The concerted addition of HBr to aryl alkynes; orthogonal pi bond selectivity

Article abstract of DOI:10.1039/b300045a

In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic π bond orthogonal to the extended aromatic π system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the β carbon. Attack on this π bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents.

Full text of DOI:10.1039/b300045a

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The concerted addition of HBr to aryl alkynes;
orthogonal pi bond selectivity†
Hilton M. Weiss,* Kim M. Touchette, Sarah Angell and Jihan Khan
Department of Chemistry, Bard College, Annandale-on-Hudson, New York, 12504, USA
Received 7th January 2003, Accepted 30th April 2003
First published as an Advance Article on the web 15th May 2003
In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-
Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that
the concerted addition is occurring on the acetylenic π bond orthogonal to the extended aromatic π system. The
electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote
bromide attack at the β carbon. Attack on this π bond is insensitive to the electronic effect of meta and para
substituents on the ring but is very (sterically) sensitive toward all ortho substituents.
of such behavior. Although oxygen was not excluded from
these reactions, there were no anti-Markovnikov products and
Introduction
In the preceding paper,1 we reported that the reaction of
no discoloration of the acid–bromide solutions over long
1-phenylprop-1-yne with 20% trifluoroacetic acid and bromide
periods of time. This was not surprising since trifluoroacetic
ion in methylene chloride leads to the addition of HBr by two
acid is weaker than hydrogen bromide and incapable of
different mechanisms. At low bromide ion concentration in a
highly acidic solution, the Markovnikov product, 1-bromo-1-
generating enough HBr to maintain a free radical mechanism.
Furthermore, our reaction produced only the (Z)-isomer of
phenylprop-1-ene is formed exclusively through the inter-
2-bromo-1-phenylprop-1-ene, a result inconsistent with a radi-
mediacy of a resonance stabilized cation. In less acidic
cal mechanism. However, as a further test of this possibility, we
solutions, the cationic mechanism becomes unavailable and the
ran the reaction in a solution containing di-t-butylmethyl-
bromide ion attack on an acid–alkyne complex becomes
phenol and found results identical to those that we had found in
increasingly competitive. This “concerted” Ad3 mechanism
the absence of this radical inhibitor.
produces significant amounts of the anti-Markovnikov prod-
uct, 2-bromo-1-phenylprop-1-ene (Scheme 1). This has been
2) Protonation in the phenyl ring
explained1,2 by the assumption that the transition state of
the concerted mechanism develops less positive charge in the
π system than in the cationic mechanism and, therefore, the
preference for resonance stabilization by the phenyl substituent
is diminished. At the highest concentrations of bromide ion,
however, the anti-Markovnikov product becomes dominant
and the reasons for this are less obvious. Furthermore, this
result calls into question the otherwise reasonable explanation
of why the anti-Markovnikov product has been increasing at
the lower acidities and higher concentrations of bromide ion.
We have considered five possible explanations for this behavior.
If the acid complexes with the phenyl ring, bromide attack
at the β position of the triple bond could produce an allene
intermediate that would subsequently rearrange to the anti-
Markovnikov product (Scheme 2). Computer simulation of this
mechanism was reasonable. However, the isomerization of the
allene intermediate would presumably generate both stereo-
isomers of the 2-bromo-1-phenylprop-1-ene and we were never
able to detect the (E)-isomer in these reactions. We synthesized
a mixture of all four isomers of the bromopropenes to insure
that all isomers were separable with our chromatographic
method. In addition, we further tested this mechanism by
performing the reaction using deuterated trifluoroacetic acid.
Negligible amounts of the doubly deuterated product were
found.
1) Free radical addition
Hydrogen bromide is renowned3 for its ability to add to alkynes
by a free radical mechanism leading to an anti-Markovnikov
product. However, our earlier work4 with terminal unconju-
gated alkynes in the same reaction media showed no evidence
3) Principal bonding by the bromide ion
As the reaction medium becomes less acidic, the acid
participation in the transition state decreases and the bromide
participation increases. Eventually, the bromide participation
† Dedicated to Frank H. Westheimer on the occasion of his 90th
birthday.
