[Pd(N,N-chelate)(olefin)] Complexes Containing Chiral Nitrogen Chelates Based on Carbohydrates – Enantioselectivity of Olefin Coordination

Abstract

Chiral N,N‐chelates of formula 6‐Me‐pyridine–2‐CH=N–R (1) and R–N=CH–CH=N–R (2) (R = 6‐deoxy‐α‐D‐glucoside or 6‐deoxy‐α‐D‐mannoside residue) and their palladium(0) complexes [Pd(N,N‐chelate)(olefin)] (I) were prepared. Symmetrical type 2 ligands induced higher enantioselectivity in the coordination of prochiral olefins. The ability of a type 1 chelate to promote a stereoselective process was also assessed, i.e. dimethylfumarate inserted into the Pd–Me bond formed upon methylation of a type I complex with 50% ee. Finally, a water‐soluble Pd0 complex was also prepared by deprotecting the alcoholic functions on the sugar residue, and its molecular structure determined through X‐ray diffractometry.

Hydrophilic Pd0 Complexes Based on Sugars for Efficient Suzuki–Miyaura Coupling in Aqueous Systems

“…Upon coordination to the Pd centre, the signals of the olefin undergo a large shift to lower frequencies. This behaviour has been reported previously7a,7c,8,9 and is attributed to the substantial degree of π back‐donation in the Pd 0 –olefin bond, which stabilizes the low oxidation state of the palladium centre. The consequent partial sp 2 →sp 3 rehybridization of the alkene carbon atoms shifts the corresponding signals towards the aliphatic region in both the 1 H and the 13 C NMR spectra.…”

“…The coordination bond lengths fall within the expected range for Pd–α‐diimine complexes12,13 and are similar to those found for [Pd(N,N‐chelate)(olefin)] complexes 7a. The length of the olefinic bond is evidence of the substantial rehybridization of the olefinic carbon atoms towards sp 3 upon coordination (1.41 and 1.47 Å for the two diastereomers vs. 1.34 Å for the free alkene).…”

“…The corresponding [Pd(PN)(fdn)] and [Pd(NN)(fdn)] complexes were prepared in good yields from fresh [Pd 2 (dba) 3 · CHCl 3 ] (dba = dibenzylideneacetone) by an established procedure (Scheme , Step i) 7a…”

“…It is worth noting that water‐soluble Pd 0 compounds are quite rare 7a,7b,10. The hydrophilic Pd 0 complexes [Pd(PN*)(fdn)] and [Pd(NN*)(fdn)] were obtained by basic hydrolysis of the acetyl and benzoyl groups (Scheme , Step ii).…”

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…X-ray diffractometric studies 10.1. Complexes bearing mono-bi-and ter-dentate nitrogen ligands A significant number of palladium(0) olefin compounds bearing the title ligands was synthesized by several authors [30,31,35,37,75,[78][79][80][81][82][83][84][85][86]. All the structures display the typical trigonal arrangement around the palladium centre with the Pd-N bond lengths being scarcely influenced by the olefin nature, spanning within 2.10 and 2.18 Å.…”