Sequential bromination reactions from beads with methyltriphenylphosphonium tribromide groups

Article abstract of DOI:10.1002/poc.1694

A reusable bromination reagent based on polystyrene beads with covalently appended methyltriphenylphosphonium tribromide groups has been developed. The results from bromination reactions of several structurally diverse unsaturated substrates with the beads and with solutions of a non-polymeric model brominating reagent, methyltriphenylphosphonium tribromide, are described. It is shown that the reactions are highly regio- and stereo-selective and can be conducted easily. Copyright

Full text of DOI:10.1002/poc.1694

Special Issue Article
Received: 3 December 2009,
Revised: 19 January 2010,
Accepted: 22 January 2010,
Published online in Wiley Online Library: 31 March 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1694
Sequential bromination reactions from beads
with methyltriphenylphosphonium tribromide
groupsz
Rodrigo Cristianoay, Andrew D. Wallsa and Richard G. Weissa
*
A reusable bromination reagent based on polystyrene beads with covalently appended methyltriphenylphospho-
nium tribromide groups has been developed. The results from bromination reactions of several structurally diverse
unsaturated substrates with the beads and with solutions of a non-polymeric model brominating reagent, methyl-
triphenylphosphonium tribromide, are described. It is shown that the reactions are highly regio- and stereo-selective
and can be conducted easily. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: bromination reagent; methyltriphenylphosphonium tribromide; phosphonium; polystyrene beads; tribromide
ions. Because Pþ is a larger cation and has lower binding energies
with anions than Nþ,[26–35] 3 is an attractive and interesting
INTRODUCTION
Brominated compounds are important intermediates in organic
synthesis because they are amenable to a vast number of organic
reactions involving functional group transformations. These
include carbon–carbon bond formations, organometallic prep-
arations (e.g., Grignard and organolithium reagents), and
metal-catalyzed cross-coupling reactions.[1–3] Brominated mol-
ecules may be prepared by free-radical processes or, more
generally, from unsaturated molecules via addition or electrophilic
substitution, using molecular bromine as a common reagent.
However, liquid bromine is difficult to handle because of its toxicity,
corrosiveness, and high volatility. Although many brominating
reagents[4–17] have been developed in recent years to minimize
these problems, new reagents are needed for specific applications,
especially if those reagents are to be environment friendly.[18] In
this regard, salts with tribromide anions (Brꢀ3 )[10–17] can provide
safer reagents because Brꢀ3 has a very low volatility and is very
stable when a part of neat salts[19] or when the salts are dissolved
in aprotic solvents.[20,21] Although addition of molecular bromine, a
dangerous and volatile liquid, to bromide anion is the usual
method for preparing Brꢀ3 , the latter can be transported and
handled easily thereafter because its vapor pressure is almost nil
and it is very stable in dry air. Recently, we reported the use of an
ionic liquid based on a tetraalkylphosphonium tribromide to
brominate a variety of substrates in a ‘stacked’ reactor that
exploited the diffusion of components and the densities and
immiscibilities of layers,[10] and several phosphonium salts with
mixed trihalide anions to effect dibromo, bromo-chloro, and
dichloro halogenations via solution phase reactions.[22]
alternative to the other polymer-bound tribromide reagents
reported thus far. It is found to be quite robust.
RESULTS AND DISCUSSION
The beads of 3 were prepared as described in Scheme 1; details
are provided in the Supporting Information. Commercially
available triphenylphosphine-functionalized polystyrene resin 1
was treated with an excess of methyl bromide to give
phosphonium salt 2. We believe that all of the triphenylphosphine
units become quaternized during the reaction based on the
following considerations: the strong nucleophilicity of the
phosphine group, the excess of the methyl bromide employed,
the ease of SN2 substitution reactions on methyl, and the presence
of an excellent leaving group, bromide. As the beads of 2 reacted
with molecular bromine dissolved in stirred dichloromethane,
they became light orange and their density increased—they fell
from the top (solvent–air interface) to the bottom of the flask.
