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

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posted on 14.04.2010, 00:00 by Changle Chen, Shuji Luo, Richard F. Jordan
The reactions of (α-diimine)PdMe+ species (1, α-diimine = (2,6-iPr2−C6H3)NCMeCMeN(2,6-iPr2−C6H3)) with vinyl ethers CH2CHOR (2ag: R = tBu (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2 (e), SiPh3 (f), Ph (g); 2ag: R = tBu (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 2ac with concomitant decomposition of 1 to Pd0. This reaction proceeds by formation of (α-diimine)PdR′(CH2CHOR)+ π complexes (R′ = Me or CH2CHMeOR from insertion), in which the vinyl ether CC bond is polarized with carbocation character at the substituted carbon (Cint). Electrophilic attack of Cint on monomer initiates polymerization. Second, 1 reacts with stoichiometric quantities of 2ag by formation of (α-diimine)PdMe(CH2CHOR)+ (3ag), insertion to form (α-diimine)Pd(CH2CHMeOR)+ (4ag), reversible isomerization to (α-diimine)Pd(CMe2OR)+ (5ag), β-OR elimination of 4ag to generate (α-diimine)Pd(OR)(CH2CHMe)+ (not observed), and allylic C−H activation to yield (α-diimine)Pd(η3-C3H5)+ (6) and ROH. Binding strengths vary in the order 2a > 2b2c > 2d2g > 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 3ag. The β-OR elimination rates vary in the order OtBu < OSiR3 < OPh. For 2dg, the insertion chemistry out-competes cationic polymerization even at high vinyl ether concentrations. β-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.