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 ar...

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Veröffentlicht in:Organometallics 2008-06, Vol.27 (11), p.2549-2559
Hauptverfasser: Gosiewska, Silvia, Martinez Herreras, Sara, Lutz, Martin, Spek, Anthony L, Havenith, Remco W. A, van Klink, Gerard P. M, van Koten, Gerard, Gebbink, Robertus J. M. Klein
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Sprache:eng
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Zusammenfassung: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 R N R N S C S C 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.
ISSN:0276-7333
1520-6041
DOI:10.1021/om701261q