*
Correspondence to: R. G. Weiss, Department of Chemistry, Georgetown
University, Washington, DC 20057-1227, USA.
E-mail: weissr@georgetown.edu
a
R. Cristiano, A. D. Walls, R. G. Weiss
Department of Chemistry, Georgetown University, Washington, DC
20057-1227, USA
Here, we describe a different strategy for sequential bromina-
tion reactions employing cross-linked polystyrene beads with
appended methyltriphenylphosphonium tribromide groups (3)
as the bromination reagent. Swellable (insoluble) solid support
reagents are advantageous for some synthetic procedures,[23]
and bromination reactions have been exploited using quaternary
ammonium[24] and pyridinium[25] tribromide polymer-bound
y
Present address: Departamento de Qu´ımica, Universidade Federal da
˜
Para´ıba, Joao Pessoa-PB, 58059-900 Brazil.
z
This article is published in Journal of Physical Organic Chemistry as a special
issue on Tenth Latin American Conference on Physical Organic Chemistry,
edited by Faruk Nome, Dept de Quimica, Universidade Federal de Santa
Catarina, Campus Universitario – Trindade 88040-900, Florianopolis-SC, Brazil.
J. Phys. Org. Chem. 2010, 23 904–909
Copyright ß 2010 John Wiley & Sons, Ltd.

SEQUENTIAL BROMINATION REACTIONS FROM BEADS
Bromination reactions were conducted in CH2Cl2, a swelling
solvent for 3 (Scheme 2). Several structurally diverse substrates,
—
—
—
containing C C, C C, methylcarbonyl, and aromatic groups
—
—
(4–10), have been employed. The high formation constant of Brꢀ3 in
chlorinated solvents[20,21] and its higher stability than Brꢀ plus Br2
(due the charge delocalization)[36] make us confident that Br3ꢀ is the
brominating reagent here. In addition, no discernible free bromine
was detectable by UV–Vis absorption spectroscopy in dichlor-
omethane that was placed in contact with 3 for several hours. A
detailed description of the experimental procedures is provided in
the Supporting Information. Reactions were performed at room
temperature in dry air atmospheres in darkened vessels (as a
precaution to avoid possible photoreactions of Brꢀ3 ).
In initial experiments, work-up procedures involved filtration and
washing of the beads on filter paper. The filtration led to significant
losses of the beads because they are electrostatically active and
adhere to many surfaces. In addition, washing the beads was not
an efficient method to remove substrate and product molecules
that were held within the polystyrene matrix because the solvent
was not in contact with the beads for sufficient periods to allow
efficient extraction. To avoid these problems, the brominating
reagent 3 was prepared, reacted, and recycled in the same flask in
sequential bromination reactions (Fig. 1). The beads can be used
for at least eight runs: no indication of bead degradation was
apparent after that number when the project was suspended.
The liquid containing the products was sucked into a syringe or
pipette with a cotton plug inserted at the tip. Thereafter, an
aliquot of neat dichloromethane was stirred with the beads and
the liquid was withdrawn as before to ensure the recovery of all
products and any remaining substrate molecules from the beads.
Using the latter procedure, the resin resided always in one flask,
even during the course of several separate bromination reactions.
After each reaction and removal of the dichloromethane liquid,
the Brꢀ3 anion was regenerated (from 2) and then a new substrate
in dichloromethane was added. To avoid the possibility that
molecular (free) bromine was present in solution, only 0.8 molar
equivalents of Br2 were added to the phosphonium groups on
the beads of 2 during each experiment.
