N. T. S. Phan et al. / Tetrahedron Letters 45 (2004) 7915–7919
7919
100% conversion for the 3-isomer and only 40% for the
2-isomer. Furthermore, 60% unreacted starting material
in the case of the 2-isomer led us to believe that this was
a rate limited selectivity based on the increased steric
bulk at the 2-position inhibiting substitution.
the catalyst during the course of the reaction. The poly-
mer-supported catalyst was considerably more active
than the homogeneous analogue. This unexpected and
surprising behaviour of the homogeneous analogue of
the immobilised complex still needs further studies and
these are underway.
In addition to the experiments reported above, we also
performed homogeneously catalysed Suzuki reactions
using the homogeneous analogue of the immobilised
complex, [Pd(salenac-OMe)] (Fig. 1). The synthesis,
purification and characterisation of this complex, and
its use as homogeneous catalyst for the Heck coupling
reaction has been published previously.8 Unfortunately,
despite the fact that the analogous heterogeneous reac-
tions proceed close to completion, the reaction using
0.5mol% [Pd(salenac-OMe)] complex with K3PO4 in
DMF at 90ꢁC for 24h resulted in only 1% conversion
of 4-bromoanisole to 4-methoxybiphenyl, while the
analogous heterogeneous reactions were performed suc-
cessfully with 99% conversion by GC. Increasing the
reaction temperature was shown to have little or no
apparent effect on the reaction rate, with only 7% con-
version being achieved at 100ꢁC. The colour of the reac-
tion solution was yellow and there was no apparent
change in colour during the course of the homogeneous
catalyst reactions. A series of bases such as NaOH,
Na2CO3, NaHCO3, Cs2CO3 and N,N-diisopropyl ethyl-
amine were also used for the homogeneous reactions but
no improvement was achieved. Changing the reaction
solvent to DME, DME/water (1:1) and DMF/water
(1:1) still failed to give an improvement in the yield of
4-methoxybiphenyl. Furthermore, the addition of
2.5mol% triphenylphosphine as a homogeneous addi-
tive had no effect on the conversion. However, it was
found that reasonable yields were obtained using a
much higher catalyst concentration. The homogeneous
reactions using 5mol% palladium complex in DME/
water (1:1) afforded 4-methoxybiphenyl in conversions
of 84%, 82% and 74% using K3PO4, NaOH and Na2CO3
as bases, respectively. This means that the Merrifield re-
sin support has a significant effect on the activity of the
catalyst. Varma et al. have reported a significant
improvement in reaction rate using a heterogeneous pal-
ladium chloride and tetraphenylphosphonium bromide
intercalated clay catalyst, when compared to the corre-
sponding homogeneous palladium chloride catalyst un-
der identical reaction conditions.21
Acknowledgements
We wish to thank the Vietnamese Government for a stu-
dentship to N.T.S.P. and the EPSRC for funding for
D.H.B.
References and notes
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In summary, the polymer-supported palladium catalyst
exhibits a high activity towards the Suzuki cross-cou-
pling reaction. The catalyst can be easily separated from
the reaction mixture by simple filtration and reused after
washing. Very little palladium leaches into solution from