| Abstract
| - The thermal decomposition of [Me2Al(μ-OCPh3)]2 to yield Ph3CMe and MAO is catalyzed by the addition of AlMe3; however, the reaction is also catalyzed by the MAO product. The overall reaction rate takes the following form: rate = kTMA[{Me2Al(μ-OCPh3)}2][AlMe3] + kMAO[{Me2Al(μ-OCPh3)}2][MAO], where kMAO ≫ kTMA. The ΔH⧧ for the AlMe3- and MAO-catalyzed reactions are similar; however, on the basis of the relative rates of reaction for [Me2Al(9-Ph-fluoroxy)]2 and [Me2Al(9-Me-fluoroxy)]2 it is proposed that the rate-determining step involves heterolytic cleavage of the O−C bond and the formation of a carbonium ion: the more stable the carbonium ion, the more facile the reaction.
- The thermal decomposition of [Me2Al(μ-OCPh3)]2, to yield Ph3CMe and methylalumoxane([MeAlO]n, MAO), is initially catalyzed by the addition of AlMe3; however, the reaction isalso catalyzed by the MAO product. The overall reaction rate takes the form: rate =kTMA[{Me2Al(μ-OCPh3)}2][AlMe3] + kMAO[{Me2Al(μ-OCPh3)}2][MAO], where kMAO ≫ kTMA. TheΔH⧧ for the AlMe3- and MAO-catalyzed reactions have been determined as 175 ± 8 and 190± 15 kJ·mol-1, respectively. Both reactions show a large positive value of ΔS⧧ (41 ± 8 euand <53 eu, respectively), indicative of a dissociative reaction. The thermal decompositionof [Me2Al(μ-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(μ-OCPh3)]2, [Me2Al(9-Ph-fluoroxy)]2 (1), and [Me2Al(9-Me-fluoroxy)]2 (2) and the MAO-catalyzed C-methylation of [Me2Al(μ-OR)]2 [R = CMePh2 (3), CMe2Ph (4), CH2Ph, C6H11,C6H4-4-tBu (5)], it is proposed that the rate-determining step for C-methylation involvesheterolytic cleavage of the O−C bond and the formation of a carbonium ion. The more stablethe carbonium ion, the faster the reaction. Additionally, it is proposed that Lewis acidcatalysis is due to the formation of an asymmetrically bridged hemi-alkoxide, whose formationis an equilibrium process such that the observed rate of the reaction will be dependent onthe equilibrium for the reaction of [Me2Al(μ-OCPh3)]2 with the Lewis acid.
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