Phase-vanishing reactions that use fluorous media as a phase screen. Facile, controlled bromination of alkenes by dibromine and dealkylation of aromatic ethers by boron tribromide

Article abstract of DOI:10.1021/ja027965y

In fluorous triphasic reactions, such as bromination of alkenes by dibromine and dealkylation of aromatic ethers by boron tribromide, the middle fluorous phase acts as a liquid membrane permitting passive transport of the reagents at the bottom to the top layer involving the substrates, thereby regulating the reactions. Copyright

Full text of DOI:10.1021/ja027965y

Published on Web 10/12/2002
Phase-Vanishing Reactions that Use Fluorous Media as a Phase Screen.
Facile, Controlled Bromination of Alkenes by Dibromine and Dealkylation of
Aromatic Ethers by Boron Tribromide
Ilhyong Ryu,*,† Hiroshi Matsubara, Shinji Yasuda, Hiroyuki Nakamura, and Dennis P. Curran
†
†
‡
‡
Department of Chemistry, Faculty of Arts and Sciences, Osaka Prefecture UniVersity, Sakai,
Osaka 599-8531, Japan, and Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
Received August 1, 2002
Ongoing work in the fluorous field focuses on the reaction and
separation features of fluorous reagents, catalysts, substrates, and
other fluorous reaction components.1 Herein we report a new
synthetic technology founded on the unusual properties of fluorous
media. We use no fluorous reaction components. We simply
capitalize on the physical phenomena that most organic and fluorous
media are immiscible, that fluorous media are generally heavier
than most organic media, but that many useful organic and inorganic
reagents are even heavier than fluorous media.
The concepts of the “phase-vanishing method” are illustrated in
Scheme 1. When a fluorous solvent such as perfluorohexanes (d
1.67) is mixed with a heavier reagent, two phases result with
Figure 1. Phase-vanishing bromination of cyclohexene. The arrows indicate
the interface of phases. (a) Triphasic system (from the bottom: bromine,
FC-72, hexane solution of cyclohexene). (b) After 2 days, the bromine layer
disappeared.
)
the reagent on the bottom. When an organic solvent containing a
substrate is added, the fluorous solvent then locates in the middle,
screening the two otherwise miscible phases containing the substrate
Table 1. Phase-Vanishing Bromination of Alkenes
2
and the reagent from each other. In this triphasic reaction, the
fluorous “phase screen” is a liquid membrane that prevents mixing
but permits passive transport of reagents from the bottom layer to
the top layer and thereby regulates the reaction. As the reagent is
consumed, the bottom phase vanishes. Our preliminary experiments
suggest that this phase-vanishing method based on spontaneous
reaction control by gravity and diffusion is indeed useful for typical
synthetic reactions such as bromination of alkenes and dealkylation
of aromatic ethers by boron tribromide.
Scheme 1. Concept of the Phase-Vanishing Method
We examined phase-vanishing bromination of alkenes as a
model. Standard bromination requires careful control of addition
rate and temperature to avoid undesirable side reactions.3 FC-72
,4
(perfluorohexanes) was placed in a test tube, and then bromine was
introduced slowly by using a glass pipet. The heavier bromine sank
to the bottom, forming two layers. A hexane solution of cyclohexene
was then slowly added. This floated on top of the FC-72 layer,
thus forming a triphasic system (Figure 1). The color of FC-72
gradually turned to wine red, and after 2 days the bromine layer
disappeared. The hexane layer was decanted, washed with aqueous
a Conditions: alkene (2 mmol), hexane (1.5 mL), Br (2 mmol), FC-72
2
(1.5 mL), room temperature, with aluminum foil protection from light.
Method A, 2 days without stirring; Method B, 4 h (5 h for entry 3) with
b
gentle stirring. Isolated yield for product from hexane layer by silica gel
chromatography. c Reaction was conducted on an 11 mmol scale. d Refer
1
to the ratio of 2R*3R* to 2R*3S*. Estimated by H NMR.
2 2 3 4
Na S O , and dried over MgSO . Purification by short column
chromatography on silica gel with hexane gave trans-1,2-dibro-
mocyclohexane in 81% yield (Table 1, entry 1).
Table 1 demonstrates the generality of the phase-vanishing
bromination of alkenes. Gentle stirring of the bromine layer, taking
*
To whom correspondence should be addressed. E-mail: ryu@ms.cias.osakafu-
u.ac.jp.
†
Osaka Prefecture University.
‡
University of Pittsburgh.
12946
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J. AM. CHEM. SOC. 2002, 124, 12946-12947
10.1021/ja027965y CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Comparison of “Phase Screen” Ability for Bromination
Scheme 2. Phase-Vanishing Demethylation by Boron Tribromide
phase screen
density (g/cm3)
methoda
yieldb
ation of anisole and two substituted anisoles also produced the
corresponding phenols in 76-98% yield.
H2O
1.0
A
B
A
A
3%
54%
32%c
81%
In summary, we have demonstrated that a fluorous phase can
screen two otherwise miscible heavier and lighter organic phases
in the bromination of alkenes and the dealkylation of aromatic ethers
by boron tribromide. The fluorous phase acts as a liquid membrane
regulating the exchange between the two organic phases by passive
diffusion. The method may be suitable for controlling heat evolution
