benzyl n-butyl ether PhCH2OCH2-
Pr (d 4.6, s; d 3.5, t) were absent. On the other hand an
equimolar mixture of benzyl butyl ether and tributylamine
under the same conditions did not give rise to benzyl alcohol
and butene. Furthermore, no traces of the corresponding
ArOCH2Pr ethers were found in the reaction mixtures of the
competitive experiments with TBA ArO2 (Table 4, Entries
4 and 5). These observations allowed us to exclude the SN2
reaction pathway for the degradation of TBA cation in the
cocatalytic PTC system and confirmed that classic Hofmann
elimination operates in this case.
Additionally a very interesting observation was made that
relation of rates of the competing reactions: b-elimination
of HBr from 1 and n-Bu3N from the TBA cation (the
Hofmann degradation) depends very much on the strength
of the basic agent (Table 4). 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. Such a
behavior suggests a principal difference between the
mechanisms of dehydrobromination of trans-b-bromostyr-
ene and the Hofmann degradation of tetrabutylammonium
cation. Whereas the Hofmann degradation of the TBA
cation most likely follows a one-step E2 mechanism,12 in
the case of trans-b-bromostyrene, for which the E2-type
mechanism seems to be unlikely due to the geometry of the
molecule, two-step E1cB mechanism might operate. That is
Since the degradation of the tetrabutylammonium cation
was monitored via the determination of tributylamine, it
should be clarified whether it is indeed produced due to
Hofmann elimination or eventually SN2 substitution which
also could produce tributylamine (Scheme 1).12 Taking into
account the observed preference for degradation by anions
of low basicity such as PhCH2O2 and ArO2, the SN2 type
reaction appeared feasible. When a solution of n-Bu4Nþ-
Table 4. b-Elimination of 1 versus Hofmann degradation of TBA cation promoted by different TBA salts at 908C
Entry
RO2
pKa of ROH
(DMSO)a
Time
Ratio PhCuCH/n-Bu3Nb
1
2
3
4
HO2
31.2
<29.8
27.0
0.5 min
0.5 min
2 min
23.8
10.0
3.9
n-BuO2
PhCH2O2
ArO2c
<18.5
60 min
0.6
a
According to Bordwell.9
b
c
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.

7300
M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 58 (2002) 7295–7301
supported by the earlier observation that in NaOH/i-PrOH
medium trans-b-bromostyrene was found to be 105 times
less reactive than the cis-isomer for which an E2-type
mechanism is feasible.13 Moreover, introduction of a
strongly electron withdrawing 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.13 This suggests that carbanionic
species are involved in the rate determining step of the base
induced dehydrobromination of the trans-isomer. Taking
into account that 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 crucial for dehydrobromination of 1 rather than for the
Hofmann degradation of TBA cation, which, if follows
concerted E2 mechanism, does not require generation of
carbanion. This phenomenon is a subject of further
investigation.
separated organic layer was washed with water (3£5 mL),
dried over MgSO4 and analyzed by GLC using chloroben-
zene, present in the mixture, as an internal standard. When
the time profile of the reaction had to be established (see
Fig. 1), small samples of the corresponding reaction
mixtures (Entries 2 and 4) were periodically taken and
analyzed by GLC after dilution with CH2Cl2 and washing
with water.
2.2. Preparation of chlorobenzene solution of TBA salts
(Tables 3 and 4)
Chlorobenzene solutions of n-Bu4NþRO2 salts (R¼H; n-
Bu; PhCH2; Ph and 2,4,6-trimethylphenyl) were prepared
according to the ion-pair extraction procedure14 from an
equimolar mixture of TBA hydrogensulfate, ROH (R–H)
and excess of 50% aq. NaOH. Thus to obtain 2.5 mL of a
solution which contains <0.5 mmol of the n-Bu4NþRO2
(R–H) a mixture of TBA hydrogensulphate (0.34 g,
1 mmol), corresponding ROH (1 mmol), chlorobenzene
(5 mL) and 50% aq. NaOH (5 mL) was stirred in a small
flask for 5 min at room temperature. After short separation
of the phases (2–3 min) 2.5 mL of the clear upper organic
layer was taken by a pipette. A solution of TBA hydroxide
(10 mL; <2.5 mmol) was obtained in a similar way: a
mixture of TBA hydrogensulphate (1.50 g, 4.4 mmol),
chlorobenzene (15 mL) and 50% aq. NaOH (10 mL) was
stirred for 5 min at room temperature and treated as above.
Concentrations of RO2 in the prepared chlorobenzene
solutions were determined by titration with 0.5 M HCl
(bromophenol blue). Under these conditions the content of
RO2 anions in the organic phase was <100% (R–H) and
<90% for OH2 anion in regard to the total amount of Qþ in
the system. The solutions of TBA hydroxide and alkoxides
in chlorobenzene are unstable and can be stored only for a
short time at low temperature.
We can conclude that the cocatalytic process operates also
for the PTC b-elimination requiring harsh conditions,
although at elevated temperature simple PTC b-elimination
proceeds satisfactorily. The Hofmann degradation of
tetrabutylammonium cation takes place much slower than
b-elimination of HBr from 1 under the simple PTC
conditions, whereas use of cocatalysts of moderate acidity
promotes the degradation.
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 RT) followed by distillation of
the crude reaction mixture (1/PhCuCH<85/15) under
reduced pressure. It was stored in a refrigerator at 0–48C.
