A mild, efficient, and selective debromination of vic-dibromides to alkenes with Cp2TiCl2/Ga system

Full text of DOI:10.5012/bkcs.2010.31.10.2757

Communications to the Editor
Bull. Korean Chem. Soc. 2010, Vol. 31, No. 10 2757
DOI 10.5012/bkcs.2010.31.10.2757
A Mild, Efficient, and Selective Debromination of vic-Dibromides
to Alkenes with Cp
2
TiCl /Ga System
2
*
Byung Woo Yoo, Seo Hee Kim, and Jun Ho Kim
Department of Advanced Materials Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea
*
E-mail: bwyoo@korea.ac.kr
Received July 29, 2010, Accepted August 24, 2010
Key Words: Debromination, Alkene, Bis(cyclopentadienyl)titanium(IV) dichloride, Gallium
The combination of bromination and reductive debromi-
nation has been used to temporarily protect double bonds, to
purify olefins, and to introduce a new double bond in organic
a controlled experiment was carried out using vic-dibromides
with Cp TiCl without gallium metal, which failed to yield any
desired product. The optimum molar ratio of vic-dibromide to
Cp TiCl to gallium (1:1:2) is found to be ideal for the complete
2
2
1
synthesis. To date, a number of methods for effecting debromi-
2
2
nation have been developed in this regard, but most of these
methods are associated with limitations regarding chemoselec-
tivity, low yields, long reaction times, and elevated tempera-
conversion of vic-dibromide 1 into alkene 2 in terms of yield
and reaction time. And THF has been found to be the most suit-
able solvent for the reaction. Under sonication the reactions
proceeded smoothly and efficiently with high yields. Thus, ultra-
sonic irradiation is necessary to accelerate the reaction. The result
of the reaction is summarized in Table 1, where we examined
a series of functionally and structurally various vic-dibromides.
It is evident from the table that the reductive debrominations
are chemoselective as aldehyde, keto, chloro, methoxy, carboxy
and ester groups remained unaffected under the reaction con-
ditions. Only trans olefins were obtained and no overreduction
2
tures. Consequently a mild and efficient procedure for de-
bromination of vic-dibromides continues to be developed. The
chemical reactivity of Cp
2
TiCl /M system (M = In, Zn, Mg,
2
Al, Sm) has been the subject of considerable interest and the
3
reducing ability of these systems has been extensively studied.
Because of the close resemblance of gallium to indium and zinc
in several respects, including first ionization potential, we con-
sidered that Cp
2
TiCl /Ga system could also serve as a protocol
2
9
for the debromination of vic-dibromides. In recent years gallium
has drawn an increasing attention for its unique properties such
as low toxicity and high stability in water and air compared with
of the produced alkene was observed with any substrate. It was
worth commenting that the sensitive carbonyl group remains
intact without any further reduction under the reaction condi-
tion (entries 4-11). In comparison with other procedures, the pre-
sent procedure reduced vic-dibromides more rapidly in higher
yields and showed a good chemoselectivity. The major advan-
tages of this methodology are mild reaction conditions, fast
reaction time, high yields, and tolerance of various functional
groups compared with existing reagents. Some of the vic-di-
bromides in the table are commercially available, or could be
prepared conveniently from the corresponding precursors th-
4
other metals. As in the case of indium, the reduction potential
o
3+
of gallium is not too negative (Ga: E , Ga /Ga = ‒0.56 V; In:
o
3+
o
2+
E , In /In = ‒0.345 V; Zn: E , Zn /Zn = ‒0.76 V) and thus it
is not sensitive to water and does not form oxides readily in air.
In continuation to our interest in exploring the utility of metal-
5
metal salt system in organic synthesis, we report herein a mild
and efficient procedure for debromination of vic-dibromides 1
to alkenes 2 with Cp
2
TiCl /Ga system. The reaction can be
2
6
10
generalized as in eq. 1. The new reagent system was generated
by the addition of gallium to a stirred solution of bis(cyclo-
rough known olefin halogenation reactions. Although the
role of bis(cyclopentadienyl)titanium(IV) dichloride is still not
clearly understood at this time, it is likely that reduction of
titanium(IV) with gallium provides low valent titanium, which
reacts with vic-dibromides 1 to give the corresponding alkenes
2. The reducing property exhibited by metal-metal salt combi-
nations proceeds through transfer of one electron from the metal
surface to the substrate. In such combinations elementary metal
part needs to be more electropositive than the metal part of the
salt. The involvement of low valent titanium resulting from
the transmetallation of bis(cyclopentadienyl)titanium(IV) di-
7
pentadienyl)-titanium(IV) dichloride in THF under sonication.
To guage the scope and limitations of this methodology, vari-
ous substituted vic-dibromides were subjected to this protocal
at room temperature. We have found that vic-dibromides on
treatment with Cp
2
TiCl /Ga system are smoothly converted
2
into the corresponding alkenes in good to high yields. It has
been reported that Cp TiCl /Zn system can be used for a reduc-
tive debromination of vic-dibromides to alkenes. In compari-
2
2
8
son with Cp
2
TiCl
2
/Zn system, the reaction using Cp
2
TiCl /Ga
2
11
system are fast and high-yielding. The high yields of the de-
bromination products and short reaction time demonstrate the
efficiency of this new method. To ensure the role of gallium,
chloride with gallium metal may be presumed. We have been
able to demonstrate the utility of Cp
2
TiCl /Ga system for effect-
2
ing chemoselective debromination of vic-dibromides under the
mild conditions.
Br Br
In summary, we have discovered that Cp
2
TiCl /gallium
2
Cp TiCl /Ga
2
2
C
C
C
C
system represents a mild and efficient protocol for reducing
vic-dibromides to the corresponding alkenes with high yields.
Although the scope and limitations were not fully established,
THF, rt
(1)
1
2

2
758 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 10
Communications to the Editor
Table 1. Reductive debromination of vic-dibromides with Cp
2
TiCl
2
/
the present procedure offers an attractive alternative to the
conventional methods with its mildness, rapidity and chemo-
selectivity as well as high yields. Further investigations of more
useful applications are in progress.
Ga system.
Time Yield
Entry
1
Substrate
Product
a
(
min) (%)
Br
Acknowledgments. This work was supported by Korea Uni-
Br
10
10
10
10
25
15
25
20
10
20
25
10
95
96
94
93
92
89
86
85
91
90
88
89
versity Grant.
Cl
Cl
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4
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