in exothermic reactions, especially on a large scale. This is usually
done by cooling and/or by slow addition of one of the components.
Cooling consumes energy, and both cooling and slow addition can
require complex engineering solutions. In contrast, heat evolution
in these fluorous phase screening reactions is regulated by the rate
of transport of the reagent, which in turn depends on readily tunable
features including its solubility in the fluorous phase, the volumes
and surface areas of the phases, and mixing method. Further
applications of this method will be reported in due course.
CH3CN
C6F14
0.79
1.67
a
Conditions: cyclohexene (4 mmol), hexane (1.5 mL), Br2 (2 mmol),
phase screen (1.5 mL), room temperature, 2 days (method A) or 4 h with
gentle stirring (method B) with aluminum foil protection from light.
Isolated yield for product from hexane layer by silica gel chromatography.
b
c
Acetonitrile layer contained 37% yield of dibromocyclohexane.
care not to mix the three layers, proved to considerably accelerate
the reaction. For example, bromination of cyclohexene was
complete within 4 h, giving trans-1,2-dibromocyclohexane in 90%
isolated yield (entry 2). Without shielding the reaction from light,
cyclohexyl bromide was generated in 36% yield along with trans-
,2-dibromocyclohexane in 60% yield. Not only cyclic alkenes
entries 1-5) but also aliphatic alkenes (entries 6-10) underwent
5
1
(
bromination to give the corresponding dibromides in high yields.
In every case, completion of the reaction was noticed by the
disappearance of the bromine phase and recovery of the transpar-
ency of the fluorous layer.6
The ability of the fluorous phase to act as a phase screen was
compared with that of acetonitrile and water, because these solvents
can also create three phase systems with bromine and hexane (Table
Acknowledgment. The Pittsburgh group thanks the National
Institutes of Health for support.
Supporting Information Available: Experimental details and
characterization data (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
2
). The use of water as a middle layer was inefficient without
stirring to give trans-1,2-dibromocyclohexane in a very poor yield
3%), suggesting that the diffusion of bromine in water is sluggish.
In the case of acetonitrile, dibromocyclohexane was obtained in
2% from the upper hexane layer, but a comparable amount of the
(
1) Recent reviews: (a) Horv a´ th, I. T. Acc. Chem. Res. 1998, 31, 641. (b)
Curran, D. P. Angew. Chem., Int. Ed. 1998, 37, 1175. (c) Cornils, B.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2057. (d) Kitazume, T. J. Fluorine
Chem. 2000, 105, 265. (e) Furin, G. G. Russ. Chem. ReV. 2000, 69, 491.
(
(
f) Curran, D. P. In Stimulating Concepts in Chemistry; V o¨ gtle, F.,
3
Stoddard, J. F., Shibasaki, M., Eds.; Wiley-VCH: New York, 2000. (g)
Symposium-in-Print on fluorous chemistry. Gladysz, J.; Curran, D. P.
Tetrahedron 2002, 58, 3823 and succeeding papers.
product was found to be dissolved in the middle acetonitrile layer.
These observations suggest that features of reagent transportation
and extrusion of organic products are superior for fluorous media.
To show that the fluorous phase screening method is applicable
to other exothermic reactions, we focused on boron tribromide
(2) (a) Nakamura, H.; Linclau, B.; Curran, D. P. J. Am. Chem. Soc. 2001,
1
23, 10119. (b) Luo, Z.; Swaleh, S. M.; Theil, F.; Curran, D. P. Org.
Lett. 2002, 4, 2585.
(3) For the previously established procedures of bromination of alkenes, see:
(
a) Org. Synth. Coll. 1941; Vol. 1, p 521. (b) Org. Synth. Coll. 1943;
Vol. 2, p 171.
(BBr
3
), which is a powerful reagent for dealkylation of ethers and
(4) For bromination in ionic liquids, see: Chiappe, C.; Capraro, D.; Conte,
V.; Pieraccini, D. Org. Lett. 2001, 3, 1061.
is heavier than FC-72. BBr
and low temperatures (-78 and 0 °C) are typically employed. A
3
dealkylations are highly exothermic,
(
5) The polybromination course is negligible in our system, which often
becomes a problematic side reaction. For a related reference, see:
MacMillen, D. W.; Grutzner, J. B. J. Org. Chem. 1994, 59, 4516.
6) Like all slow addition techniques (for example, syringe pump additions),
the maximum benefits are reaped when the reaction is much faster than
the transport as in the case of the present bromination. Because reagents
have some solubility in the fluorous phase, the disappearance of the bottom
phase may not strictly correspond to the end of the reaction even for fast
reactions. Yet it is a good qualitative guide that the reaction is approaching
completion.
7
3
dry, inert atmosphere is required because BBr is highly moisture
(
sensitive, so water and acetonitrile are not potential phase screens.
We found that the present phase-vanishing method is advantageous
for reactions using boron tribromide (Scheme 2). A triphasic mixture
consisting of a dichloromethane layer containing R-methoxynaph-
thalene, FC-72, and boron tribromide was kept overnight at room
temperature. The bottom phase vanished, and R-naphthol was
obtained from the dichloromethane layer in 94% yield. Demethyl-
(7) (a) McOmie, J. F. W.; Watts, M. L.; West, D. E. Tetrahedron 1968, 24,
289. (b) Benton, F. L.; Dillon, T. E. J. Am. Chem. Soc. 1942, 64, 1128.
2
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