Effect of cyclodextrin complexation in bromine addition to unsymmetrical olefins: Evidence for participation of cyclodextrin hydroxyl groups

Article abstract of DOI:10.1039/b418685k

Multiple recognition by cyclodextrin in a bimolecular reaction, namely bromination of styrene, methyl cinnamate, phenylacetylene and allylbenzene, has been studied. Bromohydrin is obtained as a major product along with dibromide in the bromination of styrene and methyl cinnamate. The percentage of bromohydrin decreases as the cavity size increases. With phenylacetylene, bromophenylacetylene and phenacyl bromide are obtained in addition to the dibromides. In the bromination of cyclodextrin complexes of allylbenzene, the product distribution is the same as in solution bromination. The observed results demonstrate the efficiency of cyclodextrin in stabilizing the open carbocationic intermediate and thus provide chemical evidence for the participation of cyclodextrin hydroxyl groups. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418685k

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A R T I C L E
Effect of cyclodextrin complexation in bromine addition to
unsymmetrical olefins: evidence for participation of cyclodextrin
hydroxyl groups†
Manickam C. Durai Manickam,a Subramanian Annalakshmi,b Kasi Pitchumani*b and
Chockalingam Srinivasana
a Department of Materials Science, Madurai Kamaraj University, Madurai, 625 021, INDIA
b School of Chemistry, Madurai Kamaraj University, Madurai, 625 021, INDIA.
E-mail: pit12399@yahoo.com; Fax: 91-0452-2459181; Tel: 91-0452-2458246
Received 13th December 2004, Accepted 17th January 2005
First published as an Advance Article on the web 14th February 2005
Multiple recognition by cyclodextrin in a bimolecular reaction, namely bromination of styrene, methyl cinnamate,
phenylacetylene and allylbenzene, has been studied. Bromohydrin is obtained as a major product along with
dibromide in the bromination of styrene and methyl cinnamate. The percentage of bromohydrin decreases as the
cavity size increases. With phenylacetylene, bromophenylacetylene and phenacyl bromide are obtained in addition to
the dibromides. In the bromination of cyclodextrin complexes of allylbenzene, the product distribution is the same as
in solution bromination. The observed results demonstrate the efficiency of cyclodextrin in stabilizing the open
carbocationic intermediate and thus provide chemical evidence for the participation of cyclodextrin hydroxyl groups.
Introduction
Cyclodextrins (CDs), cyclic oligosaccharides with a hydropho-
bic cavity, possess the ability to form host–guest complexes
with different guest molecules of appropriate size, resulting in
modified reactivities and selectivities.1 Many synthetic strategies
in which CDs have been used as reaction vessels to control
photochemical2 and thermal reactions3 are reported. In several
cases, reactivity inside the CD cavity is distinct from the
solution phase due to the restricted motions of the guest
molecule and the incoming reagent, which is more pronounced
in solid state.4 Selective and catalytic transformations such as
reduction,5 epoxidation,6 sulfoxidation7,8 and halogenation of
olefins9 catalyzed by CDs have been studied.
Electrophilic bromination of olefins by molecular bromine is
stereoselective and the nature of the olefin–bromine complex and
–
12
the intermediates involved have been extensively studied.10
Other notable features13,14 include dependance of stereochem-
istry on the solvent and substituents, as well as the chemos-
electivity (predominant dibromide or solvent-incorporated
adducts). The stereochemical outcome is controlled not only by
the bridged or unbridged structure of the cationic intermediate,
but also by the association with its nucleophilic partners and
its lifetime. Based on extensive product and kinetic evidence,
a mechanistic scheme may be visualized (Scheme 1) in which
preassociation, free-ion and ion-pair pathways compete. The
stereochemical outcome depends on the nucleophilic partners
of the product-forming ionic intermediates (which, depending
on the double bond substituents, can be open a-I+, fully bridged
b-I+ or weakly bridged c-I+), arising from different ionization
routes. The products are the solvent-incorporated or mixed
adducts (MA) and the dibromides (DB). They can be obtained
from free-ions (E), ion-pairs (B) or ion-dipole sandwiches (D),
formed via preassociation or ion-pair pathways.
