Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous ...

Heterogeneous Pd Catalyzed C−C Coupling Reactions Chemical Reviews, 2007, Vol. 107, No. 1 147
absorber utilized in the manufacture of sunscreen lotion
(Scheme 20).
Scheme 20. Heck Reaction of 1-Bromo-4-methoxybenzene
with Octyl Acrylate
Köhler et al. provided a very detailed investigation about
the application of Pd/C in the Heck reaction of aryl bromides
with olefins. 28,50 A variety of Pd/C catalysts differing in Pd
dispersion, Pd distribution, Pd oxidation state, and water
content were tested. The experimental results indicated a
(quasi-)homogeneous reaction mechanism, that is, the Pd
complex or colloidal particles in solution act as the catalytically
active species. E 105 CA/W 5% Pd from Degussa AG
turned out to be the best Pd/C catalyst. It has a high Pd
dispersion (36%), a low reduction degree [mainly Pd(II)],
and a high water content (∼55%). Its high catalytic activity
in N-methylpyrrolidone/sodium acetate at 140 °C allowed
working with extremely low catalyst concentrations (Table
28). An argon atmosphere helps to decrease the Pd leaching
while maintaining the high activity. Under these optimized
reaction conditions, turnover numbers (TON) up to 36 000
and turnover frequencies (TOF) up to 18 000 h -1 (for
bromobenzene) were achieved using a Pd concentration as
low as 0.0025 mol % (Table 28, entry 3). Complete
conversion was achieved for activated (electron-withdrawing
substituents) and nonactivated, as well as deactivated,
bromoarenes (electron-donating substituents) within a few
hours. The extraordinarily high activity seems to be connected
with dissolution-reprecipitation processes, where
dissolved Pd species formed at this temperature are the active
species. Much lower catalytic activity (TON and TOF
reduced by magnitudes) in Heck coupling was found with
pre-reduced Pd/C catalysts (Table 28, entries 4 and 5) in
accordance with previous findings. 11
Table 28. Effect of Reaction Conditions on the Heck Reaction of
Aryl Bromides with Styrene 81
entry R
catalyst
(mol %) T (h)
yielda conv (%)
(%) 76 77 78 TON TOF
1 Ac 0.005 4 96 90 0 5 19200 4800
2 H 0.005 1 90 83 1 6 18000 18000
3 H 0.0025 2 90 83 1 6 36000 18000
4 b H 1.0 20 58 52 0 5 58 3
5 c H 1.0 20 12 10 0 1 12