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 $\textbf{1}$ was explored with 1, 2 and 3 equivalents of the organoaluminium reagent. Spectroscopic analysis points to the formation of the adduct $\textbf{1}$ (TMA) accompanied only by the low level 1:1 production of Me${2}$AlOCEtMe${2}$ $\textbf{2}$ and Me${2}$AlOMe $\textbf{3}$ when an equimolar amount of TMA is applied. The deployment of excess TMA favours reaction to give $\textbf{2}$ and $\textbf{3}$ over $\textbf{1}$(TMA) adduct formation and spectroscopy reveals that in hydrocarbon solution substitution product $\textbf{2}$ traps unreacted TMA to yield $\textbf{2}$(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 $\textbf{3}$, $\textbf{2}$(TMA) undergoes thermally induced exchange to yield Me${4}$Al${2}$(μ$^{2}$-OMe)(μ$^{2}$-OCEtMe${2}$) $\textbf{4}$ and TMA. The reaction of methyl phenylacetate $\textbf{5}$ with TMA allows isolation of the crystalline product Me${2}$AlOCBnMe${2}$ (TMA) $\textbf{6}$(TMA), which allows the first observation of the Me${}$ 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 $\textbf{6}$(TMA) by the introduction of Et${2}$O has yielded $\textbf{6}$, which exists as a dimer.