Scheme 1. Synthesis of the brominating reagent[4]
Representative reaction conditions, products, and product
ratios are provided in Table 1.[37] In all runs, a 10–40% molar
excess of 3 (with respect to substrate) was used to aid the
complete conversion of substrates to products. The products
were isolated in good yields, many >90%. The durability of the
beads was demonstrated by the fact that subsequent runs using
the same substrate and beads led to very similar product yields
and selectivities. Only with the product mixture from phenol 10b
Scheme 2. Substrates and products from stirring their CH2Cl2 solutions
with swollen beads of 3 at room temperature
Figure 1. Schematic representation of the procedures used for bromi-
nation reactions, removal of products, and recycling of the beads
J. Phys. Org. Chem. 2010, 23 904–909
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com

R. CRISTIANO, A. D. WALLS AND R. G. WEISS
Table 1. Results from bromination reactions using beads of 3 swollen in CH2Cl2
Products and relative product
yieldsc
Entry
Substrate [Conc. (M)]
Mole % of 3a
Time (h)
Yieldb (%)
11
100
12
—
13a
13b
d
1
2
3
4 [0.09]
5a [0.10]
5b [0.10]
140
140
140
3
2
2
95
89
93
14a
100 (>99)e
2 (<1)e
14b
15a
d
4
5
6a [0.10]
7a [0.10]
140
140
1
1
87
90
—
(<1)e
98 (>99)e
15b
2
6
6b [0.09]
7b [0.09]
140
140
24
48
96
95
98
2
16
98
17
7
8
8 [0.10]
8 [0.10]
110
140
48
48
94
93
92
85
8
15
18
83 (80)e
20a
19
9
9 [0.10]
140
140
48
48
88 (97)f
92 (98)f
17 (20)e
21a
2 (1)e
21b
17
10
10a [0.10]
98 (99)e
20b
83
11
12
10b [0.10]
10b [0.10]
140
140
4
48
55g
92
50
50
All reactions were conducted at ambient temperatures (ca. 18–208C).
a Based on substrate concentration.
b Isolated yields.
1
c From H NMR spectra of product mixtures.
d None detected by NMR; limits of detection are ca. 1.4–2.4% (see Supporting Information).
e By integration of peaks in GC chromatograms without correction for different detector responses.
f Conversion % based on the amount of substrate converted to products as estimated from NMR spectra.
g Determined after separation from starting material by column chromatography.
(entries 11 and 12) was another purification step (column
chromatography) necessary after the physical separation of the
beads from the liquid. The substrates were chosen so that all of
the products and their NMR spectra are known; product ratios are
based on integration ratios from NMR spectra and, in some cases,
they were verified by GC analyses.
Reactions with diphenylethyne 8 (entries 7 and 8) yielded the
dibromo E-isomer 16, and the tetrabrominated 17. The latter
yield is increased by using a larger excess of 3, or by allowing the
reaction to continue for longer periods; the initially (and
slowly[45,46]) formed dibromo product can react again to form
the tetrabromo compound. It should be noted that rates of
conversion here are controlled by a convolution of the rates of
diffusion of the substrates to sites at which Brꢀ3 ions reside inside
(or on the surfaces of) the beads, the rates at which the substrates
add bromine, and the rates at which the products diffuse from
the reactive sites within the beads back into the dichloromethane
liquid. The rate-limiting step may differ from substrate to
substrate, and this aspect of the reactions has not been
investigated as yet. However, the importance of opening access
to the sites where the tribromide ions reside in the beads (i.e., the
rates of diffusion of substrates within the cross-linked polystyrene
networks) is evident from comparisons between reactions with
CH2Cl2 and with hexane (a less efficient swelling solvent for 3) as
the supporting liquid: reaction of 6b in hexane resulted in only
13% conversion after 4 days (see Supporting Information)
whereas a 96% yield was obtained after 24 h in dichloromethane
(Table 1).
