Aluminum alkoxides as synthons for methylalumoxane (MAO): Product-catalyzed thermal decomposition of [Me2Al(mu-OCPh3)](2)

The thermal decomposition of [Me2Al(mu -OCPh3)](2), to yield Ph3CMe and met hylalumoxane ([MeAlO](n), MAO), is initially catalyzed by the addition of A lMe3; however, the reaction is also catalyzed by the MAO product. The overa ll reaction rate takes the form: rate = k(TMA)[{Me2Al(mu -OCPh3)}(2)] [AlMe 3] + k(MAO) [{Me2Al(mu -OCPh3)}(2)] [MAO], where k(MAO) much greater than k (TMA). The DeltaH(double dagger) for the AlMe3- and MAO-catalyzed reactions have been determined as 175 +/- 8 and 190 +/- 15 kJ . mol(-1), respectivel y. Both reactions show a large positive value of DeltaS(double dagger) (41 +/- 8 eu < 53 eu, respectively), indicative of a dissociative reaction. The thermal decomposition of [Me2Al(mu -OCPh3)](2) is also catalyzed by Lewis acids, including AlCl3, AlCl2Me, and AlClMe2. On the basis of the relative rates of the AlMe3-catalyzed thermal decomposition Of [Me2Al(mu -OCPh3)](2) , [Me2Al(9-Ph-fluoroxy)](2) (1), and [Me2Al(9-Me-fluoroxy)](2) (2) and the MAO-catalyzed C-methylation of [Me2Al(mu -OR)](2) [R = CMePh2 (3), CMe2Ph ( 4), CH2Ph, C6H11, C6H4-4-Bu-t (5)], it is proposed that the rate-determinin g step for C-methylation involves heterolytic cleavage of the O-C bond and the formation of a carbonium ion. The more stable the carbonium ion, the fa ster the reaction. Additionally, it is proposed that Lewis acid catalysis i s due to the formation of an asymmetrically bridged hemi-alkoxide, whose fo rmation is an equilibrium process such that the observed rate of the reacti on will be dependent oil the equilibrium for the reaction of [Me2Al(mu -OCP h3)](2) with the Lewis acid.