Scheme 1
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 5 2 – 2 1 5 6
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 Pseudo-first order rate constants and product distributions for the addition of HBr to phenylacetylenes in 0.1 M and 1.2 M bromide
solutions
1.2 M Br
Phenylacetylene
substituent
0.1 M Br k (minϪ1
)
k × 105 (minϪ1
)
% Anti-Markovnikov product
k(α) × 105 (minϪ1
)
k(β) × 105 (minϪ1
)
p-CF3
m-CF3
p-Br
p-Cl
H
p-F
p-CH3
p-C(CH3)3
0.00265
0.00506
0.597
1.33
1.40
4.25
—
0.36
4.2
3.9
3.3
10.5
35.5
60.5
20
25
—
0.27
4.1
3.8
3.3
10.3
35.5
60.5
—
0.089
0.125
0.14
0.054
0.071
0.00
0.00
3.0
3.6
1.6
0.68
0.00
0.00
39.9
72.0
Scheme 3
Results and discussion
A reasonable number of meta and para substituted phenyl-
acetylenes were commercially available and these were reacted
with the 20% trifluoroacetic acid in methylene chloride contain-
ing 0.1 M or 1.2 M bromide ions. As with the reaction of
1-phenylprop-1-yne in the 0.1 M bromide solution, all of the
substituted phenylacetylenes produced only the Markovnikov
addition products and were assumed to react through a cationic
intermediate. The pseudo-first order rate constants for these
reactions are listed in Table 1 and their logarithms are graphed
against the σϩ values in Fig. 1. The ρ value of Ϫ4.9 supports the
idea that a significant positive charge was developing at the
benzyl-type position in the transition state of the slow step.
Scheme 2
becomes dominant and a negative charge is developed in the
π system. This negative charge is best accommodated at the α
carbon where it can be delocalized into the aromatic ring. This
predominantly nucleophilic attack thus favors formation of the
anti-Markovnikov product.
AM3 calculations supported this possibility and we pro-
ceeded to test it further. A Hammett series of ring-substituted
phenylpropynes were submitted to these reaction conditions to
ascertain whether a predominantly negative charge was devel-
oping at the α position. The outcome is discussed in the results
section.
4) Steric interaction with the aromatic ꢀ cloud
Protonation of the β carbon of the triple bond should occur in
the π system which includes the aromatic electrons, thereby
allowing the greatest stabilization of the developing positive
charge. The decreasingly acidic media demands increasing
bromide participation in the transition state drawing the brom-
ide ion closer to the π electrons of the aromatic ring thereby
increasing steric repulsion and leaving bromide to attack at the
alkyne carbon β to the aromatic ring.
In trying to model this behavior with AM3 calculations, we
found that bringing the bromide ion closer to the α carbon in
the transition state causes the carbon to rehybridize toward sp2
and leads to less steric interaction with the aromatic π electrons.
Fig. 1 Logarithm of rate constants of phenylacetylenes from Table 1
versus Hammett σϩ values.
5) Bromide attack at the ꢁ carbon occurs in the plane of the
aromatic ring leading to steric interference by an ortho hydrogen.
The reaction of the substituted phenylacetylenes with 1.2 M
bromide ion was more interesting. Although 1-phenylprop-
1-yne had been found to produce almost 50% of the anti-
Markovnikov adduct at this bromide ion concentration,
(consistent with the expectations of a concerted mechanism)
phenylacetylene was found to produce little anti-Markovnikov
adduct under these conditions. This was perhaps due to the
difficulty of developing any positive charge on the terminal
carbon of the phenylacetylene. Phenylacetylenes bearing elec-
tron withdrawing substituents formed larger proportions of the
anti-Markovnikov product and this supported the idea that
predominant bonding to the bromide ion was generating a
negative charge at the α carbon. From the relative yields of the
If the proton and the bromide ion were to add synchronously,
there would be no need to involve the aromatic ring to stabilize
charge. Indeed, the alkyne π electrons which are orthogonal to
the conjugated system might be more available for attack so as
to leave the conjugated system intact leading to a lower energy
transition state. In this situation, bromide attack on the α carb-
on would occur in the plane of the ring and might be hindered
by the nearby ortho hydrogen (Scheme 3). Larger groups in the
ortho position could be more effective in blocking the acid
and/or the bromide ion. Ortho-substituted phenylpropynes
were synthesized to test this possibility (vide infra).