As expected, the substrates that are known to be more reactive
to electrophilic addition (N. B., aliphatic alkenes; entries 1–5) were
converted to products more rapidly than aromatic alkenes and
diphenylethyne. The reactions were stereoselective, giving
almost exclusively anti-addition products. For example, only
the trans-dibromo stereoisomer was detected from cyclohexene
and cis-2-hexene (6a) and trans-2-hexene (7a) yielded the threo
and erythro dibromo products, respectively, in anti/syn addition
ratios larger than 99/1 (by GC). Although cis-stilbene (6b) reacts
faster than its trans-isomer (7b), both gave anti-addition
products, the d,l and meso stereoisomers, respectively, in high
yields. These results are indicative of a similar reaction pathway
for all of the alkene substrates, whether they involve a
bromocarbenium ion, a bridged bromonium ion,[18,38–44] or an
alkene-molecular bromine p-complex[43] in the product deter-
mining step. By contrast, bromination of 6b by a tetralkypho-
sphonium tribromide ionic liquid using 2 and 4 layer reactors
yielded the syn-addition product preferentially (but the expected
anti-addition product for 7b).[10]
The beads of 3 effect efficient and selective electrophilic
substitutions on aromatic substrates, as well (entries 9–12). In
these reactions, evolution of HBr was expected and detected
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 904–909

SEQUENTIAL BROMINATION REACTIONS FROM BEADS
19, showing that the excess of tribromide ions is able to react
with the initially formed monobrominated 18.
These results demonstrate clearly the utility of beads of 3 to
effect bromination reactions. All of the reactions reported here
were performed with the same two sets of beads. However, there
are obvious limitations. Thus, toluene, an aromatic molecule less
susceptible to electrophilic aromatic substitution than phenol or
anisole, was not brominated using the same conditions as
employed in Table 1 after 4 days.
Scheme 3. Preparation of methyltriphenylphosphonium tribromide (23)
from methyltriphenylphosphonium bromide (22)
In order to compare the results obtained here with those from
tribromide anions not attached to beads, bromination reactions
were performed using a ‘model’ reagent, methyltriphenylpho-
sphonium tribromide (23, Scheme 3),[48] in CH2Cl2 solutions. The
products and selectivities obtained for substrates 4–10 are, in
general, very similar when 23 or the beads 3 were employed
(Table 2). For example, substrates 6-7 yielded addition products
with high anti-selectivities. Also, reactions involving electrophilic
substitutions presented similar results: acetophenone (9) gave a
80:20 mixture of 18:19, anisole (10a) gave a >99:1 ratio of
20a:21a, and phenol (10b) gave a 77:23 mixture of 20b:21a after
only 2.5 h of reaction time. Thus, although the product
selectivities from reactions employing methyltriphenylphospho-
nium tribromide and 3 are similar, isolation of products using the
(qualitatively). It did not appear to affect the product
compositions—no products from ether cleavage of 10a were
found, for example.[47] As in the case of the addition reactions
to diphenylethyne, the degree of substitution on phenol 10b
depends on the reaction time; the initially formed product
appears to be 4-bromophenol—no 2-bromophenol (or 2-
bromoanisole from anisole 10a) was detected—and it is
converted slowly to 2,4-dibromophenol. This bromination is
more selective for para substitution than reaction employing
beads with tetraalkylammonium tribromide as the reagent.[24]
Similarly, acetophenone 9 yielded some dibrominated product
Table 2. Products, relative product yields, and conditions for the bromination reactions using the soluble salt, 23, in CH2Cl2 solution
Products and relative pro-
Entry
Substrate [Conc. (M)]
Mole % of 23a
Time (h)
Yieldb (%)
duct yieldsc
11
100
12
d
1
2
4 [0.10]
5b [0.10]
140
140
1
1
85
76
—
13b
15a
1 (2.5)f
98 (>99)f
15b
1
14a
3
4
6a [0.10]
7a [0.10]
140
140
1
1
75e
72g
99 (97.5)f
2 (<1)f
14b
5
6b [0.10]
7b [0.10]
140
140
12
24
91
85 (88)h,i
99
6
16
(92)j
(95)j
18
80 (79)f
20a
99
20b
77 (78)f
94
17
6
7
8 [0.10]
8 [0.10]
110
110
2
24
(15)h
(20)h
(8)j
(5)j
19
8
9 [0.10]
10a [0.10]
10b [0.10]
140
140
140
12
14
2.5
85 (90)h
(75)h
95
20 (21)f
21a
9
1
21b
23 (22)f
10
All reactions were conducted at ambient temperatures (ca. 18–208C).
a Based on substrate concentration.
b Isolated yields.
