ALPHA-DEUTERIUM AND BETA-DEUTERIUM ISOTOPE EFFECTS IN THE MGX(2) AND METHYLALUMINOXANE PROMOTED INTRAMOLECULAR OLEFIN INSERTION OF CP(2)TICLR COMPLEXES - INSIGHT INTO COCATALYST DEPENDENCE AND CHAIN-END CONTROL IN ZIEGLER-NATTA POLYMERIZATION

Participation of alpha- and beta-hydrogens in the intramolecular insertion of an alpha-olefin into a titanium-carbon bond was examined through competitive cyclization of isotopically labeled 2-alkyl-6-hepten-1-yl ligands. Comparison of cyclization rates revealed deuterium isotope effects for both the alpha and beta sites of a propagating chain model (L(n)MCH(2)-CHRCH(2)P) in which the alpha-, beta-, and gamma-positions of the ligand reflected the structural features of a propagating poly alpha-olefin chain. In particular, the polymerizations of 1-pentene (R = n-propyl) and 1-hexene (R = n-butyl) were modeled. Through the use of MgX(2) to promote alkene insertion, unusual mechanistic features of this insertion process were observed in which both alpha- and beta-agostic interactions were involved in the rate determining step of olefin insertion. The k(H)/k(D) values for the alpha-position were 1.22 +/- 0.03 and 1.28 +/- 0.03 for the n-propyl and n-butyl substrates, respectively, while the k(H)/k(D) values for the beta-position were 1.09 +/- 0.02 and 1.10 +/- 0.02 for these substrates. When both sites were labeled, alpha- and beta-hydrogens worked in concert to produce a k(H)/k(D) of 1.36 +/- 0.03. In contrast, when insertion was promoted by methylaluminoxane, an inverse deuterium isotope effect was observed for the alpha-hydrogen, with a k(H)/k(D) for the alpha-hydrogen participation of 0.88 +/- 0.04 and 0.95 +/- 0.04 for the n-propyl and n-butyl substrates, respectively. An opposite but complementary effect was observed for beta-hydrogen participation, for which a k(H)/k(D) value of 1.06 +/- 0.04 was obtained for each substrate.