Web Release Date: February 15,
Endo-Effect-Driven Regioselectivity in the Cyclopalladation of (S)-2-tert-Butyl-4-phenyl-2-oxazoline
and
Department of Chemistry, University of North Dakota, 151 Cornell Street, Stop 9024, Grand Forks, North Dakota 58202, and X-Ray Crystallographic Facility, Chemistry Department, University of Minnesota, 160 Kolthoff Hall, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455
Received December 14, 2006
Abstract:
Direct cyclopalladation of (S)-2-tert-butyl-4-phenyl-2-oxazoline using palladium acetate in acetic acid
or acetonitrile provided a mixture of two isomeric compounds, the endo 
-AcO dimeric complex with a
C(sp3)-Pd bond and the corresponding exo derivative with a C(sp2)-Pd bond, with the former being the
major product. The -AcO dimeric complexes were converted to the corresponding -Cl analogues 3
and 4 by treatment with LiCl in acetone; the latter compounds were transformed to the corresponding
PPh3 adducts 5 and 6. The NMR data suggested the puckered structure of the endo palladacycle in
complexes 3 and 5, the twisted 
(S) conformation of the oxazoline ring in 3, 4, and 6, and the 
(S)
conformation of the heterocycle in complex 5. The X-ray crystal structure of 5 confirmed the (S)
conformation of the oxazoline ring in the solid state and the twisted conformation of the palladacycle
and revealed a P-propeller chiral configuration of the PPh3 ligand. A series of ab initio quantum chemical
calculations were performed on two model compounds generated by replacing the PPh3 ligands with
NH3 in complexes 5 and 6. The structures and energies of the two model exo and endo isomers were
calculated at the RHF, BLYP, and MP2 levels of theory with a 6-31G* basis for the light atoms and
LANL2DZ ECP for the palladium and were found to be comparable. Single point coupled cluster
calculations, with single double excitations (CCSD), corroborated the results.
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