Investigations of the hydrolysis reaction mechanism of organoaluminium compounds. 1H NMR spectroscopic studies on the R3Al/H2O reaction in polar solvents

Abstract The hydrolysis of R 3 Al (R = Me, Et, i-Bu) in diethyl ether as a solvent was investigated. The molar ratios of the reagents R 3 Al and H 2 O were 1:1 and 2:1, the temperature range −70 to 0°C. The reaction course was followed by 1 H NMR spectroscopy. A several step reaction course was found: in the first step a 1:1 complex of R 2 Al with H 2 O is formed. This complex undergoes intramolecular rearrangement to give dialkylaluminium hydroxide, R 2 AlOH. In the next step R 2 AlOH forms table autoassociates (in 1:1 systems) or reacts with the excess of R 3 Al to give aluminoxanes (in 2:1 systems). The selectivity of the whole reaction (in 2:1 systems) depends on the stability of the autoassociated dialkylaluminium hydroxide and on the relatives rates of the reaction steps. The main products of the hydroylsis reaction in 2:1 systems are alkylaluminoxanes, and in the 1:1 systems dialkylaluminium hydroxides. All the intermediates, the starting materials and the final products of the reaction are complexed by diethyl ether, which forms donor-acceptor complexes with hydrogen bondings. This complexation decreases the reaction rate and allows observation of the reaction steps by 1 H NMR spectroscopy.