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Williams, B. S.; Leatherman, M. D.; White, P. S.; Brookhart, M. 2005. Reactions of Vinyl Acetate and Vinyl Trifluoroacetate with Cationic Diimine Pd(II) and Ni(II) Alkyl Complexes: Identification of Problems Connected with Copolymerizations of These Monomers with Ethylene. J. Am. Chem. Soc. 127: 5132-5146. Full Article

Vinyl acetate (VA) and vinyl trifluoroacetate (VAf) react with [(NN)Pd(Me)(L)][X] (M = Pd, Ni, (NN) = N,N'-1,2-acenaphthylenediylidene bis(2,6-dimethyl aniline), Arf = 3,5-trifluoromethyl phenyl, L = ArfCN, Et2O; X = B(Arf)4-, SbF6-) to form -adducts 3 and 5 at -40 C. Binding affinities relative to ethylene have been determined. Migratory insertion occurs in a 2,1 fashion (G = 19.4 kcal/mol, 0 C for VA, and 17.4 kcal/mol, -40 C for VAf) to yield five-membered chelate complexes [(NN)Pd(2-CH(Et)(OC(O)CH3))]+, 4, and [(NN)Pd(2-CH(Et)(OC(O)CF3))]+, 6. When VA is added to [(NN)Ni(CH3)]+, an equilibrium mixture of an 2 olefin complex, 8c, and a -oxygen complex, 8o, results. Insertion occurs from the 2 olefin complex, 8c (G = 15.5 kcal/mol, -51 C), in both a 2,1 and a 1,2 fashion to generate a mixture of five- and six-membered chelates, 92,1 and 91,2. VAf inserts into the Ni-CH3 bond (-80 C) to form a five-membered chelate with no detectable intermediate. Thermolysis of the Pd chelates results in -acetate elimination from 4 (G = 25.5 kcal/mol, 60 C) and -trifluoroacetate elimination from 6 (G = 20.5 kcal/mol, 10 C). The five-membered Ni chelate, 92,1, is quite stable at room temperature, but the six-membered chelate, 91,2, undergoes -elimination at -34 C. Treatment of the OAcf containing Pd chelate 6 with ethylene results in complete opening to the -complex [(NN)Pd(2-CH(Et)(OAcf))(CH2CH2)]+ (OAcf = OC(O)CF3), 18, while reaction of the OAc containing Pd chelate 4 with ethylene establishes an equilibrium between 4 and the open form 16, strongly favoring the closed chelate 4 (H = -4.1 kcal/mol, S = -23 eu, K = 0.009 M-1 at 25 C). The open chelates undergo migratory insertion at much slower rates relative to those of the simple (NN)Pd(CH3)(CH2CH2)+ analogue. These quantitative studies provide an explanation for the behavior of VA and VAf in attempted copolymerizations with ethylene.

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