Modifying Methylalumoxane via Alkyl Exchange

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [Me₂Al∙OMTS]⁺ [(MeAlO)₁₆(Me₃Al)₆Me]^–. We can take advantage of this transformation to examine the reactivity of a key component of MAO using electrospray ionization mass spectrometry (ESI-MS), and here we describe the reactivity of this pair of ions with other trialkyl aluminum (R₃Al) components. This alkyl exchange reaction bears relevance to the various modified methylalumoxanes (MMAOs) available, which differ from regular MAO in being adulterated with different alkyl groups. We found Et₃Al to exchange much faster and extensively (t_½ ~ 2 sec, up to 25 exchanges of Me for Et) than iBu₃Al (t_½ ~ 40 sec, up to 11 exchanges) or Oct₃Al (t_½ ~ 200 sec, up to 7 exchanges). The exchanges are reversible and the methyl groups on the cation are also observed to exchange with the added R₃Al species. These studies are the first to offer concrete insights into the solution transformations of MAO.