Scheme 1 Mechanism of bromine addition to olefins.
derivatives. With an increase in cavity diameter (from ato c-CD),
the ratio decreases. This significant reduction in stereoselectivity
is attributed to a) presence of a polar environment provided by
the secondary hydroxyl/methoxyl groups at the wider rim of the
CD cavity in stabilizing the carbocationic intermediate and b)
steric hindrance to the attack of the tribromide.16
Thus, it will be interesting to study bromination of unsym-
metrical olefins and acetylenes in the presence of CDs to gain
a better understanding of the role of the CD in influencing the
mechanism and stereochemistry of bromination. It is relevant
to note that bromine addition to styrene17 in an acetic acid
medium gives a substantial amount of acetoxybromides in
addition to the major product, namely dibromides, and the
reaction is nonstereospecific. Bromination of methyl esters of
cinnamic acid18 in aqueous acetic acid gives a mixture of
erythro-2,3-dibromo-3-phenylpropionate ester and erythro-2-
bromo-3-hydroxy-3-phenylpropionate ester, which are formed
in a completely stereo- and regiospecific anti-addition mode. In
the bromination of phenylacetylene19 in an acetic acid medium,
25% of bromoacetylene, 42% of trans-, 19% of cis-dibromide
and 14% of phenacyl bromide are obtained. In the present study,
bromination of styrene (1), methyl cinnamate (2) phenylacety-
lene (3) and allylbenzene (4) is carried out in the presence of
cyclodextrins and the observed results are discussed (Scheme 2).
In our earlier study15 on bromination of trans-stilbene in CDs,
a significant change in product composition with a consequent
loss of stereoselective trans-mode of addition was reported. The
meso/dl ratio is less than one in the bromination of 1 : 1
complexes of stilbene with CDs and their hydroxypropyl
† Electronic supplementary information (ESI) available: preparation
and characterization of cyclodextrin complexes and preparation of 5–13.
See http://www.rsc.org/suppdata/ob/b4/b418685k/
1 0 0 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 0 0 8 – 1 0 1 2
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 5
©

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that of 6 increases (Table 1). It is relevant to note here that
in a 75% CH3OH–25% water mixture as the reaction medium
no methanol incorporated product is obtained, indicating that
participation by CD-hydroxyl groups alone play a major role
in stabilizing the carbonium ion intermediate than the solvent
molecules.
To account for the observed product selectivity upon CD en-
capsulation, the following mechanism (Scheme 3) is visualized.
Inclusion into the CD cavity, followed by addition of the bromo-
nium ion results in the predominant formation of an acyclic,
open benzylcarbonium ion (in preference to fully-bridged or
weakly bridged cyclic intermediate), which can be stabilized by
the aryl ring and also by the secondary hydroxyl groups present
in the rim of the CD cavity. The resultant intermediate (attempts
to isolate this intermediate were unsuccessful) may be attacked
ꢀ
by either the Br3 (leading to dibromide) or by water molecules
leading to bromohydrin (which may be either the cyclodextrin-
bound high energy water included into the CD cavity at the time
of complex formation, or the solvent water used for extracting
the substrate from the complex). As the size of the CD cavity
becomes smaller, as in a-CD, not only is the attack by the second
bromine more hindered, but formation of the intermediate
between the hydroxyl group of CD and the open carbonium ion
can also take place more readily. Consequently, in these cases,
the formation of bromohydrin takes place more readily.
Scheme 2 Structures of reactants and products in the bromination
of styrene, methyl cinnamate, phenylacetylene and allylbenzene by
molecular bromine.
Results and discussion
Bromination of 1 in CCl4 (without CD) yields the dibromide 5 as
the exclusive product. However, a remarkable change in product
selectivity is observed when the a-CD complex of styrene is
brominated and bromohydrin 7 is obtained as the exclusive
product. In bromination with the b-CD complex, the percentage
of 7 decreases and the two products 5 and 7 are formed in equal
amounts. This trend continues in the bromination of the c-CD
complex also. As the distance between the carbonium ion to the
secondary hydroxyl group increases, as one goes from a- to c-CD
(Table 1), the proportion of bromohydrin decreases and that of
dibromide increases.
A similar trend is also observed in the bromination of another
aryl olefin, methyl cinnamate 2. When bromination of 2 is carried
out in aqueous methanol or CCl4 (Table 1), 6 is obtained as
the only product and the mechanism as depicted in Scheme 1
may be visualized. However, in bromination of CD complexes
of 2, in addition to the dibromide 6 a significant amount of 8
is formed (as in the case of bromination of styrene). With an
increase in size of the CD, the percentage of 8 decreases and
Scheme 3 Mechanism of bromination of CD-included styrene and
methyl cinnamate.
