Synthesis, Structure, and Catalytic Performance of Diastereopure Five-Coordinated NCN-Pincer Palladium(II) Complexes Bearing Bulky Amino Acid Substituents

New chiral NCN-pincer palladium complexes containing bulky diphenylhydroxymethyl pyrrolidinyl moieties as chiral auxiliaries have been synthesized. Oxidative palladium addition to ligand 2 ( 2 = 2,6-bis{[( S )-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl}-1-bromobenzene) initially yielded neutral arylpalladium bromide complex 3 in a moderate yield as a consequence of the bulky pyrrolidinyl functional groups. Performing the palladation reaction under microwave irradiation for only 5 min gave 3 in 79% yield. Abstraction of the bromide ion from 3 subsequently yielded the cationic complexes [ 4 ]PF 6 and [ 4 ]BF 4 . Palladium complexes 3, [ 4 ]PF 6, and [ 4 ]BF 4 are all formed as single diastereoisomers with an R N R N S C S C configuration. X-ray crystal structure determinations of [ 4 ]PF 6 and [ 4 ]BF 4 revealed an unusual κ 5 - N, C, N, O, O coordination around palladium, in which the normal meridional κ 3 -coordination mode of the pincer framework is complemented by two rather long Pd−O interactions (2.622(2)–2.649(2) Å). Theoretical calculations (Mulliken population analysis and “atoms in molecules” analysis) confirmed that there is a coordinative interaction between the palladium and the oxygen atoms of both hydroxyl groups despite the long Pd−O distance. Complexes [ 4 ]PF 6 and [ 4 ]BF 4 catalyze the aldol condensation between α-methyl isocyanoacetate and various aromatic aldehydes with enhanced regio- (up to 70%) and stereoselectivity (up to 42% ee) for the cis -oxazolines. These observations are in contrast to prior literature results on other pincer systems, where the trans -oxazolines were the main product, and point to a deeper chiral cavity pointing away from palladium toward the coordinated isocyanoacetate substrate.