Reactions of Trimethylaluminium: Modelling the Chemical Degradation of Synthetic Lubricants

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 Me(2)AlOCEtMe(2) 2 and Me(2)AlOMe 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). The (1)H 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 Me(4)Al(2)(μ(2)‐Me)(μ(2)‐OCEtMe(2)). In the presence of 3, 2(TMA) undergoes thermally induced exchange to yield Me(4)Al(2)(μ(2)‐OMe)(μ(2)‐OCEtMe(2)) 4 and TMA. The reaction of methyl phenylacetate 5 with TMA allows isolation of the crystalline product Me(2)AlOCBnMe(2)(TMA) 6(TMA), which allows the first observation of the Me(4)Al(2)(μ(2)‐Me)(μ(2)‐OR) motif in the solid state. Distances of 2.133(3) Å (Al−Me(bridging)) and 1.951 Å (mean Al−Me(terminal)) are recorded. The abstraction of TMA from 6(TMA) by the introduction of Et(2)O has yielded 6, which exists as a dimer.