Abstract
Two families of biarylphosphine ligands were prepared for use
in the Pd-catalyzed amination reaction. The first series investigated
was derived from N -phenylpyrroles, and
the second, was derived from N -phenylpyrazoles.
While the pyrrole ligands were not as general in substrate scope,
one of the readily prepared pyrazole ligands (R = t -Bu) was found to have a fairly broad
substrate scope.
Key words
palladium - catalysis - ligands - biaryls - aminations
References
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5 Various alkyllithiums were screened
for the halogen-metal exchange, including s -BuLi
and none provided significantly improved branching ratios. Organomagnesium
derivatives were prepared as well but these had significantly poorer reactivity
and required copper salts to obtain product. Ultimately, ligand 2 was used impure since the contaminants were
not removed easily by recrystallization and were not considered
impediments to the reaction.
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7 Reactions in toluene with 3 or 4 and
a weak base (such as Cs2 CO3 ) turned yellow-green
and resulted in poor reaction conversions. Adding t -BuONa
led to restarting of the reactions and drove them to completion,
suggesting t -BuONa can break aggregation
of a Pd complex that is not competent in the catalytic cycle.
8 When carrying out the coupling of morpholine
and 4-tert -butylphenylbromide in toluene
with 1% Pd catalyst, the reaction only reached 22% conversion,
while the same reaction in tert -amyl
alcohol reached completion in under 2 h. Use of butan-2-ol accelerated
the reaction further, but significant dehalogenation of the aryl
bromide was observed (due to β-hydride elimination).
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