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 5 2 – 2 1 5 6
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Table 2 Pseudo-first order rate constants and product distributions from phenylpropynes in 0.1 M, 1.2 M and 2.0 M bromide solutions
0.1 M Br
1.2 M Br
2.0 M Br
1-Phenylprop-1-yne
substituent
via AdE2 k/
E : Z
ratio
via Ad3 k × 105/
% Anti-Markovnikov
Product
k(β) × 105/
% Anti-Markovnikov
product
minϪ1
minϪ1
minϪ1
m-CF3
p-Cl
H
p-F
p-CH3
o-Cl
0.00027
0.068
0.516
0.254
21.6
—
1.36
1.8
3.45
3.6
4.0
3.25
6.6
0.94
4.3
6.2
14
21
0.32
6.5
93
80
46
55
12a
24
<1
0.87
3.4
2.9
7.7
2.5
0.08
<0.06
100
98
86
92
60
—
—
o-CH3
8.0
a % anti-Markovnikov product from Ad3 mechanism based upon a 24% yield of the E1B product arising from the cationic AdE2 mechanism being
accompanied by 8% of the Z1B product from this AdE2 mechanism. The remaining Z1B yield was ascribed to the concerted Ad3 route.
two products, the pseudo-first order rate constants could be
split into rate constants for the formation of each product and
plotted separately against σϩ. These data are shown in Table 1
and graphed in Fig. 1. A ρ value of Ϫ2.9 was found for
the formation of the α products. A ρ value of Ϫ0.05 was found
for the formation of the β-bromostyrenes suggesting that
little if any positive (or negative) charge was developing at
the benzyl-type position in the transition state leading to the
anti-Markovnikov products.
A number of these substituted phenylacetylenes were con-
verted into phenylpropynes and reacted with the 0.1 M, 1.2 M
and 2.0 M bromide ion solutions. In the most acidic
0.1 M bromide solutions, all of the substrates still formed the
Markovnikov products consistent with a cationic mechanism.
Furthermore, the constant E : Z ratio of the Markovnikov
products with the majority of para-substituted phenylpropynes
supports our idea that the free cation was involved in all but the
most deactivated of these cases. These substituents were neither
inducing an alternate mechanism nor modifying the degree of
bromide association in their transition states. The ρ value for
the phenylpropynes in 0.1 M bromide was Ϫ5.5 also supporting
the idea of a cationic intermediate. The rate and product data
are listed in Table 2 and graphed in Fig. 2.
The data graphed in Figs. 1 and 2 make it clear that the
anti-Markovnikov products are not arising from electron with-
drawing groups promoting nucleophilic attack at the β position
but, rather, from the electron withdrawing groups discouraging
protonation at the β position, thus causing production of the
anti-Markovnikov adducts by default. Indeed, the meta and
para substituents had very little effect on the rate of formation
of the anti-Markovnikov products (ρ = Ϫ0.5). This suggests that
in the transition state of this reaction, there was little charge built
up in the conjugated π system and that the proton and bromide
were adding quite simultaneously. However, if there was mini-
mal charge development, there was little reason for the attack to
occur in the conjugated system. Attack on the isolated, orthog-
onal π bond became preferred since this allowed the conjugated
system to remain intact. Furthermore, this orthogonal π bond
was little affected by the aryl substituents and a low ρ value
should be expected. HBr addition in the plane of the aromatic
ring should be subject to significant steric effects leading to a
preference for the anti-Markovnikov product. In addition, the
small positive charge expected in this transition state should
be better stabilized by the methyl group than by the coplanar
aromatic ring. Therefore, electronic effects in the isolated π
bond would also favor anti-Markovnikov addition. Indeed, the
anti-Markovnikov addition to the phenylpropyne occurs by a
Markovnikov addition to the isolated π bond.
We synthesized 1-(2-methylphenyl)-prop-1-yne and sub-
jected it to our reaction conditions. The results were striking.