1
c From H NMR spectra of product mixtures.
d None detected by NMR.
e Ca. 8% of unidentified materials also present according to GC analyses.
f By integration of peaks in GC chromatograms without correction for different detector responses.
g Ca. 3% of unidentified materials also present according to GC analyses.
h Conversion % based on the amount of substrate converted to products as estimated from NMR spectra.
1
i Detected 4% of cis-stilbene 6b and 8% of trans-stilbene 7b by H NMR spectroscopy.
j Approximated due to overlap between peaks of the dibromo product and starting material.
J. Phys. Org. Chem. 2010, 23 904–909
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R. CRISTIANO, A. D. WALLS AND R. G. WEISS
former reagent requires column chromatography in all cases and
the methyltriphenylphosphonium bromide 22 is much more
difficult to isolate and reuse than is 2. Additionally, the yields and
purities of products from reaction with methyltriphenylpho-
sphonium tribromide were lower than those from the beads of 3
in several of the reactions. The experiments with 23 also provide
support for our contention that a very small amount of free Br2 is
not responsible for the brominations. Were it the case, the
brominations in solution using 23 should have been very slow;
they were not when the substrates were very reactive toward
polar addition reactions.
(99%) was from ACROS. Phenol (crystal) was from J. T. Baker.
Toluene (99.9%, Aldrich) was dried by refluxing over sodium
metal for 5 h followed by distillation. Dichloromethane (CH2Cl2,
Fisher Scientific) was distilled from calcium hydride.
Preparation of the brominating beads (3)
Resin-bound methyltriphenylphosphonium bromide (2)
Triphenylphosphine resin (1; 2.00 g, 3 mmol of triphenylpho-
sphine) was swollen in 10 ml of dry toluene under a N2
atmosphere. The flask was sealed with a rubber septum and the
mixture was cooled in an ice-salt bath (ꢀ58C). Bromomethane
(2 M in tert-butyl methyl ether solution; 3 ml, 6 mmol) was added
via syringe and the mixture was stirred at room temperature for
24 h. The resin 2 was filtered and washed three times each with
15 ml aliquots of toluene, CH2Cl2, and ethyl ether. Finally, the resin
was dried under vacuum, yielding 2.22 g (97%), corresponding to
a calculated loading of approximately 1.3 mmol of phosphonium
salt groups per gram of beads.
CONCLUSIONS
In conclusion, we have developed a new brominating reagent
consisting of polymer beads with appended phosphonium
tribromide groups. When swelled, the beads of 3 are able to
brominate efficiently and selectively a variety of substrates. A very
important attribute of this reagent is its ability to be used
repeatedly without removing it from a reaction flask. It is a
rechargeable and robust reactor. Because the products are easily
separated from the beads, work-up procedures are simple and
rapid. Some improvements (e.g., substitution of the liquid for one
which is not halogenated and immobilization of the beads on a
column so that the various steps can be conducted simply by
passing different liquids through it) are possible, and they will be
explored in the future. Other modifications to be explored
include changing the polymer support and the nature of the
phosphonium salt. However, even in their present state, the
beads reported here are a useful, simple tool for bromination
reactions in any laboratory.
Resin-bound triphenylmethylphosphonium tribromide (3)
Methyltriphenylphosphonium bromide resin (2) (0.95 g, 1.2 mmol
of phosphonium groups) was swollen in 10 ml CH2Cl2 for 60 min
and then bromine (0.19 g, 1.2 mmol) dissolved in 1 ml of CH2Cl2
was added. The resulting mixture was stirred for 6 h at room
temperature in a sealed flask. The beads were filtered and
washed three times with CH2Cl2 (20 ml). After being dried in
vacuo, 1.01 g (88%) of orange beads (ꢁ1.1 mmol of tribromide per
gram of beads) were obtained.
