Facile and efficient method for the debromination of vic-dibromides to alkenes with BiCL3/Ga system

Article abstract of DOI:10.1080/00397911.2010.542568

BiCl₃/Ga was found to be a mild, efficient, and chemoselective system for the debromination of a series of functionally and structurally various vic-dibromides. A broad range of functional groups (ester, carboxy, aldehyde, methoxy, chloro, and ketone) was tolerated under the reaction conditions and only trans olefins were obtained in good yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.542568

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Facile and Efficient Method for the
Debromination of vic-Dibromides to
Alkenes with BiCl /Ga System
3
a
a
a
Byung Woo Yoo , Seo Hee Kim & Young Kwang Park
a
Department of Advanced Materials Chemistry , Korea University ,
Jochiwon , Chungnam , Korea
Accepted author version posted online: 17 Nov 2011.Published
online: 03 Feb 2012.
To cite this article: Byung Woo Yoo , Seo Hee Kim & Young Kwang Park (2012) Facile and Efficient
Method for the Debromination of vic-Dibromides to Alkenes with BiCl /Ga System, Synthetic
3
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:11, 1632-1636, DOI: 10.1080/00397911.2010.542568
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Synthetic Communications , 42: 1632–1636, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.542568
FACILE AND EFFICIENT METHOD FOR THE
DEBROMINATION OF vic-DIBROMIDES TO ALKENES
WITH BiCl /Ga SYSTEM
3
Byung Woo Yoo, Seo Hee Kim, and Young Kwang Park
Department of Advanced Materials Chemistry, Korea University,
Jochiwon, Chungnam, Korea
GRAPHICAL ABSTRACT
3
Abstract BiCl /Ga was found to be a mild, efficient, and chemoselective system for the deb-
romination of a series of functionally and structurally various vic-dibromides. A broad
range of functional groups (ester, carboxy, aldehyde, methoxy, chloro, and ketone) was tol-
erated under the reaction conditions and only trans olefins were obtained in good yields.
Keywords Alkene; bismuth; debromination; gallium
INTRODUCTION
The combination of bromination and reductive debromination has been used
to temporarily protect double bonds, to purify olefins, and to introduce a new double
[1]
bond in organic synthesis. Although reductive debromination of vic-dibromides
with various agents has been the subject of many reports, some of these methods
are associated with limitations regarding poor yields, prolonged reaction times,
[2]
and harsh reaction conditions. Consequently, an efficient and mild procedure for
debromination of vic-dibromides continues to be developed. The chemical reactivity
of the BiCl =M system (M=Zn, Fe, Al, Mg, Sm) has been the subject of considerable
3
[
3]
interest, and the reducing ability of these systems has been extensively studied.
Because of the close resemblance of gallium to zinc in several respects, including first
ionization, we reasoned that a BiCl =Ga system could serve as a protocol for the
3
debromination of vic-dibromides. In recent years, gallium metal has drawn increas-
ing attention for its unique properties such as low toxicity and high stability in water
[4]
and air compared with other metals. Bismuth(III) chloride is inexpensive, relatively
Received August 24, 2010.
Address correspondence to Byung Woo Yoo, Department of Advanced Materials Chemistry,
Korea University, Jochiwon, Chungnam 339-700, Korea. E-mail: bwyoo@korea.ac.kr
1
632