In the 0.1 M bromide mixture, the Markovnikov products
were evidence of a cationic intermediate much as had been
seen with the 1-(4-methylphenyl)-prop-1-yne. No evidence
of a steric effect was seen. However, in the less acidic 1.2 M
bromide solution, the ortho methyl group appeared to block
all reaction of the in-plane π bond and less than 1% of the
anti-Markovnikov product was found. The 1-(4-methylphenyl)-
prop-1-yne had produced approximately 12% of the anti-
Markovnikov product.
We also synthesized and reacted 1-(2-chlorophenyl)-prop-1-
yne. In the 1.2 M bromide, the corresponding para isomer had
produced 80% of the anti-Markovnikov HBr adduct with an
overall rate constant of 4.3 × 10Ϫ5 min.Ϫ1. In contrast, the ortho
isomer produced only 24% of the anti-Markovnikov product
with a rate constant of 3.2 × 10Ϫ6 min.Ϫ1. Clearly, the ortho
chlorine hinders reaction at the π bond coplanar with the ring.
Fig. 2 Logarithm of rate constants of phenylpropynes from Table 2
versus Hammett σϩ values.
In the less acidic solutions containing 1.2 M bromide, the
anti-Markovnikov product was found in every case. For the
phenylpropyne with the most electron withdrawing substituent
reacting with the 1.2 M bromide, the anti-Markovnikov product
comprised 93% of the adducts. The reason for this result may
be seen in a comparison of the curves of Figs. 1 and 2. While
the slopes of these lines are quite similar for the acetylenes and
the propynes, the rates of formation of the β-bromopropenes
are more than ten times the values for the corresponding
β-bromoethylenes.
Conclusion
Electrophilic attack on the unconjugated π bonds of phenyl-
acetylenes and phenylpropynes has been suggested previously
but has not been universally accepted. Coplanar iodonium
ions have been postulated in an addition of IN3 to phenyl-
propyne5 but the idea was criticized6 for the experimental
absence of stereochemically pure anti addition. An epi-
sulfonium ion was also suggested7 to result from the attack
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 5 2 – 2 1 5 6
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of dinitrobenzenesulfenyl chloride on the π bond in the plane
of the ring of phenylacetylene. The activation parameters and
secondary kinetic isotope effects for this reaction were more
similar to the addition to unconjugated alkynes than to styrene.
The ρ value was also less negative than those for other electro-
philic additions to phenylacetylenes. The exclusive Marko-
vnikov addition was explained by the significant positive charge
on the carbons of the episulfonium ion being better stabilized
next to the aromatic ring.
We feel that the HBr additions at high bromide concen-
trations present a clearer case for this behavior. The absence of
syn addition products at bromide ion concentrations above one
molar is strong evidence for the Ad3 mechanism at these
concentrations. As Okuyama et al.7 pointed out, the greater
reactivity of the propynes over the ethynes is further evidence
for reaction via the π complex. Such a complex at the conjugated
π bond of the alkyne would be heavily polarized toward posi-
tive charge developing adjacent to the aromatic ring and the
resulting transition state would be strongly affected by substi-
tuents in the ring. Nucleophilic attack would occur at the pos-
ition α to the ring. Furthermore, this transition state would
suffer attack and rehybridization in the plane orthogonal to the
plane of the ring and show minimal steric interaction with the
phenyl ring. We believe that this transition state predominates
at intermediate bromide concentrations and that it leads to
exclusive formation of the Z1B Markovnikov products. The
ρ for Z1B formation at 1.2 M bromide is Ϫ2.9, consistent with
this model. If any anti-Markovnikov products were coming
from addition to the conjugated π system, we would expect to
see it reflected in the ρ value for the formation of those
products.
showing that both regioisomers were similarly promoted by
the higher nucleophilicity of the iodide ion. Therefore, both
products were generated by nucleophilic attack on an acid–
alkyne complexes.
