Zirconium-Catalyzed Carboalumination of α-Olefins and Chain Growth of Aluminum Alkyls: Kinetics and Mechanism

A mechanism based on Michaelis-Menten kinetics with corn petitive inhibition is proposed for both the Zr-catalyzed carboalumination of alpha-olefins and the Zr-catalyzed chain growth of aluminum alkyls from ethylene. AlMe3 binds to the active catalyst in a rapidly maintained equilibrium to form a Zr/Al heterobimetallic, which inhibits polymerization and transfers chains from Zr to Al. The kinetics of both carboalumination and chain growth have been studied when catalyzed by [(EBI)Zr(mu-Me)(2)AlMe2][B(C6F5)(4)]. In accord with the proposed mechanism, both reactions are first-order in [olefin] and [catalyst] and inverse first-order in [AlR3]. The position of the equilibria between various Zr/Al heterobimetallics and the corresponding zirconium methyl cations has been quantified by use of a Dixon plot, yielding K = 1.1(3) x 10(-4) M, 4.7(5) x 10(-4) M, and 7.6(7) x 10(-4) M at 40 degrees C in benzene for the catalyst species [rac-(EBI)Zr(mu-Me)(2)AlMe2][B(C6F5)(4)], [Cp2Zr(mu-Me)(2)AlMe2] [B(C6F5)(4)], and [Me2C(Cp)(2)Zr(mu-Me)(2)AlMe2] [B(C6F5)(4)] respectively. These equilibrium constants are consistent with the solution behavior observed for the [Cp2Zr(mu-Me)(2)AlMe2] [B(C6F5)(4)] system, where all relevant species are observable by H-1 NMR. Alternative mechanisms for the Zr-catalyzed carboalumination of olefins involving singly bridged Zr/Al adducts have been discounted on the basis of kinetics and/or H-1 NMR EXSY experiments.