Cationic Polymerization and Insertion Chemistry in the Reactions of Vinyl Ethers with (α-Diimine)PdMe+ Species

The reactions of (alpha-diimine)PdMe+ species (1, alpha-diimine = (2,6-Pr-i(2)-C6H3)N=CMeCMe=N (2,6-Pr-i(2)-C6H3)) with vinyl ethers CH2=CHOR (2a-g: R = Bu-t (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2 (e), SiPh3 (f), Ph (g); 2a-g: R = Bu-t (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2 (e), SiPh3 (f), Ph (g)) were investigated. Two pathways were observed. First, 1 initiates the cationic polymerization of 2a-c with concomitant decomposition of 1 to Pd-0. This reaction proceeds by formation of (alpha-diimine)PdR'(CH2=CHOR)(+) pi complexes (R' = Me or CH2CHMeOR from insertion), in which the vinyl ether C=C bond is polarized with carbocation character at the substituted carbon (C-int). Electrophilic attack of C-int on monomer initiates polymerization. Second, 1 reacts with stoichiometric quantities of 2a-g by formation of (alpha-diimine)PdMe(CH2=CHOR)(+) (3a-g), insertion to form (alpha-diimine)Pd(CH2CHMeOR)(+) (4a-g), reversible isomerization to (alpha-diimine)Pd(CMe2OR)(+) (5a-g), beta-OR elimination of 4a-g to generate (alpha-diimine)Pd(OR)(CH2=CHMe)(+) (not observed), and allylic C-H activation to yield (alpha-diimine)Pd(eta(3)-C3H5)(+) (6) and ROH. Binding strengths vary in the order 2a > 2b similar to 2c > 2d similar to 2g > 2e > 2f. Strongly electron-donating OR groups increase the binding strength, while steric crowding has the opposite effect. The insertion rates vary in the order 3a < 3b < 3c < 3d < 3e < 3f < 3g; this trend is determined primarily by the relative ground-state energies of 3a g. The beta-OR elimination rates vary in the order (OBu)-Bu-t < OSiR3 < OPh. For 2d g, the insertion chemistry out-competes cationic polymerization even at high vinyl ether concentrations. beta-OR elimination of 4/5 mixtures is faster for SbF6- salts than B(C6F5)(4)(-) salts. The implications of these results for olefin/vinyl ether copolymerization are discussed.