Action of Organoaluminum Reagents on Esters: Alkene Production and the Degradation of Synthetic Lubricants

Abstract

Reactions of methylaluminum reagents with ester-based lubricating oils are mimicked through the reaction of trimethylaluminum (TMA) with tetraesters C­(CH2OC­(O)­R)4 (R = C5H11 4 Pent, Bn 4 Bn). Using a 2:1 stoichiometry gave adduct 4 Pent(TMA)4. NMR spectroscopy on 1:1–12:1 TMA/4 Pent systems suggested 4 Pent gave dimethylated adduct C5H11CMe2OAlMe2(TMA), 2 Pent(TMA). Similar combination of TMA with 4 Bn at raised temperatures transformed 4 Bn into C­(CH2OAlMe2)4(2 Bn)4 5(2 Bn)4 by sequential reaction of each ester group. Doubly reacted {BnC­(O)­OCH2}2C­(CH2OAlMe2)2(2 Bn)2 7 Bn(2 Bn)2 was isolated and characterized. A Mitsubishi molecule could also be isolated, its formation rationalized by the elimination of 2 Bn and TMA from 5(2 Bn)4. The action of Me n AlCl3–n (n = 1, 1.5, 2) was studied initially on monoester BnC­(O)­OMe 1 Bn. Combining excess Me2AlCl with 1 Bn gave adduct 1 Bn(Me2AlCl) and small amounts of dimethylated BnCMe2OAlMe2(Me2AlCl), 2 Bn(Me2AlCl), and MeOAlCl2 10. 2 Bn(Me2AlCl) was fully characterized and, in the presence of 10, acted as a source of 2 Bn(MeAlCl2). From this species, a mixture of alkenes could be generated by formal elimination of Me3Al2(OH)­Cl2 13, the decomposition of which was presumed to also explain MeH observation. Replacing Me2AlCl with aluminum sesquichloride or MeAlCl2 led to progressively more sluggish but similar reactions. Using Me n AlCl3–n (n = 1, 1.5, 2) with tetraesters suggested similar reactivity to monoesters.