T. Weskamp et al. / Journal of Organometallic Chemistry 585 (1999) 348–352
349
aryl halide and 1.2 equivalents tributylphenyltin in
toluene or xylene at 110°C (see Table 2). The prelimi-
nary results show that turnovers are moderate to good
with bromoarenes. At the same time the stability of the
complexes 2 is excellent during catalysis, i.e. no palla-
dium black precipitation occurs. This is not observed to
be the case with the corresponding bis(phosphine) com-
plexes under the same reaction conditions. The varia-
tion of the phosphine in compounds 2 shows the need
for less basic triarylphosphines like PPh3 as PCy3 per-
forms significantly worse (Entries 13 and 14). Dissocia-
tive ligands such as tris(2-furyl)phosphine P(Fur)3 and
triphenylarsine AsPh3 can result in largely improved
rates and yields [16] but this was not observed in our
case although P(Fur)3 was the second best phosphine
we tested (Entries 15 and 16). Thus, we used PPh3 as
the standard ligand because of its economical advan-
tage. The addition of Cu(I) salts does not show any
promoting effect on the turnover numbers with our
system although these salts have shown to be beneficial
in other cases [17]. The transformation of chloroarenes
is not possible under the described conditions (Entry
21).
lin-2-ylidene)dipalladium(II) 1 by addition of the ap-
propriate phosphine (Scheme 1). NMR experiments of
complexes 2 show only one set of signals in a symmetri-
cal surrounding indicating the exclusive formation of
the trans-isomer. This observation can be readily ex-
plained by considering the sterical bulk of the 1,3-di(1%-
(R)-phenylethyl)imidazolin-2-ylidene.
Palladium-catalyzed Suzuki and Stille cross-coupling
reactions have both emerged powerful methods for the
synthesis of unsymmetrical biaryls [2,3,13], which repre-
sent important structural units in drug intermediates
[14] and in non-linear optical materials [15].
The new complexes 2 were tested in the Suzuki cross
coupling of aryl halides and organoboronates using
phenylboronic acid as the standard substrate and differ-
ent aryl halides as the coupling partners (see Table 1).
With bis(carbene) complexes of palladium(II) the use of
potassium carbonate in toluene at 120°C results in
optimized yields [7c]. Thus, similar conditions were
applied to the mixed NHCꢀphosphine complexes: 1.0
equivalent aryl halide, 1.2 equivalents phenylboronic
acid and 1.5 equivalents K2CO3 in xylene at 130°C
work well both with bromoarenes and chloroarenes.
Changing the base to potassium phosphate results in
similar yields, but also produces early precipitation of
palladium black. Sodium acetate or sodium fluoride do
not give satisfactory results. Only the use of Cs2CO3
does result in higher yields, especially in the case of
non-activated chloroarenes (Entries 8 and 9). Our pre-
liminary results show that turnover numbers of up to
1000 [mol product/mol palladium] can be achieved with
p-bromoacetophenone (Entry 2). Comparison with
bis(1,3-dimethylimidazolin-2-ylidene)dipalladium(II) di-
iodide stresses the need for one phosphine ligand for
the complex to be an active catalyst: the bis(carbene)
complex does not show any catalytic activity with
p-chloroanisole in this reaction under the conditions
described above (Entry 11). The corresponding bis(phos-
phine) complexes (R3P)2PdX2 are known to readily
decompose to palladium black at elevated reaction
3. Conclusions
UsingmixedNHCꢀphosphinecomplexesofpalladium-
(II) for CC-coupling reactions is an excellent means of
triggering both activity and stability of the catalytically
active species. Our experiments demonstrate the appli-
cability of these catalysts in Suzuki and Stille biaryl
formation reactions with boronic acids and tributyl-
stannanes. An important feature of the pre-catalysts is
the necessity for bulky N-heterocyclic carbene ligands,
whereas the phosphine ligand can be varied from tri-
aryl- to trialkyl phosphines depending on the reaction
and the substrate. To our knowledge, this is one of
the most effective palladium(II) catalyst systems for
these reactions and it is the first time that palladium
complexes of N-heterocyclic carbenes have been re-
ported to catalyze the Stille reaction. Only palladium(0)
catalyst systems have shown significantly higher activi-
ties in the Suzuki reaction but have not been able to
temperatures
whereas
the
stability
of
the
NHCꢀphosphine complexes 2 is comparable to the one
of the bis(carbene) complexes, i.e. palladium black pre-
cipitation only occurs after prolonged reaction times.
Variation of the phosphine in the catalysts 2 exhibits
only little effect with bromoarenes but results in dra-
matic changes in yield with chloroarenes. In the latter
cases, only the basic tricyclohexylphosphine PCy3
achieved high turnover numbers (Entries 7 and 8).
Thus, with bromoarenes the economical triphenylphos-
phine PPh3 is good enough whereas with chloroarenes
the more expensive PCy3 is to be used.
The Stille cross coupling of aryl halides and organos-
tannanes was tested using tributylphenyltin as the stan-
dard substrate. Coupling with different aryl halides was
achieved with the best conditions being 1.0 equivalent
Scheme 1. Preparation of trans-(1,3-di(1%-(R)-phenylethyl)imidazolin-
2-ylidene)(phosphine)palladium(II) diiodide 2, see Section 4.
Weskamp, Thomas
