Synthesis, structure, and catalytic performance of diastereopure five-coordinated NCN-pincer palladium(II) complexes bearing bulky amino acid substituents

Abstract

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]PF6 and [4]BF4. Palladium complexes 3, [4]PF6, and [4]BF4 are all formed as single diastereoisomers with an RNRNSCSC configuration. X-ray crystal structure determinations of [4]PF6 and [4]BF4 revealed an unusual κ5-N,C,N,O,O coordination around palladium, in which the normal meridional κ3-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]PF6 and [4]BF4 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.