To gain further evidence for participation of CD hydroxyl
groups in stabilizing the intermediate acyclic carbonium ion,
bromination of 1 and 2 is also extended to studies of CD
Table 1 Product distribution in bromination of styrene 1/methyl cinnamate 2 in solution and in CD complexa
Percentage of:
Medium
Substrate
Conversion (%)
Dibromide (5/6)
Bromohydrin (7/8) Dibromide/bromohydrin ratio
CCl4
a-CD
b-CD
c-CD
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
95.5
100.0
89.0
100.0
100.0
100.0
90.7
100.0
90.0
100.0
84.0
90.8
94.1
69.2
85.9
92.9
88.3
95.5
—
48.0
62.0
—
—
High
∼ 0.01
1.17
100.0
41.0
38.0
100.0
100.0
65.9
—
1.61
HP-a-CDb
HP-b-CDb
DM-b-CDc
TRIME-b-CDd
CCl4
∼ 0.01
∼ 0.01
0.37
High
High
1.10
0.82
1.37
2.43
2.09
—
24.8
100.0
90.0
52.5
37.8
52.6
66.7
46.8
64.4
64.5
88.3
—
75% CH3OH–H2O
a-CD
b-CD
47.5
46.2
38.2
27.4
22.4
21.5
28.4
—
c-CD
HP-a-CDb
HP-b-CDb
DM-b-CDc
TRIME-b-CDd
3.00
2.27
High
a Analysed by GC, error limit 2%. b Randomly hydroxypropyl-a/b-cyclodextrin. c Heptakis (2,6-di-O-methyl)-b-cyclodextrin. d Heptakis (2,3,6-tri-
O-methyl)-b-cyclodextrin.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 0 0 8 – 1 0 1 2
1 0 0 9

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derivatives (Table 1). With hydroxylpropyl-a-CD (HP-a-CD)
and HP-b-CD (both containing only one hydroxyl group in the
secondary rim), bromination of 1 produces only bromohydrin
(7). With DM-b-CD (which also contains only one hydroxyl
group in the secondary rim), a high yield of bromohydrin
is obtained. However, bromination with permethylated-b-CD
(TRIME-b-CD) only dibromide is obtained as the exclusive
product, clearly indicating that the secondary hydroxyl group
of b-CD plays a decisive role in stabilizing the carbocationic
intermediate. A similar trend is also obtained in bromination of
2 in presence of cyclodextrin derivatives (Table 1).
It is also interesting to note that in an earlier report,20
asymmetric bromination, when carried out in microcrystalline
CD complexes of 1 in the gas–solid state, produces predom-
inantly (−)-1,2-dibromo-1-phenylethane. Chiral induction for
the reaction of the a-CD complex leads to a nine-fold increase
compared to that of the the b-CD complex, reflecting the more
compact and tight CD complex in the former. Bromination
in a homogeneous solution containing a- or b-CD complexes
gives no 5 but racemic 7. This chiral induction in the gas–solid
halogenation using the solid CD complexes is attributed to the
ability to hold rigidly a chiral conformation in the crystalline
state. In the present study, it is not attempted to measure precisely
the extent of chiral induction taking place inside the CD cavity.
To substantiate our generalization that the hydroxyl group
of CD is actively involved and stabilizes the carbonium ion
intermediate bromination is extended to phenylacetylene 3, in
which the hindrance to attack of the second bromide ion is
expected to be less as the carbon–carbon triple bond is axially
oriented with respect to the CD cavity. The product distribution
and their relative percentages are given in Table 2 and the
structures of different products obtained in bromination of 3
are given in Scheme 1.
The data presented in Table 2 show that while bromination of
3 in CCl4 yields the trans-dibromide 9 as the major product, the
reaction when carried out in acetic acid yields a mixture of 9 and
10, together with a small amount of 11. This is in accordance
with a previous report19 on bromination of phenylacetylene in
an acetic acid medium. However, when CD complexes of 3 are
brominated, several interesting features are observed in contrast
to solution bromination: a) cis-dibromide 10 is obtained in larger
amounts than the trans-dibromide 9, b) significant amounts of
bromophenylacetylene 11 are obtained and c) phenacyl bromide
12 (formed from the ketonisation of bromohydrin of 3) is also
obtained in good yield.
