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

Diazaphospholidine-sulfonato Pd­(II) complexes [{κ2-P,O-(N-Ar2C2H4N2P)­C6H4SO3}­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-Me3C6H2, 1e: Ar = 2,6-iPr2C6H3, 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 [(P∧O)­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 [(P∧O)­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-Ar2C2H4N2P)­C6H4SO3}­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 Mn up to 35 000 g mol–1. In copolymerization experiments, norbornene was incorporated in
up to 6.1 mol % into the polyethylene backbone.