Diastereopure Cationic NCN-Pincer Palladium Complexes with Square Planar η4-N,C,N,O Coordination

Abstract

Neutral NCN-pincer Pd bromide complex 2 contg. the monoanionic, enantiopure pincer ligand 2,6-bis{[(S)-2-hydroxymethyl-1-pyrrolidinyl]methyl}phenyl bromide (1) with bis-ortho-(S)-prolinol substituents was synthesized and isolated as a mixt. of three stereoisomers [(SN,SN,SC,SC), (RN,SN,SC,SC), and (RN,RN,SC,SC)] in a 1:1:1 ratio. Upon abstraction of the bromide ion from the unresolved mixt. of 2, single diastereoisomers of the cationic complexes [3]BF4 and [3]PF6, resp., were formed with a unique η4-N,C,N,O coordination mode of ligand 1. X-ray crystal structure detn. established the intramol. η4-N,C,N,O coordination of 1 to Pd where the typical mer-η3-N,C,N pincer coordination is accompanied by coordination of one of the hydroxy groups of the (S)-prolinol moieties. The H2O mol. that was cocrystd. in the crystal structure of [3]PF6 does not coordinate to Pd, but instead is involved in a H bonding network. The catalytic potential of both cationic complexes, [3]BF4 and [3]PF6, was tested in an aldol reaction of aldehydes with Me isocyanoacetate to yield the oxazoline products as racemic mixts. [on SciFinder(R)]