At the lowest concentration of bromide ion, attack on
the conjugated π bond produces a free cation leading to the
Markovnikov product primarily by a syn addition. As the
bromide ion increases, the less acidic solutions require bromide
participation in the transition state causing a polarized
concerted addition in the conjugated π system and more
Markovnikov anti addition. Higher concentrations of bromide
ion caused decreased acidity and a less polarized transition
state. This leads to the concerted reaction occurring preferen-
tially in the isolated π bond of the alkyne and exclusive form-
ation of the anti-Markovnikov anti addition product.
Experimental
The HBr additions were performed as described in the preced-
ing paper.1 Product identification was based upon their relative
retention times in the chromatographic analysis and the relative
abundance of the molecular ion to the (Mϩ–Br) peak in the
mass spectra. In questionable cases, isomeric mixtures were
synthesized and analyzed by NMR and GC/MS. The results
of these analyses identified the chromatographic peaks and
supported our assumption of similar detector responses for
isomeric products. NMR spectra were recorded on a Varian
Mercury 300 MHz spectrometer utilizing a deuterium lock and
TMS as internal reference. Mass spectra and chromatographic
analyses were performed on a Hewlett Packard 5890 Chrom-
atograph with a 12 m HP-1 capillary column and a 5971A
mass selective detector.
As the concentration of bromide ion increases, the acidity of
the solution drops and the polarized transition state with its
strong bonding to the acid, becomes less feasible. Because the
weaker acid requires more bonding from the nucleophilic brom-
ide, the Ad3 transition state becomes less polarized and little
charge builds up on the acetylenic carbon atoms. A π complex
between an acid and an unconjugated alkyne has been shown8
to distort the alkyne structure very little and presumably to
develop little charge on these carbons. With little charge to
stabilize, reaction at the isolated π bond becomes favored as it
leaves the conjugated π system intact. This π bond, orthogonal
to the conjugated π system, is well insulated from the electronic
effects of the phenyl substituents. Neither the rate of reaction
nor the regiochemistry at this π bond should be strongly influ-
enced by meta or para substituents in the ring. The rates of
formation of the anti-Markovnikov products were very similar
for all members of the phenylpropyne series and for the phenyl-
acetylene series at 1.2 M bromide concentration leading to
ρ values close to zero. The greater rates for the phenylpropyne
series derived from the added methyl group stabilizing the
π complex and/or stabilizing the positive charge building at
the β carbon in the transition state.
General synthesis of substituted phenylpropynes9
To a flame dried flask under N2 atmosphere was added 60 mL
of a THF solution containing 8.6 mmol of the phenylacetylene.
The solution was cooled to Ϫ20 ЊC. Methyl lithium 13.0 mL of
a 1.4 M solution in ether (2.1 eq.) was added dropwise to
the cooled solution over a 10 minute period. After allowing
the reaction to stir at Ϫ20 ЊC for 30 minutes, methyl iodide
(1.15 mL, 2.1 eq.) was added. The reaction was allowed to pro-
ceed for 30 minutes at Ϫ20 ЊC, slowly allowed to warm to room
temperature over the next 30 minutes and then quenched with
water. The product was extracted with hexanes, washed with
water and brine and dried over anhydrous sodium sulfate. Flash
chromatography on silica gel with hexanes yielded a 66–95%
yield of a colorless liquid. Any unreacted starting material was
removed by treatment with 1% silver nitrate solution in ethanol.
1-(4-Chlorophenyl)-prop-1-yne
Rf = 0.43, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.02
(3 H, s), 7.20–7.29 (4 H); m/z 150 (Mϩ, 54%), 115 (100).
On the other hand, the aromatic ring should have a strong
steric effect on this π bond, hindering bromide attack at the α
position and promoting the production of anti-Markovnikov
adducts. At the highest concentration of bromide ion, the anti-
Markovnikov adduct was the principal product from all of the
phenylpropynes tested. We therefore believe that the attack at
the isolated π bond leads to exclusive formation of the anti-
Markovnikov product. Ortho substituents on the phenyl ring
further hinder attack in this plane, leading to a rate retardation
and increased Markovnikov products coming from the slow
attack on the conjugated system.
1-(4-Fluorophenyl)-prop-1-yne
Rf = 0.41, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.01
(3 H, s), 6.94 (2 H), 7.33 (2 H); m/z 133 (Mϩ, 100%), 134 (67),
107(10).
