Diastereopure Cationic NCN‐Pincer Palladium Complexes with Square Planar η 4 ‐ N , C , N , O Coordination

Neutral NCN‐pincer palladium bromide complex 2 containing 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 mixture 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 )] in a 1:1:1 ratio. Upon abstraction of the bromide ion from the unresolved mixture of 2 , single diastereoisomers of the cationic complexes [ 3 ]BF 4 and [ 3 ]PF 6 , respectively, were formed with a uniqueη 4 ‐ N , C , N , O coordination mode of ligand 1 . X‐ray crystal structure determination established the intramolecularη 4 ‐ N , C , N , O coordination of 1 to palladium where the typical mer ‐η 3 ‐ N , C , N pincer coordination is accompanied by coordination of one of the hydroxy groups of the ( S )‐prolinol moieties. The water molecule that was cocrystallized in the crystal structure of [ 3 ]PF 6 does not coordinate to palladium, but instead is involved in a hydrogen bonding network. The catalytic potential of both cationic complexes, [ 3 ]BF 4 and [ 3 ]PF 6 , was tested in an aldol reaction of aldehydes with methyl isocyanoacetate to yield the oxazoline products as racemic mixtures.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)