56 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 56†
Debromination of a-Bromo Ketones using Polymer-
supported Triphenylphosphine†
Sameer P. Dhuru, Kamlesh J. Padiya and M. M. Salunkhe*
Department of Chemistry, The Institute of Science, 15, Madam Cama Road, Mumbai - 400 032,
India
An effective method for the debromination of a-bromo ketones using polymer-supported triphenylphosphine is
described.
The use of polymeric reagents in organic synthesis has been a
subject of substantial interest in recent years.1 Among the
several advantages offered by these reagents,2 the one most
frequently utilized is the ease of work-up, which often con-
sists of a simple filtration. This feature is of more importance
especially when the reaction products are toxic or noxious
and when they are unstable to lengthy and tedious work-ups.
We have developed a mild and high-yielding method for the
debromination of a-bromo ketones using a polymer-sup-
ported reagent. Polymer-supported triphenylphosphine has
been earlier used in a number of reactions such as in the
conversion of alcohols into alkyl halides,3 the Wittig reaction4
and the conversion of aromatic disulfides into thiols.5 The
polymeric phosphine oxide obtained as a by-product can be
recycled after reduction to phosphine.3,4
The debromination of a-bromo ketones has been exten-
sively studied and the mechanism carefully detailed.6–8 How-
ever, separation of the ketone from the triphenylphosphine
oxide by-product and any excess of phosphine is difficult.
Also, the yields obtained in the above reactions are con-
siderably low.
Table 1 Debromination of a-bromo ketones
Time
a-Bromo ketones
Product
(t/min) % Yielda
Phenacyl bromide
Acetophenone
45
96 (64)
85 (53)
89 (62)
97 (51)
p-Nitrophenacyl bromide p-Nitroacetophenone 30
2-Bromocyclohexanone Cyclohexanone
3-Bromocamphor Camphor
50
40
aFigures in parentheses indicate percentage yields in
solution-phase reactions.
Various a-bromo ketones were investigated, as outlined in
Table 1. The yields indicated are for isolated products with-
out additional purification. In all cases the isolated products
were found to be homogeneous by TLC and were charac-
terised by IR and NMR spectroscopy.
In summary, the use of polystyryldiphenylphosphine repre-
sents a very effective procedure for the debromination of
a-bromo ketones. The reaction offers improved yields and a
convenient isolation procedure.
We report here the debromination of a-bromo ketones
using polymer-supported triphenylphosphine as shown in
Scheme 1. To a solution of the bromo ketone in anhydrous
benzene is added the insoluble phosphine reagent. This
results in the formation of the phosphonium salt which is
decomposed by alcohol to yield the corresponding ketone.
The yield of product and the rate of reaction, when carried
out in toluene, acetonitrile, THF, etc. were found to be very
low. Polymeric phosphine oxide is obtained as a by-product,
which is separated by filtration. Removal of the solvent under
reduced pressure yields the pure ketone.
Experimental
General Procedure for the Debromination of a-Bromo Keto-
nes.sThe a-bromo ketone (0.125 mmol) was dissolved in anydrous
benzene (10 ml). Polymer-bound triphenylphosphine (50 mg; 2%
crosslinked with DVB, 3 mmol gꢀ1) was added to form the phos-
phonium salt after which methanol (1 ml) was added and the
reaction mixture was refluxed. When the reaction was complete
(TLC), the reaction mixture was cooled, polymeric phosphine
oxide was filtered off and the solvent was removed under reduced
pressure to give the product ketone in pure form.
We are thankful to Dr N. V. Thakkar, Institute of Science,
Mumbai, for helpful discussions.
R
C
O
+
PPh2
P
Received, 7th July 1997; Accepted, 29th September 1997
Paper E/7/04778I
CH Br
R′
R
O–
R′
R
References
+
+
C
+
PPh2O
C
PPh2 CH
P
P
[
PPh2Br]
or
P
1 For recent reviews see: Polymers as Aids in Organic Chemistry, ed.
N. K. Mathur, C. K. Narang and R. E. Williams, Academic Press,
New York, 1980; Polymer Supported Reactions in Organic Synthesis,
ed. P. Hodge and D. C. Sherrington, Wiley, London, 1980; A.
Akelah and D. C. Sherrington, Chem. Rev., 1981, 81, 1557; A.
Akelah, Synthesis, 1981, 413; Functionalised Polymers and Their
Applications, ed. A. Akelah and A. Moet, Chapman and Hall,
1990.
CH
CH
C
O
R′
R′
Br–
R
Br–
MeOH
O
MeOH
R
C
O
+
PPh2 + MeBr
P
CH2
2 J. M. J. Frechet, Tetrahedron, 1981, 37, 663.
R′
3 L. R. Steven and P. L. Dan, J. Org. Chem., 1975, 40, 1669.
4 M. Bernard and W. T. Ford, J. Org. Chem., 1983, 48, 326.
5 R. A. Amos and S. M. Fawcett, J. Org. Chem., 1984, 49, 2637.
6 I. J. Borowitz and L. I. Grossmann, Tetrahedron Lett., 1962, 11,
471.
Scheme 1
*To receive any correspondence.
7 I. J. Borowitz and R. Virkhaus, J. Am. Chem. Soc., 1963, 85,
2183.
†This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
8 P. A. Chopard, R. F. Hudson and G. Klopman, J. Chem. Soc.,
1965, 1379.