Mechanistic Studies of Palladium(II)-α-Diimine-Catalyzed Polymerizations of cis- and trans-2-Butenes

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Abstract

Complexes [(N∧N)Pd(CH3)(L)]BAr‘4 (3, 4: L = NCAr‘; 5: L = OEt2) (N∧N ≡ ArNC(R)−C(R)NAr, 3, 5: Ar = 2,6-C6H3(Me)2; 4: Ar = 2,6-C6H3(iPr)2; Ar‘ ≡ 3,5-C6H3(CF3)2) catalyze the polymerization of trans- and cis-2-butene. Both the productivity and the molecular weight of poly(cis-2-butene) are much lower than those of poly(trans-2-butene). The polymers exhibit atactic regioregular microstructure with a methyl group on every third backbone carbon. Low-temperature NMR studies show that migratory insertion in the η2-butene complexes [(N∧N)Pd(CH3)(CH3CHCHCH3)]+ (6 and 11) occurs to give isomerized alkyl olefin complexes [(N∧N)Pd(CH2CH2CH(CH3)2)(CH3CHCHCH3)]+ (8 and 12). The first insertion barrier of trans-2-butene (19.1 kcal/mol) is slightly lower than that of cis-2-butene (19.3 kcal/mol), while the subsequent insertion barrier of cis-2-butene (20.2 kcal/mol) is 0.6 kcal/mol higher than that for trans-2-butene (19.6 kcal/mol). The isopentyl palladium complexes 14a and 14b, which most closely model the propagating species, exhibit nearly equal binding affinities for cis- and trans-2-butene.

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