First enantiopure phosphapalladacycle with a palladium bonded stereogenic carbon as the sole chirality source

First enantiomerically pure phosphapalladacycle bearing an asymmetric carbon directly bonded to a palladium atom as a single chirality source was prepared by the resolution of racemic dimer {Pd(η2-L)(μ-Cl)}2 (1), where L=2-Bu2tPC6H4CHMe, using (R,R)-stilbendiamine as an auxiliary ligand. Double recrystallization of a 1:1 mixture of diastereomers (S,RR)-4a and (R,RR)-4b from MeCN affords one of them, (S,RR)-4a, as a single isomer (>98% de, 31P{1H} NMR data). Another diastereomerically pure complex, (R,SS)-4c, was isolated by the same method using (S,S)-stilbendiamine as a resolving agent. This diastereomer (R,SS)-4c was converted into the enantiopure dimer (R,R)-1 (>98% ee, 31P{1H} NMR data) by the diamine protonation with dilute acetic acid at ∼5 °C. Dimer (R,R)-1 is configurationally stable at high temperatures up to at least 110 °C. The absolute configuration of the C∗-stereocenter in dimer (S,S)-1 was established by an X-ray diffraction study of its precursor (S,RR)-4a.

The first enantiopure CP-palladacycle with the metal directly bonded to the asymmetric carbon atom as a single chirality source was prepared via separation of its diastereomeric derivatives with diamine (SS)-Stien. The procedure for auxiliary ligand removal from the intermediate diamine adduct (R,SS)-4c without enantiopurity decreasing was elaborated. The configurational stability of dimer (R,R)-1 at 110 °C was established. The absolute (S)-configuration of the carbon stereocenters in the enantiopure dimer 1 was determined by X-ray diffraction study of its diamine precursor (S,RR)-4a.Figure optionsDownload as PowerPoint slide