Kinetic resolution of N-methyl-2-phenylpyrrolidine by cyclopalladation in the presence of an optically active base

Abstract

The possibility of the kinetic resohaion of the racemic C-chiral ligand in the course of its orthopalladation in the presence of an optically active base was shown Jbr the first time using a tertiary heterocyclic amine as the ligand. The absolute configuration of the C'-stereocenter in the predominant enantiomer of the dimeric complex was established by the X-ray diffraction of (S)-prolinate derivative.Asymmetric activation of C-H bonds seems a highly promising method for the synthesis of optically active metallacycles. However, this approach has remained little studied up to the present, and is only utilized for systems with planar chirality. After the pioneering investigations of Sokolov and coworkers on the asymmetric cyclopalladation of ot-ferrocenylalkylamines [ I-3] and their ruthenocenyl analogs [4] in the presence of optically active bases (as the external inductor of chirality), a series of publications on the diastereoselective cyclopalladation of optically active ferrocenylimines [5][6][7] and ferrocenylhydrazones [8,9], containing the C -stereocenter in the N-substituent as the internal inductor of chirality, appeared only recently.The present work presents preliminary results of investigation into the asymmetric cyclopalladation of racemic N-methyl-2-phenylpyrrolidine (I) in the presence of optically active bases, as well as the determination of the absolute configuration of the enantiomer formed preferentially on account of kinetic resolution using the method of X-ray diffraction analysis. It should be noted that only one case of kinetic resolution in the course of cyclopalladation of a racemic ligand has been described until now, using the example of the system containing elements of planar and central chirality simultaneously [2].We previously published [10] the synthesis of the racemic dimeric ortho-palladated complex di-~t-chlorobis[2-(l-methylpyrrolidin-2-yl)phenyl-C,N]dipailadium(II) (1I) and its optical resolution using (S)-prolinate as the auxiliary chiral ligand.

Asymmetric exchange of cyclopalladated ligands: A new route to optically active phosphapalladacycles

“…According to TLC, the reaction mixture contained only the adduct of the starting dimer 1a and thioamide HL 11 (1e); R f 0.38 (ben zene-acetone, 5 : 1). 1 * To control the course of the reaction by TLC, the sample was treated with a solution of LiCl in acetone.…”

Determination of the Absolute Configuration of CN-Palladacycles by 31P{1H} NMR Spectroscopy Using (1R,2S,5R)-Menthyloxydiphenylphosphine as the Chiral Derivatizing Agent: Efficient Chirality Transfer in Phosphinite Adducts

“…In contrast to the vast number of diverse methods employed to estimate the AC of organic compounds, only X-ray crystallography has been previously used for the AC determination of the new CN - and CP -palladacycles, prepared by racemic CPC resolution, diastereoselective cyclopalladation of optically active ligands, or asymmetric C–H bond activation . This method is the best route for the unambiguous establishment of the AC; however its mandatory requirements of good quality crystals have essentially restricted its applications.…”

New principle for palladacycle resolution: diastereoselective monomer to dimer conversion

“…The third method based on the asymmetric activation of the C-H bond under control of an optically active base [34][35][36][37][38][39] or ligand in the palladation agent [40][41][42] is not so popular and used mainly for prochiral substrates. Except in a few cases, 41 the latter approach does not provide palladacycles in enantiomerically pure form and requires further minor enantiomer removal at the final stages.…”