The analyses of the reaction mixtures were performed using
‘Shimadzu GC-14A’ gas chromatograph; injection port
2.3. Competitive b-elimination of HBr from 1 and 2
promoted by different TBA salts in the two-phase system
(Table 3(a))
10 mL of a freshly prepared solution of the corresponding
TBA salt (2.5 mmol) in chlorobenzene was added under
vigorous stirring to the thermostated at given temperature
mixture of the bromides 1 and 2 (0.915 and 0.815 g, 5 mmol
each), durene (<0.3 g) and 50% aq. NaOH (10 mL). The
mixture was stirred for a given time. A sample of the
organic phase was washed with equal volume of saturated
aqueous NaI in order to remove any residual basic species,
dried with Na2SO4 and analyzed by GLC using durene as an
internal standard.
1
temperature 1208C; injection time 1 min. H NMR spectra
were recorded on Varian Gemini (200 MHz) spectrometer.
All the experiments were carried out using a magnetic stirrer
equipped with a temperature-controlled oil bath.
2.1. General procedure for b-elimination of HBr from
trans-b-bromostyrene (1) or cyclohexyl bromide (2)
under phase-transfer catalysis conditions at 908C (Tables
1 and 2)
2.4. Competitive b-elimination of HBr from 1 and 2
promoted by TBA hydroxide in a homogeneous system
(Table 3(b))
A mixture of the bromide(s) (20 mmol; 3.66 g of 1 and/or
3.26 g of 2), chlorobenzene (2.00 g), and the corresponding
Y–H (1 mmol) (when indicated) was added to a 25 mL flask
charged with concentrated aqueous solution of NaOH
(<200 mmol; 10 mL of 50% aq. NaOH or 70% aq. NaOH
prepared from 5 mL of 50% aq. NaOH and 5.1 g of solid
NaOH) thermostated at 908C. Then powdered QþBr2
(1 mmol) was added, the flask was tightly closed with a
stopper using a metallic clip. The reaction mixture was
stirred for a given time at 90^18C. The flask was cooled,
CH2Cl2 (25 mL) was added to the reaction mixture and the
Ten milliliters of a freshly prepared solution of TBA
hydroxide (2.5 mmol) in chlorobenzene was rapidly added
under vigorous stirring to the thermostated at 258C mixture
of the bromides 1 and 2 (0.915 and 0.815 g, 5 mmol each)
and durene (<0.3 g). After certain time intervals small
samples of the organic phase were taken, rapidly mixed with
saturated aqueous NaI (<1:1 v/v) and vigorously shaken.

M. Ma˛kosza, A. A. Chesnokov / Tetrahedron 58 (2002) 7295–7301
7301
Tetrahedron 1986, 42, 3569–3574. (b) Shavanov, S. S.;
Tolstikov, G. A.; Shutienkova, T. V.; Viktorov, G. A. Zh.
Obsch. Khim. 1987, 57(7), 1587–1594. (c) Shavanov, S. S.;
Tolstikov, G. A.; Shutienkova, T. V.; Riabova, N. A. Zh. Org.
Khim. 1989, 25(9), 1867–1875. (d) Ido, T.; Matsuura, Y.;
Goto, S. Kagaku Kogaku Ronbunshu 1988, 2, 174–181;
Chem. Abstr., 1988, 109, 72972.
After separation from the aqueous phase the samples were
dried with Na2SO4 and analyzed by GLC using durene as an
internal standard.
2.5. Competitive b-elimination of HBr from 1 and
Hofmann degradation of TBA cation promoted by
different TBA salts at 908C (Table 4)
3. Ma˛kosza, M.; Chesnokov, A. A. Tetrahedron 2000, 56,
3553–3558.
Freshly prepared chlorobenzene solution of the correspond-
ing TBA salt (2.5 mL, 0.5 mmol) was rapidly added with
vigorous stirring to the thermostated at 908C 10 mL flask
charged with 1 (0.366 g, 2 mmol), durene (<0.1 g) and 50%
aq. NaOH (3 mL). The flask was tightly closed with a
stopper using a metallic clip and the mixture was stirred for
a given time at 90^18C. Samples of the organic phases were
rapidly mixed with saturated aqueous NaI (<1:1 v/v) and
vigorously shaken. After separation from the aqueous phase
the samples were dried with Na2SO4 and analyzed by GLC
4. Hessler, J. C. Organic Syntheses, Blatt, A. H., Ed.; Wiley:
New York, 1941; Collect. Vol. 1, p 438.
5. Dehmlow, E. V.; Lissel, M. Tetrahedron 1981, 37,
1653–1658.
6. Vinczer, P.; Kovacs, T.; Novak, L. OPPI 1989, 2, 232–236.
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1
and H NMR using durene as an internal standard.
9. (a) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463. (b)
Bordwell, F. G.; Liu, W. Z. J. Am. Chem. Soc. 1996, 118,
8777–8781.
Acknowledgments
10. Lasek, W.; Ma˛kosza, M. J. Phys. Org. Chem., 1993, 6(7),
412–420.
This work was partially supported by the Foundation for
Polish Science (Fundacja na Rzecz Nauki Polskiej).
11. Albanese, D.; Landini, D.; Maia, A.; Penso, M. Ind. Engng
Chem. Res. 2001, 40, 2396–2401.
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