Modifying methylalumoxane via alkyl exchange.

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [MeAl·OMTS] [(MeAlO)(MeAl)Me]. 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 (RAl) components. Using continuous injection methods, we found EtAl to exchange much faster and extensively at room temperature in fluorobenzene (t∼2 s, up to 25 exchanges of Me for Et) than iBuAl (t∼40 s, up to 11 exchanges) or OctAl (t∼200 s, up to 7 exchanges). The exchanges are reversible and the methyl groups on the cation are also observed to exchange with the added RAl species. These results point to the reactive components of MAO having a structure that deviates significantly from the cage-like motifs studied to date.