H. Lahrache et al. / Tetrahedron Letters 46 (2005) 1635–1637
1637
Table 2. Reaction of a,b-acetylenic phosphonic acid monoesters with (biscollidine)bromine(I) and iodine(I) hexafluorophosphate
Entry
a,b-Acetylenic phosphonate 5
Products 10, 11
Reaction with HBB (yield, %)
Reaction with HBI (yield, %)
O
a
(44)
(23)
I
Br
P OEt
OH
O
b
(17)
Br
NR
C5H11
C5H11
P OEt
OH
R1
R2
X
coll
O P O
(coll)2X+ PF6
-
R1
R2
+
X
R1
OEt
- PO2OEt
R1
R2
O
OEt
OEt
OH
P
P
O
R2
X
X
+
HO
R1
R2
O
OEt
P
HO
Scheme 3.
and iodine(I) hexafluorophosphate were carried out in
methylene chloride at room temperature.6 After reac-
tion, the unsaturated halides were isolated by liquid
chromatography over silica gel (Scheme 2) and charac-
terized from their spectral data and by comparison with
those reported in the literature. The instable ethyl meta
phosphonate could not be isolated. Our results are col-
lected in Table 1.
substituents. In particular, formation of b-lactones was
observed when the substituents were alkyl groups and
spontaneous elimination of carbon dioxide occurred in
the case of aryl groups.7 With phosphonates we were
never able to detect the oxaphosphetane intermediate
formation. However, we cannot completely exclude
their formations due to their probably low stability at
room temperature. The intervention of carbocations as
intermediates is also possible (Scheme 3). Further exper-
iments are necessary to choose between these two
mechanisms.
With 2,2-disubstituted a,b-ethylenic phosphonates 2a–d
we observed the formation of vinyl halides in satisfac-
tory yields (entries a–d). In the particular case of phos-
phonate 2c, the reaction with (biscollidine)bromine(I)
hexafluorophosphate led to the formation of three prod-
ucts (Scheme 2), two of them corresponding to a double
reaction of the bromonium reagent. These side products
were partially avoided when the reaction was carried out
in the presence of 3,5-ditert-butylphenol (5 mol %).
Reaction of vinyl bromide 6c with the bromonium
reagent did not lead to the formation of compounds 8
or 9. This result suggests that compound 8 is probably
formed first by allylic bromination of the phosphonate
2c, followed by a dephosphorylation, the dibromo com-
pound 9 being probably formed by a radical induced
allylic rearrangement of the dibromo compound 8. In
the case of 1-monosubstituted a,b-ethylenic phospho-
nates 2e–g low yields were observed if the substituents
were alkyl or alkenyl groups (entries f and g). However,
yield was satisfactory if the substituent was an aryl
group (entry e). These dephosphorylations were found
to be diastereoselective (entries c and h–i). We checked
also the reactivity of 1-alkynyl phosphonates. Our re-
sults are reported in Table 2. Low yields were obtained
in our conditions. These results are very different to
those that we published in the case of carboxylic acids.1
In conclusion we report for the first time the halo-
dephosphorylation of a,b-unsaturated phosphonate
monoesters. These reactions led to the formation of
a,b-unsaturated bromides or iodides and can probably
be useful in synthesis.
References and notes
1. Homsi, F.; Rousseau, G. Tetrahedron Lett. 1999, 40, 1495–
1498.
2. Kiddle, J. J.; Babler, J. H. J. Org. Chem. 1993, 58, 3572–
3574.
3. Rabinowitz, R. J. Am. Chem. Soc. 1960, 82, 4564–
4567.
4. Harris, R. L. N.; McFadden, H. G. Aust. J. Chem. 1984, 37,
417–424.
5. Yokomatsu, T.; Shioya, Y.; Iwasawa, H.; Shibuya, S.
Heterocycles 1997, 46, 463–472.
6. Representative procedure: To a solution of phosphonate
monoester 2 (1 mmol) in methylene chloride (10 mL) was
added over 2 h a methylene chloride solution (10 mL) of
bis(collidine)bromine(I) hexafluorophosphate (1.3 mmol).
After complexion of the addition, the solvent was removed
under vacuum and the residue purified by liquid chroma-
tography over silica gel (petroleum ether–ether).
7. Homsi, F.; Rousseau, G. J. Org. Chem. 1999, 64, 81–
85.
We found that the mechanism of the halodecarboxyla-
tion of carboxylic acids depends on the nature of the