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44% at 90 1C to 99% at 100 1C. The catalyst derived from this ligand is
completely soluble in water at room temperature, but partitions into the
substrate phase upon heating to 100 1C, leaving a colorless water layer. Upon
cooling to room temperature, the catalyst partitions back into the water
phase and the water-insoluble product precipitates. In Sonogashira coup-
ling, the temperature effect was even more dramatic: at 72 1C, a 30% yield
was obtained in the coupling of 4-iodonitrobenzene and phenylacetylene,
whereas at 74 1C, the yield was 70% under otherwise identical conditions.
The catalyst system could be used for four cycles before a significant de-
crease in activity occurred in both the Suzuki and Sonogashira reactions.
14.5 Greener Substrates for Cross-Coupling
Reactions
As a substitution reaction, the traditional cross-coupling of an organic halide
with a nucleophile is inherently atom inecient. The resulting halide salt
by-product represents a stoichiometric waste stream that must be treated.
One approach would be to use leaving groups that leave innocuous by-
products. Even more attractive would be if the leaving group were to act as a
base in the reaction, thus eliminating the need for a stoichiometric base.
The use of no leaving group would be the most atom economical approach.
14.5.1 Carboxylic Acid Derivatives as Coupling Partners
Decarboxylative or decarbonylative coupling of benzoic acid derivatives
would release either CO
2
or CO as gaseous by-products, thus avoiding the
production of metal halide waste. Benzoic acid derivatives can be used as
either the nucleophile or electrophile in cross-coupling reactions. In a few
cases, cross-coupling of different benzoic acid derivatives has been reported.
Work on the development of this class of cross-coupling reaction has been
reviewed previously,
192
so just key examples are highlighted here.
Stephan and co-workers first reported a cross-coupling reaction without
salt by-product formation.
193
Palladium(II) chloride (0.25 mol%) catalyzed
the Heck coupling of benzoic anhydride with alkenes in NMP at 140 1Cto
afford the arylated alkene (Scheme 14.63). Palladium inserts into the an-
hydride to give benzoate and a benzoyl-palladium intermediate that elim-
inates CO to give the aryl-palladium intermediate in the catalytic cycle. The
benzoate released upon oxidative addition serves as the base in the reaction.
In principle, the benzoic acid could be recovered and converted back to the
O
O
CO
2
Bu
O
CO
2
Bu
PdCl
2
(0.25 mol %)
CO
2
Bu
+ CO +
+
O
HO
CO
2
Bu
NMP, 190 °C, 3 h
73%
7:2
E/Z
Scheme 14.63
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