tetrafluoroborates11 and H-phosphonates. The reaction pro-
ceeds well at room temperature to afford coupling products
in good yields.12,13 The starting (E)-vinyliodoniums 2a-e
were easily prepared from the vinyltrimethylsilyl derivatives
1a-d as shown in Table 1 using a modified Ochiai’s
method.11a
followed by silica gel chromatography. Using these condi-
tions, however, 2d remained contaminated by ca. 15% of
impurities.15 Iodonium salts 2f-h were too unstable to be
purified and/or handled satisfactorily.
With the required iodonium salts 2a-e in hand, we turned
our attention to their coupling with various H-phosphonates,
at room temperature, to afford 2-arylvinylphosphonates. As
a model, we first studied the coupling of dimethyl phosphite
with 2a using the conditions recommended for coupling
H-phosphonates with aryl iodides (presence of a base and
DMF-containing solvent). In contrast to the latter reaction
which proceeds under heating,16 we were pleased to observe
that, as shown by 31P NMR monitoring, significant conver-
sion already occurred after 4h at 20 °C (Table 2, entry 1),
Table 1. Preparation of (E)-2-Arylvinyl(phenyl)iodonium
Tetrafluoroborates
Table 2. Optimization of the Reaction Conditionsa
purification
procedurea
yieldb
(%)
entry
R reactant
H, 1a
o-F, 1b
o-Me, 1c
m-OMe, 1d
p-CHdCHSiMe3, 1e
p-OMe, 1f
p-NO2, 1g
product
1
2
3
4
5d
6
7
8
A
A
A
A
B
C
C
C
2a
2b
2c
2d
2ee
2f
82
79
80
66c
85
<20f
0g
entry
catalyst (equiv)
Pd(PPh3)4 (0.1)
CuI (0.3)
CuI (0.3)
CuI (0.3)
CuI (0.3)
ligand
base
yieldb (%)
2g
2h
1
2
3
4
5
6
7
K2CO3
20
0
0
o-Br, 1h
0g
a A, trituration of the crude residue obtained after DCM evaporation in
Et2O then in Cy followed by SiO2 column chromatography; B, washing of
the precipitate with CH2Cl2; C, trituration in Cy. b Yield in product obtained
after the purification procedure. c Accompanied by ca. 15% of impurities.
d Three equivalents of PhIO and Et3O+ BF4- were used. e The bis-iodonium
derivative was obtained (see Scheme 1). f Decomposed rapidly at room
temperature. g Decomposed totally during the purification.
Lc
TMEDA
TMEDA K2CO3 >95 (80)
>95 (82)
CuI (0.3) Pd(PPh3)4 (0.1) TMEDA K2CO3
TMEDA
10
0
a Reaction conditions: iodonium 2a (1.5 equiv), room temperature, DMF/
THF (1/4), 4 h. b Conversion in 31P NMR, isolated yield of 3a after
chromatography in brackets. c L ) trans-diaminocyclohexane or dimeth-
ylethylenediamine.
Silanes 1a-h14 consisted of E/Z mixtures (E/Z, ca. 95/5)
but, as reported by Ochiai et al. for the preparation of 2a
(see the equation in Table 1) only the (E)-isomers reacted,
leading to stereochemically pure (E)-iodonium salts.
Except for the bis-iodonium salt 2e which readily pre-
cipitated, purification of tetrafluoroborates 2a-h proved to
be difficult. Ochiai’s purification procedure, which involves
precipitation of the salts by addition of diethyl ether and/or
cyclohexane precipitation was not effective in our case. With
the exception of 2d, pure 2a-e were obtained by trituration
of the oily residue obtained after removal of the solvent at
the end of the reaction with cyclohexane and diethyl ether
under Pd(PPh3)4 catalysis. Instead of investigating other
palladium-based conditions, we switched to copper iodide-
mediated conditions as previously reported for the cross-
couplings (under heating) of simple vinylhalides to H-phos-
phonates.10 After negative results using copper iodide either
alone or in the presence of excess dimethylethylenediamine
or trans-diaminocyclohexane as recommended10b (entries 2
and 3), we were pleased to discover that using tetramethyl-
ethylenediamine (TMEDA) resulted in an almost quantitative
conversion after 4h at 20 °C (entry 4). Neither the addition
of K2CO3 nor the presence of Pd(PPh3)4 as a second catalyst
proved beneficial (entries 5 and 6) and, as expected, a blank
experiment run with TMEDA alone yielded no traces of the
desired vinylphosphonate (entry 7).
(11) (a) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.;
shiro, M.; Fujita, E. Tetrahedron 1988, 44, 4095-4112. (b) Stang, P. J.
Angew. Chem., Int. Ed. Engl. 1992, 31, 274-285.
(12) Hypervalent iodine derivatives have been successfully used in
various transition metal-catalyzed couplings near room temperature but, to
the best of our knowledge, the method has not been applied to the
preparation of vinylphosphonates (a) Kang, S.-K.; Lee, H.-W.; Jang, S.-B.;
Ho, P.-S. J. Org. Chem. 1996, 61, 4720-4724. (b) Moriarty, R. M.; Epa,
W. R. Tetrahedron Lett. 1992, 33, 4095-4098.
(13) Coupling of hypervalent iodine derivatives with sodium dialkyl-
phosphonates has been used for the preparation of alkynyl- and arylphos-
phonates. The reaction proceeds at 70-90 °C in DMF. (a) Liu, Z.-D.; Chen,
Z.-C. Synthesis 1993, 373-374. (b) Zhang, J.-L.; Chen, Z.-C. Synth.
Commun. 1998, 28, 175-179.
(14) Vinyltrimethylsilyl derivatives are directly issued from the DIBAL-H
reduction of the corresponding alcynyl compounds as described previ-
ously: Eisch, J. J.; Foxton, M. W. J. Org. Chem. 1971, 36, 3520-3526.
Finally, the reaction of (2-iodo-vinyl)-benzene with di-
methyl phosphite in the presence of Pd(PPh3)4 and K2CO3
at room temperature (same conditions as in Table 2, entry
(15) The presence of these impurities did not appear as an handicap when
iodonium 2d was engaged in a coupling reaction with dimethyl phosphite
(Table 3, entry 9).
(16) (a) Hirao, T.; Masunaga, T.; Yamada, N.; Ohshiro, Y.; Agawa, T.
Bull. Chem. Soc. Jpn. 1982, 55, 909-913. (b) Hirao, T.; Masunaga, T.;
Ohshiro, Y.; Agawa, T. Synthesis 1981, 56-57.
682
Org. Lett., Vol. 7, No. 4, 2005