New Aryl α‑Diimine Palladium(II) Catalysts in Stereocontrolled CO/Vinyl Arene Copolymerization

The
effect of the catalyst structure on the stereoselectivity of CO/vinyl
arene copolymerization has been studied with the aim of developing
catalytic systems able to improve the yields while maintaining the
high degree of copolymer isotacticity previously obtained using achiral
nitrogen ligands. Aryl α-diimine ligands having extended aromatic
rings (Ar)2DABMe2, with Ar = 1-C10H7 (e), 1-C14H9 (f), 9-C14H9 (g), have been
synthesized, and α-diimine coordination to cationic methylpalladium
complexes has been investigated in solution, by means of NMR spectroscopy,
and in the solid state for [Pd­(Me)­(NCMe)­((9-C14H9)2DABMe2)]­[PF6] (2g). The performance of these catalysts in CO/vinyl arene copolymerization,
under mild conditions, was analyzed in terms of productivity and degree
of stereoregularity of the resulting polyketones. In comparison with
previous results, a remarkable enhancement in the yield of isotactic
copolymer was observed using the new achiral 9-anthryl α-diimine
ligand g, confirming that the ortho disubstitution and
the extended aromatic rings play key roles in obtaining good stereoselectivity
and good productivity. To perform a structural analysis of the first
steps of the CO/p-methylstyrene copolymerization,
complex [Pd­(Me)­(CO)­((9-C14H9)2DABMe2)]­[BAr′4] (3g) was used as
a starting point: NMR investigation reveals the stereoselective formation
of the olefin/CO/olefin insertion product (6g), which
prevalently exists in solution in only one diastereoisomeric form,
thus justifying the observed high polymer isotacticity.