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

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 enantiornerically 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)]-CHCl3, were isolated as mixtures of three stereoisomers (SNSNSCSC, RNSNSCSC and RNRNSCSC), 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 (SNSNSCSC)-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 carbortyl group to the metal has a substantial influence on the stereochemistry and results in the formation of a single diastereoisomer, having the (RNRNSCSC)-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)(OH2)](+) complexes was anticipated. In preliminary tests of these cationic complexes as catalysts in the enantioselective aldol condensation of benzalclehyde with methyl isocyanoacetate, modest selectivities were observed. (c) 2006 Elsevier Ltd. All rights reserved.