1-(4-Methylphenyl)-prop-1-yne
Rf = 0.48, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.01
(3 H, s), 2.30 (3 H), 7.04 (2 H, d), 7.25 (2 H, d); m/z 130 (Mϩ,
100%), 115 (83).
As we reported in our preceding paper,1 a competition
experiment using equimolar bromide and iodide ions produced
90% iodide adducts indicating the involvement of the halide
ions in the rate and product-determining step. More signifi-
cantly, the ratio of anti-Markovnikov to Markovnikov products
was little affected by this increased nucleophilicity of iodide
1-(3-Trifluoromethylphenyl)-prop-1-yne
Rf = 0.40, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.06
(3 H, s), 7.34–7.62 (4 H); m/z 184 (Mϩ, 100%), 165 (16), 115
(95).
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 5 2 – 2 1 5 6
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1-(2-Chlorophenyl)-prop-1-yne10
MHz; CDCl3; Me4Si) 1.42 (3 H, d, J 7.2), 2.23 (3 H, s), 6.24
(1 H, q, J 7.2), 7.1–7.4 (4 H); m/z 198, 196 (Mϩ, 48%), 115
(100);
To a flame dried 3-neck flask under N2 atmosphere 1.757 g
(1.5 eq.) of anhydrous zinc bromide was added followed by
15 mL of anhydrous THF. The resulting solution was cooled in
an ice bath to 5 ЊC. The propargyl Grignard solution, 15 mL of
a 0.5 M solution in THF (1.5 eq.), was added dropwise to the
stirred solution of zinc bromide over a fifteen minute period.
The ice bath was removed and the reaction mixture was allowed
to warm to room temperature. After 15 minutes at room tem-
perature, 1.233 g (5.17 mmol) of o-chloroiodobenzene dissolved
in 5 mL of THF was added to the stirred reaction mixture
followed by 0.293 g (0.05 eq.) of palladium tetrakis(triphenyl-
phosphine) dissolved in 10 mL of THF. The reaction was
allowed to proceed at room temperature for two hours and then
was quenched with water. The crude product was extracted with
ether, washed with brine and dried over anhydrous sodium
sulfate. The solvent was removed by rotary evaporation. The
residue was taken up in hexanes and the resulting slurry was
filtered to remove the catalyst. Flash chromatography on silica
gel with hexanes yielded 0.690 g, 92% yield of a colorless liquid.
Rf = 0.50, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.11
(3 H, s), 7.14–7.16 (2 H), 7.17–7.32 (1 H), 7.39–7.42 (1 H);
m/z 150 (Mϩ, 61%), 115 (100).
(Z)-1-(2-methylphenyl)-1-bromoprop-1-ene 17.3%; δH(300
MHz; CDCl3; Me4Si) 1.89 (3 H, d, J 6.6), 2.32 (3 H, s), 6.24
(1 H, q, J 6.6), 7.1–7.4 (5 H); m/z 198, 196 (Mϩ, 47%), 115
(100);
(E)-1-(2-methylphenyl)-2-bromoprop-1-ene 28.0%; δH(300
MHz; CDCl3; Me4Si) 2.23 (3 H, s), 2.28 (3 H, d, J 1.5), 6.90
(1 H, s), 7.1–7.4 (4 H); m/z 198, 196 (Mϩ, 70%), 115 (100);
(Z)-1-(2-methylphenyl)-2-bromoprop-1-ene 44.7%; δH(300
MHz; CDCl3; Me4Si) 2.23 (3 H, s), 2.46 (3 H, d, J 1.5), 6.66
(1 H, s), 7.1–7.4 (4 H); m/z 198, 196 (Mϩ, 75%), 115 (100).