Bromination reactions and recycling. General procedures
Beads of 2 (1.0 g, 1.3 mmol) and CH2Cl2 (6 ml) were stirred in a
closed 25 ml round-bottom flask for 60 min. Then, bromine
(0.17 g, 1.06 mmol, representing ca. 80% mol equivalent of the
phosphonium groups in 2) was added and the mixture was
stirred at room temperature for 6 h in the closed flask (to generate
1.06 mmol of resin 3). A substrate (see Table 1 for quantities) was
added to each flask and the reaction mixtures were stirred for
periods that were based upon TLC analyses of periodically
removed aliquots of liquid (TLC: silica gel and hexanes containing
0–10% ethyl acetate depending on the molecules being
separated) as eluent. The liquid was sucked into a pipette or
syringe through a cotton plug and the beads were slurried with
several aliquots of dichloromethane, each of which was removed
as described for the reaction liquid. The combined CH2Cl2 liquids
were reduced to residue (i.e., product and unreacted substrate)
on a rotary evaporator. Finally, the beads were reswollen in CH2Cl2
and reconverted to 3 as described above for the next run. The
sequence of procedures is shown schematically in Fig. 1.
EXPERIMENTAL SECTION
General
1H NMR (referenced to internal TMS) and 13C NMR (referenced to
CDCl3) were recorded on Varian 400 MHz spectrometers inter-
faced to a Sparc UNIX computer using Mercury Software. Gas
chromatography (GC) was performed on a Hewlett-Packard 5890
gas chromatograph equipped with a flame ionization detector, a
0.25 mm Alltech DB-5 (30 m, 0.25 mm) column and a Hewlett-
Packard 3393A integrator. All products reported in the bromina-
tion reactions have been reported in the literature. The yields
reported are based on isolated products and diastereomeric
ratios of the products were determined by comparison of the
areas of diagnostic peaks in 1H NMR spectra that were identified
from literature assignments (see Supporting Information for
references) and, in some cases, also verified by GC analyses by
comparison of retention times with authentic samples.
Reactions with methyltriphenylphosphonium tribromide
(23)
Materials
Unless stated otherwise, reagents were used as received.
Triphenylphosphine resin (1.5 mmol/g of polystyrene with 1%
divinylbenzene as a cross-linker) was from Advanced Chem Tech.
Bromine (ACS grade), bromomethane (2.0 M in anhydrous
tert-butyl methyl ether), cis-stilbene (96%), trans-stilbene (96%),
trans-2-hexene (97%), cis-2-hexene (97%), diphenylacetylene
(98%), styrene (99%), 1-octene (98%), methyltriphenylphonium
bromide, and anisole (99.7%) were from Aldrich. Cyclohexene
Methyltriphenylphosphonium tribromide (23)
Methyltriphenylphosphonium bromide (22; 0.500 g, 1.4 mmol),
bromine (0.224 g, 1.4 mmol), and 14 ml of dichloromethane were
allowed to stand for 6 h in a closed flask. The solvent was
removed at room temperature on a rotary evaporator and the
residue was dried under house vacuum over 12 h to yield 0.65 g
(89% yield) of a dark orange powder. After recrystallization from
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J. Phys. Org. Chem. 2010, 23 904–909

SEQUENTIAL BROMINATION REACTIONS FROM BEADS
dichloromethane, the powder was a yellow flaky solid, mp
143–1478C (dec.).
[17] A. Podgorsek, M. Eissen, J. Fleckenstein, S. Stavber, M. Zupan, J. Iskra,
Green Chem. 2009, 11, 120–126.
[18] M. Eissen, D. Lenoir, Chem. Eur. J. 2008, 14, 9830–9841 and references
cited therein.
General procedure for bromination reactions with
methyltriphenylphosphonium tribromide (23) made and used
in situ
[19] H. Slebocka-Tilk, R. G. Ball, R. S. Brown, J. Am. Chem. Soc. 1985, 107,
4504–4508.
[20] R. Bianchini, C. Chiappe, J. Org. Chem. 1992, 57, 6474–6478.
[21] C. Chiappe, D. Pieraccini, J. Org. Chem. 2004, 69, 6059–6064.