Scheme 4 Mechanism of bromination of phenylacetylene included
inside the CD cavity.
to solution bromination. In addition, phenacyl bromide 12 is
also formed and these results can be rationalized as shown
in Scheme 4. The a-bromocarbonium ion may be stabilized
(route (d)) by interaction with the secondary hydroxyl groups
of CD and the resultant intermediate may undergo subsequent
hydrolysis (route (f)). Direct attack by water on the open
carbonium ion is also likely (route (e)) which takes place more
readily with c-CD.
To gain additional support for our generalization that the
active participation of secondary hydroxyl groups of CD stabi-
lizes the initially formed a-bromocarbonium ion intermediates,
bromination of allylbenzene 4 as its CD complex is carried out.
Only the dibromide 13 is observed, analogous to the correspond-
ing solution bromination. This result is not unexpected. Though
the aryl ring of 4 penetrates the CD cavity, the site of bromination
is farther removed from the influence of CD and its hydroxyl
groups. Thus, complexation by CD affects neither the rate nor
the product distribution in bromination of 4. This result also
provides evidence for the absence of any inclusion of bromine
into the CD cavity under the present experimental conditions.
It is relevant to note here that in ring bromination of phenols
catalyzed by CDs, bromine complexed in CD is proposed21 to
account for the observed catalytic activity.
The observed product distribution in the bromination of 3
can be explained (Scheme 4) as follows: in route (a), inclusion
into the CD cavity followed by addition of bromine leads to
preferential formation of open carbonium ion, which on further
reaction (route (b)) yields dibromides 9 and 10. As attack by the
second bromine from the rear is sterically hindered by the CD
hydroxyls (which stabilize the open carbonium ion), more of cis-
dibromide 10 is formed when compared to the trans-dibromide
9. Substitution of acetylenic hydrogen by bromine (route (c))
leads to bromophenylacetylene 11. As this is the least hindered
attack, yields of 11 are substantial in CD complexes compared
To understand the interaction between CD hydroxyls and the
carbonium ion intermediate, molecular modeling studies22 are
carried out with CD complexes (Fig. 1). The results presented
in Table 3 show that the distance between the a-carbon atom of
the substrate and the oxygen of the secondary hydroxyl in CD
Table 2 Products distribution in bromination of phenylacetylenea,b
˚
is within 4 A in all the olefins and also the distance increases as
the size of the CD cavity increases. This explains the decrease in
the formation of bromohydrin with an increase in the size of the
CD cavity. Complexation energies of CD–olefin complexes are
also calculated. These values, presented in Table 3, show that
a-CD forms stronger complexes indicating a more stable and
deep binding. As the size of the CD increases, the complexation
tends to be less pronounced.
Percentage of:
Medium
3
9
10
11
12
trans/cis (9/10) ratio
CCl4
Acetic acid
a-CD
b-CD
c-CD
54.4 41.7 2.9
1.0
—
14.40
1.83
—
—
—
—
55.0 30.0 12.0 3.0
21.5 44.3 21.2 13.0 0.49
15.3 40.1 26.0 18.6 0.38
7.2
19.3 27.0 46.5 0.30
Conclusions
a Analysed by GC, error limit 2%. b For structures of 3 and 9–12 refer
The results observed in the present study into bromination of
CD complexed olefins 1–4 amply demonstrate the efficiency of
to Scheme 2.
1 0 1 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 0 0 8 – 1 0 1 2

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Table 3 Calculated complexation energies (kcal mol−1) and distance between the a- and b-carbon atom of the olefins with the oxygen atom of the
secondary hydroxyl group of CD
Complexation energiesa
Distance between a-C and 2◦ OH/A
Distance between b-C and 2◦ OH/A
˚
˚
Complex
a-CD + Styrene
−27.33
−24.16
−17.86
−26.70
−31.49
−28.96
−44.20
−22.21
−16.52
3.45
3.61
4.64
4.01
4.10
4.38
3.38
4.32
4.37
3.40
3.68
3.69
3.85
3.76
4.25
3.36
3.84
3.47
b-CD + Styrene
c-CD + Styrene
a-CD + Methyl cinnamate
b-CD + Methyl cinnamate
c-CD + Methyl cinnamate
a-CD + Phenylacetylene
b-CD + Phenylacetylene
c-CD + Phenylacetylene
a Complexation energy (Ecomplex − Ehost − Eguest) obtained by CVFF force field, a RMS derivative for each substrate of 0.001 is achieved.