HBr in acetic acid addition to 1-(2-chlorophenyl)-prop-1-yne
1H-NMR and GC/MS analysis determined the products to be:
(E)-1-(2-chlorophenyl)-1-bromoprop-1-ene 0.30%, m/z 198,
196 (Mϩ, 48%), 115 (100);
(Z)-1-(2-chlorophenyl)-1-bromoprop-1-ene 2.2%, m/z 198,
196 (Mϩ, 47%), 115 (100);
(E)-1-(2-chlorophenyl)-2-bromoprop-1-ene 26.8%; δH(300
MHz; CDCl3; Me4Si) 2.35 (3 H, d, J 1.5), 6.97 (1 H, q), 7.1–7.4
(3 H), 7.88–7.92 (3 H); m/z 198, 196 (Mϩ, 70%), 115 (100);
(Z)-1-(2-chlorophenyl)-2-bromoprop-1-ene 70.7%; δH(300
MHz; CDCl3; Me4Si) 2.51 (3 H, d, J 1.5), 6.77 (1 H, q), 7.1–7.4
(3 H), 7.61–7.66 (1 H); m/z 198, 196 (Mϩ, 75%), 115 (100).
1-(2-Methylphenyl)-prop-1-yne
Rf = 0.45, silica, hexanes; δH(300 MHz; CDCl3; Me4Si) 2.07
(3 H, s), 2.40 (3 H, s), 7.05–7.14 (3 H), 7.33 (1 H); m/z 130
(Mϩ, 100%), 115 (84).
Acknowledgements
This work was supported by the Petroleum Research Fund of
The American Chemical Society. Portions of this work were
reported at the Northeast Regional Meeting of the American
Chemical Society in 2001 at Durham, NH.
Synthesis of all four isomers of phenylbromopropene
1-Phenylprop-1-yne (0.644 g, 5.55 mmol) was cooled in an ice
bath. To the stirred reaction 5.5 mL of ice cold 30% HBr in
acetic acid solution was added. The reaction was allowed to
proceed for fifteen minutes at 5 ЊC and then was quenched with
water. The mixture of products was extracted with hexanes,
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39, 1124.
3 F. A. Carroll, Perspectives on Structure and Mechanism in Organic
Chemistry, Brooks/Cole, Pacific Grove, CA, 1998.
4 H. M. Weiss and K. M. Touchette, J. Chem. Soc., Perkin Trans. 2,
1998, 1523.
5 A. Hassner, R. J. Ishister and A. Friederang, Tetrahedron Lett.,
1969, 10, 2939.
6 B. Capon and C. W. Rees, Organic Reaction Mechanisms 1969,
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and GC/MS analysis determined the products to be:
(E)-1-phenyl-1-bromoprop-1-ene 3.0%; δH(300 MHz; CDCl3;
Me4Si) 1.64 (3 H, d, J 6.8), 6.24 (1 H, q, J 6.8), 7.24–7.53 (5 H);
m/z 198, 196 (Mϩ, 48%), 115 (100);
(Z)-1-phenyl-1-bromoprop-1-ene 20.2%; δH(300 MHz;
CDCl3; Me4Si) 1.93 (3 H, d, J 6.6), 6.24 (1 H, q, J 6.6), 7.24–7.53
(5 H); m/z 198, 196 (Mϩ, 47%), 115 (100);
(E)-1-phenyl-2-bromoprop-1-ene 14.6%, m/z 198, 196 (Mϩ,
70%), 115 (100);
(Z)-1-phenyl-2-bromoprop-1-ene 62.1%; δH(300 MHz;
CDCl3; Me4Si) 2.48 (3 H, s), 6.68 (1 H, s), 7.24–7.35 (4 H),
7.51–7.53 (1 H); m/z 198, 196 (Mϩ, 75%), 115 (100).
7 T. Okuyama, K. Izawa and T. Fueno, J. Org. Chem., 1974, 39,
351.
HBr in acetic acid addition to 1-(2-methylphenyl)-prop-1-yne
1H-NMR and GC/MS analysis determined the products to be:
8 D. Mootz and A. Deeg, J. Am. Chem. Soc., 1992, 114, 5887.
9 Z. Xu, H.-S. Byun and R. Bittman, J. Org. Chem., 1991, 56,
7183.
(E)-1-(2-methylphenyl)-1-bromoprop-1-ene 10.0%; δH(300
10 E. Negishi, M. Kotoro and C. Yu, J. Org. Chem., 1997, 62, 8957.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 5 2 – 2 1 5 6
2156