Unique syndio-selectivity in CO/styrene copolymerization reaction catalyzed by palladium complexes with 2-(2\u2032-oxazolinyl)-1,10-phenanthrolines

The reaction of the neutral Pd complex [Pd(CH3)Cl(cod)] with the potentially terdentate 2-oxazolinyl phenanthroline ligands 1-3 affords the corresponding cationic dinuclear Pd-complexes 1a-3a which can be isolated in the solid state in good yields. By treatment with AgPF6 the complexes 1a-3a were converted into the corresponding hexafluorophosphate derivatives 1b-3b where both the ligand units feature a terdentate coordination around the two Pd-centres with the phenanthroline fragment of each unit displaying a chelate coordination to one Pd-centre while the corresponding oxazolinyl pendant acts as a bridging ligand towards the second Pd-centre. The persistence of this dimeric structure of 1b-3b in CD2Cl2 solution is confirmed by 15N-NMR experiments at natural abundance, which clearly show the binding to the metal of all the nitrogen donors as well as the overall C2 symmetry of the compound. In consequence of the different strength of the relevant ion-pair, the dimeric structure of the complex undergoes partial fragmentation in the case of the chloride derivatives 1a-3a as evidenced from the 15N-NMR spectra. Complexes 1b-3b are active catalysts in styrene alternate carbonylation, where, under very mild conditions (30 \ub0C and 1 atm of CO), they provide oligomers with 3-5 repetitive units as the exclusive or prevailing product. When traces of the CO/styrene polyketones are also formed, their 13C-NMR characterization shows that they are stereochemically homogeneous with a unique syndio-tacticity. This result implies that Pd-complexes able to induce a complete enantioface discrimination in the insertion step of the alkene during the catalytic cycle of the styrene alternate carbonylation have been produced for the first time

Addressing the poly- to oligo-ketone selectivity in styrene carbonylation catalyzed by Pd/bpy complexes. Effect of the 6-alkyl substitution

Two series of organometallic Pd-complexes, namely i. [Pd(CH3)(CH3CN)(N-N')][PF6] and ii. [Pd(CH3)(N-N')2][PF6], with a range of 6-alkyl-substituted-2,2'-bipyridine ligands, including the new 6-(1-methoxyethyl)-2,2'-bipyridine, both in racemic and enantiopure form, and 2-(methoxymethyl)-6-(1H-1,2,3-triazol-1-yl)pyridine, have been studied. 6-(1-methoxyethyl)-2,2'-bipyridine was synthesized both in racemic and in the opposite homochiral enantiomeric forms by two stereocomplementary chemoenzymatic procedures. The characterization of the new complexes, both in solid state and in solution, provides evidence for the formation of a unique isomer featured by the methyl ligand trans to the Pd-N bond of the substituted pyridine ring. For the complex with the bpy ligand having the sec-butyl substituent a cyclometallation reaction with the release of methane occurs, leading the substituted bpy to act as a terdentate N-N'-C ligand. Complexes of series ii. feature one chelate N-N' ligand, while the other one is coordinated to Pd in a monodentate fashion. In solution a fluxional process that makes equivalent the two N-N' ligands is present and the static 1H NMR spectra correspond to an averaged structure where palladium is a stereogenic center. All these complexes behave as catalysts for the styrene carbonylation yielding CO/styrene oligoketones, which are optically active when catalysts containing chiral, enantiomerically pure, ligands are applied. For both series of complexes the reactivity with labeled CO has been investigated leading to the formation of the corresponding Pd-acetyl species, that for complexes of series ii. is featured by both N-N' molecules bonded to the same metal center