Fig. 1 CVFF-optimized cyclodextrin inclusion complexes; a) a- and b- CD complexes of styrene, b) a- and b- CD complexes of phenylacetylene.
CDs in stabilizing acyclic, open carbocationic intermediates at
the expense of bridged bromonium ion and, hence, modify the
reactivity of the included guest olefins. Results with various
CDs and CD derivatives provide chemical evidence for the
participation of the secondary hydroxyl groups of cyclodextrin
in the present study. Also, the possibility of direct attack of inner
cavity water molecules on the carbocation intermediate to yield
bromohydrins directly (without participation by CD hydroxyl
groups) may be ruled out as the yield of bromohydrin is found
to depend on the size of the CD cavity.
purifications and a stock solution of bromine was prepared by
dissolving 74 mL of bromine in 100 mL of CCl4 and making up
to 250 mL in a standard measuring flask. 1 : 1 CD complexes
were prepared† as reported earlier.24
Complex formation of the substrates 1–4 with cyclodextrins
was inferred by calculating the formation constants (Kf ) using
Benasi–Hildebrand method.25 With a-, b- and c- CDs the
formation constants (Kf per mol−1 dm3) for 1 are 290, 345
and 375 respectively; for 2 215, 227 and 285 respectively; for
3 185, 265 and 305 respectively and for 4 225, 315 and 320
respectively. The values are fairly high indicative of the formation
of strong complexes between the CDs and the guest. The small,
yet significant increase in Kf values with increase in CD size
may be rationalised due to formation of small amount of higher
order complexes also, in addition to 1 : 1 complexes. a-, b- and
c- CD complexes of substrates 1–4 were also characterized by
their 1H-NMR spectra.26
Experimental
Materials
Cyclodextrins a- and c- (American Maize Products, Indiana),
b- (Aldrich), hydroxypropyl-b- (DS 6, randomly hydroxypropy-
lated, a gift-sample from Cerestar, USA), hydroxylpropyl-a- (Ni-
hon Shokuhin Kako Co. Ltd., Japan) and dimethyl-b- (DS 1.8,
non-recrystallizable, randomly methylated, a gift-sample from
Wacker-Chemie, Germany) were used as received. TRIME-b-
CD (heptakis (2,3,6-tri-O-methyl)-b-cyclodextrin) was prepared
from b-cyclodextrin in dry DMF at 0 ◦C with excess methyl
iodide in the presence of NaH.23 Bromine (Merck), styrene
(Aldrich), methyl cinnamate (Merck), phenylacetylene (Aldrich)
and allylbenzene (Aldrich) were used as received without further
General procedure for bromination of substrates 1–4 in solution
and in CD complexes
Substrates 1–4 were dissolved in 20 mL of CCl4 taken in a conical
flask. To this solution, 12 mL of a stock solution of bromine was
added dropwise with constant stirring at 0 ◦C. After the reaction
was over in about 30 min, the excess bromine was removed with
sodium thiosulfate solution and the products were analyzed. In
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 0 0 8 – 1 0 1 2
1 0 1 1

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a typical CD meditated bromination reaction, 0.1 g of the 1 :
1 complex was thoroughly mixed with an equimolar amount of
bromine in CCl4 and kept at 0 ◦C for 3 h. After completion of the
reaction the excess bromine was removed and the complex was
dissolved in water, the products extracted with hot CHCl3 and
analyzed by capillary GC (Shimadzu 17A, SE-30 (5%) column
with high purity N2 as carrier gas). The products were identified
by coinjection with authentic samples prepared by reported
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procedures17
and also by their 1H-NMR spectra.†
Molecular modeling studies
Molecular mechanics calculations22 were carried out for all the
olefins inside a, band cCDs using Insight II Discover program in
IRIX system. Calculations are done in a vacuum and structures
are minimized using a CVFF force field and the RMS derivative
0.001 is achieved in each case.
17 J. H. Rolston and K. Yates, J. Am. Chem. Soc., 1969, 91, 1469.
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(b) Discover User Guide, Accelrys Inc., 2001.
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Acknowledgements
Financial assistance from Council of Scientific and Industrial
Research (CSIR), New Delhi is gratefully acknowledged. K. P.
thanks UGC-DRS support to MKU for GC-MS facility.
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1 0 1 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 0 0 8 – 1 0 1 2