[22] R. Cristiano, K. Ma, G. Pottanat, R. G. Weiss, J. Org. Chem. 2009, 74,
9027–9033.
[23] F. Z. Do¨rwald, Organic Synthesis on Solid Phase, Wiley-VCH, Weinheim,
2000.
[24] K. Smith, D. M. James, I. Matthews, M. R. Bye, J. Chem. Soc. Perkin Trans.
1 1992, 1877–1878.
Bromine (0.224 g, 1.4 mmol) in 1 ml of CH2Cl2 was added to a
solution of methyltriphenylphosphonium bromide (22; 0.50 g,
1.4 mmol) in 7 ml of CH2Cl2 in a 50 ml round-bottom flask. The
flask was closed and the solution was left in the dark for 6 h. Then,
a substrate (see Table S2 of Supporting Information for quantities)
in 1 ml of CH2Cl2 was added and the flask was reclosed and left
without stirring usually until no starting reactant was detectable
by TLC analyses of aliquots (silica gel, with 9/1 hexanes/ethyl
acetate as an eluent). Products were separated by column
chromatography (silica gel, with 9/1 hexanes/ethyl acetate as an
eluent). After removal on a rotory evaporator of the eluent from
fractions containing products, the residues were left under house
vacuum overnight.
´
[25] J. M. J. Frechet, M. J. Farrall, L. J. Nuyens, J. Macromol. Sci. Chem. 1977,
A11, 507–514.
[26] D. J. Abdallah, R. E. Bachman, J. Perlstein, R. G. Weiss, J. Phys. Chem. B
1999, 103, 9269–9278.
[27] D. J. Abdallah, A. Robertson, H.-F. Hsu, R. G. Weiss, J. Am. Chem. Soc.
2000, 122, 3053–3062.
[28] D. J. Abdallah, R. G. Weiss, Chem. Mater. 2000, 12, 406–413.
[29] D. J. Abdallah, L. Lu, M. T. Cocker, R. E. Bachman, R. G. Weiss, Liq. Cryst.
2000, 27, 831–837.
[30] C. Hui, D. C. Kwait, Z. S. Go¨nen, B. T. Weslowski, D. J. Abdallah, R. G.
Weiss, Chem. Mater. 2002, 14, 4063–4072.
[31] D. J. Abdallah, H. C. Wauters, D. C. Kwait, C. L. Khetrapal, G. A.
Nagana Gowda, A. Robertson, R. G. Weiss, Ionic Liquids IIIB: Funda-
mental, Progress, Challenges and Opportunities (ACS Symposium Series
902) (Eds: R. D. Rogers, K. R. Seddon ), American Chemical Society,
Washington, DC, 2005, Chapter 21, pp 303–320.
Acknowledgements
The authors thank the US National Science Foundation for its
˜
support of this research and the Coordenac¸ao de Aper-
feic¸oamento de Pessoal de N´ıvel Superior (CAPES) of Brazil for
[32] K. Ma, B. S. Somashekhar, G. A. Nagana Gowda, C. L. Khetrapal, R. G.
Weiss, Langmuir 2008, 24, 2746–2758.
a fellowship to R. C.
[33] K. Ma, K.-M. Lee, L. Minkova, R. G. Weiss, J. Org. Chem. 2009, 74,
2088–2098.
[34] A. A. Shahkhatuni, K. Ma, R. G. Weiss, J. Phys. Chem. B 2009, 113,
4209–4217.
[35] K. Ma, A. A. Shahkhatuni, B. S. Somashekhar, G. A. Nagana Gowda,
Y. Tong, C. L. Khetrapal, R. G. Weiss, Langmuir 2008, 24, 9843–
9854.
[36] J. Grodkowski, P. Neta, J. Phys. Chem. A 2002, 106, 11130–11134.
[37] A more complete table, showing the sequence of runs and workup
procedures, is provided in the Supporting Information.
[38] J. R. Chretien, J.-D. Coudert, M.-F. Ruasse, J. Org. Chem. 1993, 58,
1917–1921.
