Phthalic anhydride metal catalysis

In trying to formulate a reaction mechanism for the catalysis, the well known reaction of aluminum alcoholates with carboxylic anhydrides was used as a basis (14). For example, phthalic anhydride, maleic anhydrides, and carbon dioxide, will react so that the acid anhydride pushes itself between the metal atom and the alkoxy groups, thus separating them. This results in forming neutral aluminum salts of the monoalkyl carboxylic acids. It is possible, of course, for the conversion to proceed incompletely, having two alkoxy groups bound to the aluminum, thus having only 1 mole of acid anhvdride react on 1 mole of aluminum alcoholate. 

Breslow and coworkers420 have studied the hydrolysis of the anhydrides (128) and (129) in both the presence and absence of zinc(II) (and other metal ions). In the the absence of metal ion, hydrolysis of the anhydrides is independent of pH in the region 1.0-7.5, as also occurs with phthalic anhydride.421 However, in the presence of zinc(II), the hydrolysis is first order in hydroxide above pH 5. Table 28 lists values of kobs for the various substrates at pH 7.5 (for the metal complex k0bs = kOH[0H-]). The catalytic effects are of the order of 103. The pH dependence of the zinc(II) catalysis is consistent with attack by external hydroxide on a complex such as (130) where the metal acts as a Lewis acid catalyst. A further possibility involves attack by coordinated hydroxide on an uncoordinated anhydride carbonyl (131), and there is some evidence that this is indeed the process which does occur. 

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