as catalysts for cross coupling and olefin metathesis reac-
tions.6 We recently reported a highly efficient Suzuki-
Miyaura boron crossed coupling method with aryl diazonium
fluoroborates under imidazolium-palladium catalysis at low
temperature.7 The most active bis-2,6-diisopropylphenyl
dihydroimidazolium chloride ligand in that series was
selected for this Heck coupling investigation. A catalytic
amount of the 4,5-dihydroimidazolium chloride (2 mol %
based on the substrate), readily prepared according to the
literature,8 was treated with palladium(II) acetate (2 mol %)
in THF. Added base was not needed to form the active
palladium carbene complex as found previously for the
Suzuki reaction.7 Others have shown that added base, tert-
butoxide,9 carbonate,10 or fluoride is required.11 Alternatively,
Cavell has shown that palladium carbenes can be formed
by treating imidazolium salts first with silver(I) oxide
followed by transfer of the carbene ligand to palladium
acetate without additional base being added.12 Couplings
were reported, but added base was needed for successful
outcomes in that case. Stilbene was now formed from phenyl
diazonium fluoroborate and styrene (Table 1) after 4.5 h at
Palladium(II) acetate (2 mol %) used alone without added
imidazolium ligand, in THF at room temperature for 12 h,
gave a much reduced 21% yield of stilbene product. Use of
tetrakis(triphenylphosphine) palladium as catalyst in THF or
toluene, resulted in no product formation even after an
extended 36 h time period and gave only a 12% yield of
product after 12 h in THF at reflux. Acetate appears to
function as base in this case to generate the active palladium
carbene catalyst under the conditions investigated. As
previously reported,7 an equimolar CDCl3 solution (0.8 M)
of the imidazolium chloride and palladium acetate monitored
1
by H NMR showed complete disappearance of the diag-
nostic proton located on the iminium carbon found at 8.15
ppm.13 Use of toluene and the other solvents shown gave
product with lower yields, including methanol, previously
the solvent of choice for diazonium couplings.4,5
Reaction conditions were explored with various aryl
diazonium fluoroborates and two styrenes (X ) H, -OMe)
at 1.2:1 stoichiometry (Table 2). The catalyst load could be
lowered to 0.1% without significant loss in yield in the
Table 2. Heck Coupling with Styrene
Table 1. Effect of Solvent and Ligand
solvent
time, h
temp, °C
yield, %a
THF
4.5
6
5
7
6
rt
rt
110
rt
rt
77 (57)b
46
57
57
61
toluene
toluene
dioxane
ether
MeOH
5
rt
5Y
a Yields are reported for isolated, chromatographed materials. b The
analogous dehydroimidazolium chloride lignad was used.
room temperature in the presence of the imidazolium-
palladium catalyst. TLC indicated consumption of the starting
materials, and the product was isolated in 77% yield
following silica gel chromatography. When the analogous
aromatic (4,5-dihydro) bis-2,6-diisopropylphenyl imidazo-
lium ligand was used, the product yield was significantly
lower at 57%. Recently Nolan and co-workers reported the
use of a chelating phosphine-imidazolium-palladium cata-
lyst that required carbonate base in N,N-dimethylacetamide
at 120 °C using less reactive aryl bromides.
(6) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953. (b) Lee, H. M.; Nolan, S. P. Org. Lett. 2000, 2, 2053. (c) Yang, C.;
Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511. (d) Fu¨rstner, A.; Leitner,
A. Synlett 2001, 290-292.
(7) Andrus, M. B.; Song, C. Org. Lett. 2001, 3, 3761-3764.
(8) (a) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J. Unverzagt, M. Tetrahedron 1999, 55, 14523.
(b) Saba, S.; Brescia, A. M.; Kaloustian, M. K. Tetrahedron Lett. 1991,
32, 5031.
a Reactions were performed on 0.1 mmol scale at 0.2 M concentration.
b Isolated yields using 0.1 mol % palladium acetate-imidazolium catalyst.
2080
Org. Lett., Vol. 4, No. 12, 2002