REFERENCES
[1] A. De Meijere, F. Diederich (Eds), Metal-catalyzed Cross-coupling
Reactions, 2nd edn, Wiley-VCH, Weinheim, 2004.
[2] Q. Yao, E. P. Kinney, Z. Yang, J. Org. Chem. 2003, 68, 7528–7531.
[3] D. Seyferth, Organometallics 2009, 28, 1598–1605.
[4] P. Rajasingh, R. Cohen, E. Shirman, L. J. W. Shimon, B. Rybtchinski, J.
Org. Chem. 2007, 72, 5973–5979.
[5] P. Suresh, S. Annalakshmi, K. Pitchumani, Tetrahedron 2007, 63,
4959–4967.
[6] Y. Okada, M. Yokozawa, M. Akiba, K. Oishi, K. O-kawa, T. Akeboshi, Y.
Kawamura, S. Inokuma, Y. Nakamura, J. Nishimura, Org. Biomol. Chem.
2003, 1, 2506–2511.
[7] J. P. Das, S. Say, J. Org. Chem. 2002, 67, 7861–7864.
[8] S. Adimurthy, G. Ramachandraiah, A. V. Bedekar, S. Ghosh, B. C. Ranu,
P. K. Ghosh, Green Chem. 2006, 8, 916–922.
[39] G. Bellucci, C. Chiappe, G. Lo Moro, J. Org. Chem. 1997, 62, 3176–3182.
[40] R. S. Brown, Acc. Chem. Res. 1997, 30, 131–137.
[41] M. F. Ruasse, Adv. Phys. Org. Chem. 1993, 28, 207–291.
[42] D. Lenoir, C. Chiappe, Chem. Eur. J. 2003, 9, 1037–1044.
[43] C. Chiappe, F. Del Moro, M. Raugi, Eur. J. Org. Chem. 2001,
3501–3510.
[44] T. X. Wang, M. D. Kelley, J. N. Cooper, R. C. Beckwith, D. W. Margerum,
Inorg. Chem. 1994, 33, 5872–5878.
[9] V. Nair, S. B. Panicker, T. Augustine, T. G. George, S. Thomas, M.
Vairamani, Tetrahedron 2001, 57, 7417–7422.
[45] G. Melloni, G. Modena, U. Tonellato, Acc. Chem. Res. 1981, 14,
227–233.
[10] K. Ma, S. Li, R. G. Weiss, Org. Lett. 2008, 10, 4155–4158.
[11] M. A. Zolfigol, G. Chehardoli, S. Salehzadeh, H. Adams, M. D. Ward,
Tetrahedron Lett. 2007, 48, 7969–7973.
[12] C. Chiappe, E. Leandri, D. Pieraccini, Chem. Commun. 2004,
2536–2537.
[13] S. P. Borikar, T. Daniel, V. Paul, Tetrahedron Lett. 2009, 50, 1007–1009.
[14] G. Bose, E. Mondal, A. T. Khan, M. J. Bordoloi, Tetrahedron Lett. 2001,
42, 8907–8909.
[15] V. Kavala, S. Naik, B. K. Patel, J. Org. Chem. 2005, 70, 4267–4271.
[16] J. Salazar, R. Dorta, Synlett 2004, 7, 1318–1320.
[46] R. Bianchini, C. Chiappe, G. Lo Moro, D. Lenoir, P. Lemmen, N.
Goldberg, Chem. Eur. J. 1999, 5, 1570–1580.
[47] A base, such as Na2CO3 or NaHCO3, is commonly added in similar
reactions to deactivate the HBr byproduct.15,16 Some cleavage of the
methyl ether part of anisole was observed when a tetraalkylpho-
sphonium tribromide was the brominating reagent.10
[48] Prepared from reaction of methyltriphenylphosphonium bromide
and Br2 in CH2Cl2 (see Supporting Information). Refer to H.-J. Cristau,
A. Bazbouz, P. Morand, E. Torreilles, Tetrahedron Lett. 1986, 27,
2965–2966 and references cited therein.
J. Phys. Org. Chem. 2010, 23 904–909
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