DEBROMINATION OF vic-DIBROMIDES
1633
nontoxic, fairly water insensitive, and environmentally benign and has been used as a
mild Lewis acid catalyst for synthetic transformations.
RESULTS AND DISCUSSION
In continuation of our interest in exploring the utility of metal–metal salt sys-
[5]
tem in organic synthesis, we report here an efficient and mild method for debromi-
nation of vic-dibromides 1 to alkenes 2 with BiCl =Ga system in tetrahydrofuran
3
(THF) at room temperature. The reaction can be generalized as in Eq. (1).
The new reagent system was generated by the addition of gallium metal to a stirred
solution of bismuth(III) chloride in THF under sonication. Sonication were carried
out in a BRANSONIC ultrasonic cleaner bath, which delivered a 47 kHz wave, with
a fixed electrical power of 125 Watts. To gauge the scope and limitations of this
methodology, various substituted vic-dibromides were subjected to this protocal at
room temperature. We found that vic-dibromides on treatment with BiCl =Ga sys-
3
tem in THF solution are smoothly converted into the corresponding alkenes in good
yields. The good yields of the debromination products demonstrate the efficiency of
this new method. To ensure the role of gallium, a controlled experiment was carried
out using vic-dibromides with BiCl without gallium metal, which failed to yield any
3
desired product. A 2:1 ratio of gallium and BiCl was the best ratio in terms of yield
3
and reaction time. THF has been found to be the most suitable solvent for the reac-
tion. The results of the reaction are summarized in Table 1, which shows a series of
functionally and structurally various vic-dibromides. Clearly, a broad range of func-
tional groups (ester, carboxy, aldehyde, methoxy, chloro, and ketone) were tolerated
under the reaction conditions and only trans olefins were obtained. The sensitive car-
bonyl group remains intact without any further reduction under the reaction con-
dition (entries 4–10). In comparison with other procedures, the present procedure
reduced vic-dibromides in better yields and showed a good chemoselectivity under
mild conditions. The notable advantages of this methodology are mild reaction con-
ditions, simple manipulation, good yields, and tolerance of various functional
[
6]
groups. No overreduction of the produced alkene was observed with any substrate.
Some of the vic-dibromides in the table are commercially available or could be pre-
pared conveniently from the corresponding precursors through known olefin hal-
ogenation reactions.
[7]
Although the reaction mechanism is still not clearly
understood at this time, the reaction can be envisaged to proceed in two stages. In
the first, bismuth(III) chloride is probably reduced by gallium to form a low-valent
bismuth species, which in the subsequent step would debrominate vic-dibromides 1
to give the corresponding alkenes 2 through a SET (single-electron transfer) process.
The reducing property exhibited by metal–metal salt combinations proceeds
through transfer of one electron from the metal surface to the substrate. In such

1634
B. W. YOO, S. H. KIM, AND Y. K. PARK
3
Table 1. Reductive debromination of vic-dibromides with BiCl =Ga system
a
Entry
1
Substrate
Product
Time (h)
Yield (%)
93
0.5
2
3
0.5
0.5
96
94
4
1.5
88
5
6
1.5
1.0
91
87
7
0.5
0.5
86
94
8
9
0.5
1.0
0.5
91
85
92
1
1
0
1
a
Isolated yields. The products are commercially available and were characterized by comparision of
their spectral data with authentic samples.

DEBROMINATION OF vic-DIBROMIDES
1635
combinations, the elementary metal part needs to be more electropositive than the
metal part of the salt. The involvement of zero-valent bismuth resulting from the
[
8]
transmetallation of bismuth chloride with gallium metal may be presumed.
We have been able to demonstrate the utility of the BiCl =Ga system for effecting
chemoselective debromination of the vic-dibromides.
3
CONCLUSION
In summary, we have demonstrated that the BiCl =Ga system mediates an
3
efficient and mild conversion of vic-dibromides to the corresponding alkenes.
Although the scope and limitations were not fully established, the present method
could be a practical alternative to the conventional method. Further work on the
application of the BiCl =Ga system is in progress.
3
EXPERIMENTAL
All reagents were used as received. THF was distilled from sodium benzophe-
1
none immediately prior to use. The H NMR spectra were recorded on a FT-Bruker
1
13
AF-300 instrument (300 MHz for H NMR; 75 MHz for C NMR) using tetra-
methylsilane (TMS) as an internal standard. Infrared (IR) spectra were obtained
on a Perkin-Elmer 16 F PC FT-IR Shimadzu. Thin-layer chromatrographic (TLC)
analysis was performed on silica-gel plates (Merck, 60 F-254). All products were pur-
ified by flash column chromatography using silica gel 60 (79–230 mesh, Merck).
A typical procedure for the debromination of vic-dibromides is as follows: Gal-
lium (140 mg, 2.0 mmol) and bismuth(III) chloride (315 mg, 1.0 mmol) were mixed in
THF (4 mL). The resulting mixture was stirred at room temperature for 0.5 h under
[
8]
sonication to produce a solution of the low-valent bismuth–gallium complex.
,2-Dibromo-1,2-diphenylethane (170 mg, 0.5 mmol) was then added to this sol-
1
ution. The mixture was stirred for 0.5 h at room temperature, and the progress of
the reaction was followed by TLC. After completion of the reaction, the solvent
was removed under reduced pressure. The residue was extracted with ether and
washed successively with brine. The organic layer was separated and dried over
anhydrous Na SO . The crude product was purified by silica-gel column
2
4
chromatography (hexane–ethyl acetate ¼ 15:1) to afford trans-stilbene (85 mg, 94%).
ACKNOWLEDGMENT
This work was supported by a Korea University Grant.
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