Norbornadiene as a Building Block for the Synthesis of Linked Benzazocinones and Benzazocinium Salts through Tetranuclear Carbopalladated Intermediates

The
six-membered C,N-palladacycle
[Pd­(C,N-C6H4CH2CMe2NH2-2)­(μ-Cl)]2 (A) derived from phentermine reacts with norbornadiene
to give a di- or tetranuclear complex arising from the double insertion
of the same molecule of the strained alkene into one or two distinct
Pd–aryl bonds. The tetranuclear complex has been characterized
by X-ray diffraction studies and exhibits a very unusual cisoid geometry
in both the disposition of the C,N-chelate ligands and the position of the palladium centers. The newly
formed Pd–alkyl bonds are still reactive toward the insertion
of unsaturated molecules, and the tetranuclear complex reacts with
CO or isocyanides to give double benzazocinones or benzazocinium salts
with a cisoid geometry, after depalladation of the corresponding organometallic
intermediates which have been isolated in some cases. When the related
palladacycles derived from phenethylamine or N-methylphenethylamine
are used as starting materials, polymeric compounds are obtained,
from which double benzazocinones or benzazocinium salts with a transoid
geometry are obtained after CO or RNC insertion and subsequent depalladation.
The presence of substituents on the α-carbon atom of the chelated
amine influences the regiochemistry of the double carbopalladation
of the norbornadiene.