Controlled acrylate insertion regioselectivity in diazaphospholidine- sulfonato palladium(II) complexes

Article abstract of DOI:10.1021/om300755j

Diazaphospholidine-sulfonato Pd(II) complexes [{κ²-P,O-(N- Ar₂C₂H₄N₂P)C₆H ₄SO₃}PdMe(L)] 1-L (L = dmso, pyridine, lutidine, or μ-LiCl(solvent); 1a: Ar = Ph, 1b: Ar = 2-MeC₆H₄, 1c: Ar = 2-MeOC₆H₄, 1d: Ar = 2,4,6-Me₃C ₆H₂, 1e: Ar = 2,6-iPr₂C₆H ₃, 1f: Ar = 2,6-(p-tolyl)₂C₆H₃) were prepared and structurally characterized. The regioselectivity of methyl acrylate (MA) insertion into the Pd-Me bond is entirely inverted from >93% 1,2-insertion for bulky substituents (1d-f, yielding the insertion products [(PO)Pd{κ²-C,O-CH₂CHMeC(O)OMe], 12) to the usual electronically controlled 2,1-insertion (>95%) for the less bulky Ar = Ph (1a, yielding the insertion product [(PO)Pd{κ²-C,O- CHEtC(O)OMe], 11, and β-H elimination product methyl crotonate). DFT studies underline that this is due to a more favorable insertion transition state (2,1- favored by 12 kJ mol^-1 over 1,2- for 1a) vs destabilization of the 2,1-insertion transition state in 1d,e. By contrast, MA insertion into the novel isolated and structurally characterized hydride and deuteride complexes [{κ²-P,O-(N-Ar₂C ₂H₄N₂P)C₆H₄SO ₃}PdR(lutidine)] (Ar = 2,6-iPr₂C₆H₃; 9e: R = H, 10e: R = D) occurs 2,1-selectively. This is due to the insertion occurring from the isomer with the P-donor and the olefin in trans arrangement, rather than the insertion into the alkyl from the cis isomer in which the olefin is in proximity to the bulky diazaphospholidine. 1a-f are precursors to active catalysts for ethylene polymerization to highly linear polyethylene with M _n up to 35 000 g mol^-1. In copolymerization experiments, norbornene was incorporated in up to 6.1 mol % into the polyethylene backbone.