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

Abstract

The reactions of (alpha-diimine)PdMe(+) species (1, alpha-diimine = (2,6-(i)Pr(2)-C(6)H(3))N=CMeCMe=N(2,6-(i)Pr(2)-C(6)H(3))) with vinyl ethers CH(2)=CHOR (2a-g: R = (t)Bu (a), Et (b), SiMe(3) (c), SiMe(2)Ph (d), SiMePh(2) (e), SiPh(3) (f), Ph (g); 2a-g: R = (t)Bu (a), Et (b), SiMe(3) (c), SiMe(2)Ph (d), SiMePh(2) (e), SiPh(3) (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'(CH(2)=CHOR)(+) pi complexes (R' = Me or CH(2)CHMeOR 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(CH(2)=CHOR)(+) (3a-g), insertion to form (alpha-diimine)Pd(CH(2)CHMeOR)(+) (4a-g), reversible isomerization to (alpha-diimine)Pd(CMe(2)OR)(+) (5a-g), beta-OR elimination of 4a-g to generate (alpha-diimine)Pd(OR)(CH(2)=CHMe)(+) (not observed), and allylic C-H activation to yield (alpha-diimine)Pd(eta(3)-C(3)H(5))(+) (6) and ROH. Binding strengths vary in the order 2a > 2b approximately 2c > 2d approximately 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 O(t)Bu < OSiR(3) < 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 SbF(6)(-) salts than B(C(6)F(5))(4)(-) salts. The implications of these results for olefin/vinyl ether copolymerization are discussed.