Three different mechanisms for azo-ether hydrolyses in aqueous acid

Abstract

It has been shown recently that most ethers hydrolyze in aqueous acid media not by the traditional A1 or A2 process, but by a mechanism involving rate-determining proton transfer to the substrate, concerted with C–O bond cleavage. The reactions of azoethers are more complicated, because the azo group can be protonated in the acid reaction medium as well. This protonation has to be accounted for in the kinetic analysis. Often it simply ties up the substrate in an unreactive form; the hydrolysis reaction slows down as a result of the azo-protonated compound not being the reactant in the hydrolysis. However, there are other possibilities. If the ether group is suitably located in the substrate the azo-protonated compound can react with three water molecules (a “water wire”) in a fast reaction, and the alkoxy group is lost as a result. Depending on the acidity, in this mechanism either the initial three-water attack, or the breakup of the resulting intermediate, can be rate-determining, and both of these were observed. A third possibility is that ring protonation of suitable substrates can occur, giving delocalized carbocations that can form a hydrolysis product in subsequent fast reactions. Thus, three different hydrolysis mechanisms for azoethers in acidic media can be observed. Six azoethers were studied, one of which contained two methoxy groups. Both of these hydrolyzed, but by different mechanisms.