Reactions of Trimethylaluminium: Modelling the Chemical Degradation of Synthetic Lubricants

Abstract

In investigating and seeking to mimic the reactivity of trimethylaluminium (TMA) with synthetic, ester-based lubricating oils, the reaction of methyl propionate 1 was explored with 1, 2 and 3 equivalents of the organoaluminium reagent. Spectroscopic analysis points to the formation of the adduct 1(TMA) accompanied only by the low level 1:1 production of Me2AlOCEtMe22 and Me2AlOMe 3 when an equimolar amount of TMA is applied. The deployment of excess TMA favours reaction to give 2 and 3 over 1(TMA) adduct formation and spectroscopy reveals that in hydrocarbon solution substitution product 2 traps unreacted TMA to yield 2(TMA). The1H NMR spectroscopic observation of two Al−Me signals not attributable to free TMA and in the ratio 1:4 suggests the formation of a previously only postulated, symmetrical metallacycle in Me4Al2(μ2-Me)(μ2-OCEtMe2). In the presence of 3, 2(TMA) undergoes thermally induced exchange to yield Me4Al2(μ2-OMe)(μ2-OCEtMe2) 4 and TMA. The reaction of methyl phenylacetate 5 with TMA allows isolation of the crystalline product Me2AlOCBnMe2(TMA) 6(TMA), which allows the first observation of the Me4Al2(μ2-Me)(μ2-OR) motif in the solid state. Distances of 2.133(3) Å (Al−Mebridging) and 1.951 Å (mean Al−Meterminal) are recorded. The abstraction of TMA from 6(TMA) by the introduction of Et2O has yielded 6, which exists as a dimer.