Nucleophilic addition to
(η3-2-chloropropenyl)palladium complexes
1 with stabilized
carbanions such as dialkyl malonates was studied. These complexes
are used as probes to
determine whether nucleophilic attack occurs at the central or terminal
carbon of the π-allyl
group. Attack at the central carbon leads to substitution of
chloride via a palladacyclobutane
intermediate. The regiochemistry of the reaction (central versus
terminal attack) is controlled
by proper choice of ligands. Thus, σ-donor ligands direct the
attack of the nucleophile to
the central carbon (C-2) of the allyl group whereas π-acceptor
ligands direct the attack to
the terminal carbons (C-1 or C-3). It was found that there is a
correlation between the
relative rate of central versus terminal attack and the 13C
NMR shifts of the allyl group.
The shift difference between the central and terminal carbons,
Cc − Ct, can be used to predict
the site of attack. Ab initio calculations were
performed on (π-allyl)palladium complexes as
well as on the postulated palladacyclobutane. The calculations
support the experimental
results, and for the π-allyl complexes with the σ-donor ligands the
LUMO from the
calculations is the symmetrical orbital with a large coefficient at the
central carbon.
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