The Metal Alkyl — Lewis Base Interaction

The cocatalyst made by the combination of an Al-alkyl and a Lewis base is actually not a simple binary system since the two components interact chemically giving rise to new products. In the case of carboxylic esters, it has been known for a long time 56) that the reaction with Al-alkyl yields Al-alkoxides from which alcohols are obtained by hydrolysis. Based on the numerous studies conducted in the last twenty years 57 72), it is known that the reaction occurs through the preliminary formation of an acid-base complex which, thereafter, undergoes rearrangement which is more or less rapid, depending on the reaction conditions. 

The true nature of the acid-base complex is still in question. In fact, a great deal of spectroscopic evidence, especially the strong shift in the C=0 stretching frequency from 1720 1730 cirT1 in the free ester to 1655-1670 cm 1 in the ester combined with 

Nevertheless, it is not yet clear whether one or two Al-alkyl molecules are involved. In fact, many authors believe only 1 1 complexes of the A type are possible, while others have different opinions. For example, based on NMR measurements, Allen63) believes that in the case of TEA and MMA a 2 1 complex of the following type may also exist  

Based on NMR and IR measurement Chien70) and Tashiro71) believe that, in the presence of excess Al-alkyl, 2 1 complexes having the structure below may coexist with the A type complexes  

On the other hand, Spitz 72) observed that upon addition of increasing amounts of EB (ethyl benzoate) to AIEt3 the concentration of the AlEt3 EB complex (by IR spectroscopy) increases until a EB/TEA ratio of 1 2 is reached and then remains constant, and he concluded that only 1 2 complexes exist with the possible structures D and E  

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