Modifying Methylalumoxane via Alkyl Exchange

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [Me<sub>2</sub>Al∙OMTS]<sup>+</sup> [(MeAlO)<sub>16</sub>(Me<sub>3</sub>Al)<sub>6</sub>Me]<sup>–</sup>. 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 (R<sub>3</sub>Al) 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 Et<sub>3</sub>Al to exchange much faster and extensively (<i>t</i><sub>½</sub> ~ 2 sec, up to 25 exchanges of Me for Et) than <i>i</i>Bu<sub>3</sub>Al (<i>t</i><sub>½</sub> ~ 40 sec, up to 11 exchanges) or Oct<sub>3</sub>Al (<i>t</i><sub>½</sub> ~ 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 R<sub>3</sub>Al species. These studies are the first to offer concrete insights into the solution transformations of MAO.