Cationic polymerization and insertion chemistry in the reactions of vinyl ethers with (alpha-diimine)PdMe(+) species.

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.

Multiple insertion of a silyl vinyl ether by (alpha-diimine)PdMe+ species.

This paper reports that (alpha-diimine)PdMe+ species (1) (alpha-diimine = (2,6-iPr2 C6H3)N CMeCMe N(2,6-iPr2 C6H3)) undergo multiple insertions of CH2 CHOSiPh3 (2), ultimately forming Pd allyl products. The reaction of (alpha-diimine)PdMeCl, [Li(Et2O)2.8][B(C6F5)4] (1 equiv), and 2 (8 equiv) in CH2Cl2 yields [(alpha-diimine)Pd{eta3-CH2CHCHCH(OSiPh3)CH2CH(OSiPh3)Me}][B(C6F5)4] (7-B(C6F5)4) in 83% NMR yield. 7 is formed as a 95/5 mixture of isomers (7a/b), which converts to an equilibrium 40/60 mixture at 23 degrees C. X-ray diffraction analysis established that the configuration of the kinetically favored isomer 7a is S,S,S (ent-R,R,R), where the descriptors refer to the configurations of the substituted allyl carbon and the side chain methine carbons, respectively. 7b has an R,S,S (ent-S,R,R) configuration and differs from 7a in that the Pd is coordinated to the opposite allyl enantioface. The reaction of (alpha-diimine)PdMeCl, Ag[SbF6] (1 equiv), and 2 (8 equiv) under the same conditions yields [(alpha-diimine)Pd{(eta3-CH2CHCHCH(OSiPh3)Me)}][SbF6] (8-SbF6) in 90-100% NMR yield. 8 is formed as a 90/10 mixture of isomers (8a/b) that differ in the configuration of the substituted allyl carbon and evolve to a 70/30 equilibrium mixture at 23 degrees C. These results are consistent with a mechanism involving generation of 1 and insertion of 2 to give (alpha-diimine)PdCH2CH(OSiPh3)Me+ (4). In the absence of 2, 4 undergoes beta-OSiPh3 elimination and allylic C-H activation to give (alpha-diimine)Pd(eta3-CH2CHCH2)+ (6) and Ph3SiOH. In the presence of excess 2, 4 undergoes a second insertion of 2 to form (alpha-diimine)Pd{CH2CH(OSiPh3)CH2CH(OSiPh3)Me+ (9), which can undergo beta-OSiPh3 elimination/allylic C-H activation to form 8, or insert a third equivalent of 2 ultimately leading to 7. Further chain growth is probably disfavored by steric crowding.

Copolymerization of silyl vinyl ethers with olefins by (alpha-diimine)PdR+.

This paper reports that (alpha-diimine)PdMe+ catalyzes the copolymerization of olefins and silyl vinyl ethers. The reactions of (alpha-diimine)PdMe+ (alpha-diimine = (2,6-iPr2-C6H3)N=CMe-CMe=N(2,6-iPr2-C6H3)) with excess vinyl ethers CH2=CHOR (1a-d: R = tBu (a), SiMe3 (b), SiPh3 (c), Ph (d)) in CH2Cl2 at 20 degrees C afford polymers for 1a (rapidly) and 1b (slowly) but not for 1c or 1d. The structures of poly(1a,b) indicate a cationic polymerization mechanism. The reaction of (alpha-diimine)PdMe+ with 1-2 equiv of 1a-d proceeds by sequential C=C pi-complexation to form (alpha-diimine)PdMe(CH2=CHOR)+ (2a-d), 1,2 insertion to form (alpha-diimine)Pd(CH2CHMeOR)+ (3a-d), reversible isomerization to (alpha-diimine)Pd(CMe2OR)+ (4a-d), beta-OR elimination to generate (alpha-diimine)Pd(OR)(CH2=CHMe)+ (not observed), and allylic C-H activation to yield (alpha-diimine)Pd(eta3-C3H5)+ (5) and ROH. The reaction of (alpha-diimine)PdMe+ with 1-hexene/1b and 1-hexene/1c mixtures in CH2Cl2 at 20 degrees C affords copolymers containing up to 20 mol % silyl vinyl ether. The copolymers were purified to be free of any -[CH2CHOSiR3]n- homopolymer. The copolymer structures are similar to that of homopoly(1-hexene) generated under the same conditions. The major comonomer units are CH3CH(OSiR3)CH2-, CH2(OSiR3)CH2- and -CH2CH(OSiR3)CH2-. The 1-hexene/CH2=CHOSiR3 copolymers can be desilylated to give 1-hexene/CH2=CHOH copolymers. The results of control experiments argue against cationic and radical mechanisms for the copolymerization, and an insertion/chain-walking mechanism is proposed.