Controlled Acrylate Insertion Regioselectivity in Diazaphospholidine-Sulfonato Palladium(II) Complexes

Diazaphospholidine-sulfonato Pd(II) complexes [{κ 2 - P,O- ( N -Ar 2 C 2 H 4 N 2 P)C 6 H 4 SO 3 }PdMe(L)] 1- L (L = dmso, pyridine, lutidine, or μ-LiCl(solvent); 1a: Ar = Ph, 1b: Ar = 2-MeC 6 H 4, 1c: Ar = 2-MeOC 6 H 4, 1d: Ar = 2,4,6-Me 3 C 6 H 2, 1e: Ar = 2,6- i Pr 2 C 6 H 3, 1f: Ar = 2,6-( p -tolyl) 2 C 6 H 3 ) 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 [(P ∧ O)Pd{κ 2 - C,O - C H 2 CHMeC( O )OMe], 12 ) to the usual electronically controlled 2,1-insertion (>95%) for the less bulky Ar = Ph ( 1a, yielding the insertion product [(P ∧ O)Pd{κ 2 - C,O - C HEtC( 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 [{κ 2 - P,O- ( N -Ar 2 C 2 H 4 N 2 P)C 6 H 4 SO 3 }PdR(lutidine)] (Ar = 2,6- i Pr 2 C 6 H 3; 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.