Novel enantiopure non- C2-symmetric NCN-pincer palladium complexes with l-proline chiral auxiliaries: mer η3- N, C, N versus square planar η4- N, C, N, O coordination

Abstract

New chiral NCN-pincer palladium complexes containing proline ester moieties as chiral auxiliaries have been synthesized. The parent ligands 2,6-bis{[( S)-2-(methoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L Me and 2,6-bis{[( S)-2-(benzoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L Bn were prepared in a single synthetic step and were obtained enantiomerically pure. Neutral arylpalladium bromide complexes 1a and 1b, formed upon treatment of the respective ligands L Me and L Bn with [Pd 2(dba) 3]·CHCl 3, were isolated as mixtures of three stereoisomers ( S N S N S C S C, R N S N S C S C and R N R N S C S C). The ratio of stereoisomers is approximately 1:1:0.6 in the case of methyl ester derivative 1a, whereas the bulkier benzyl ester derivative 1b predominantly forms the ( S N S N S C S C)-stereoisomer. Upon abstraction of the bromide ion from unresolved mixtures of 1a and 1b, cationic complexes 2 and 3, respectively, form as single diastereoisomers in which one of the ester prolinate carbonyl groups is coordinated to palladium according to X-ray crystal structure determination. This coordination of a carbonyl group to the metal has a substantial influence on the stereochemistry and results in the formation of a single diastereoisomer, having the ( R N R N S C S C)-configuration, regardless of the stereochemistry or ratio of stereoisomers of the starting bromide compound. The structures of compounds 2 and 3 were somewhat unexpected since formation of the corresponding cationic [Pd(NCN)(OH 2)] + complexes was anticipated. In preliminary tests of these cationic complexes as catalysts in the enantioselective aldol condensation of benzaldehyde with methyl isocyanoacetate, modest selectivities were observed.