New optically active camphor-derived cyclopalladated complexes with an asymmetric carbon bonded to the metal

Abstract

N,N-Dimethylhydrazone of D-camphor 1 was obtained as the single E isomer in 80% yield by treating the enantiopure ketone with N,N-dimethylhydrazine in the presence of an equimolar amount of p-toluenesulfonic acid in ethanol. Direct cyclopalladation of hydrazone 1 was accomplished at the C(3)H2 group using [Pd(MeCN)2Cl2] and NaOAc in MeCN at the reflux temperature. The product of the reaction, dinuclear cyclopalladated complex 2, was isolated in 89% yield as a mixture of diastereoisomers, which differ by the absolute configuration of the chiral carbon bound to the metal. Compound 2 was converted to the mononuclear complexes 3 and 4 by treating the dimer with PPh3 and Na(acac), respectively. Compounds 3 and 4 were also mixtures of two diastereomers with the Pd atom either in endo or exo position at the new chiral center in the norbornane moiety. Isomers of complex 3 were partly separated by column chromatography to obtain samples of (1S,2S,4R)-3 and (1S,2R,4R)-3 with 96% and 86% de, respectively. The structures of all new compounds were supported by 1H, 13C{1H}, and 31P{1H} spectra as well as 1D NOE experiments. X-ray crystallographic data of complexes 2, (1S,2S,4R)-3 and (1S,2R,4R)-3 supported the cyclopalladation at the C(3)H2 group of the ligand and the absolute configuration of the Pd-bound chiral carbon in both endo and exo palladacycles.