Dipole cascade, azomethine ylide generation

Dipoles can also be generated from rearrangements that take place after the formation of an initial rhodium carbenoid product ]40, 70, 71]. One example of this type of transmutation, also known as a dipole cascade process, involves the formation of an azomethine ylide via the initial formation of a carbonyl ylide [72]. This process was... 

In the dipole cascade reaction, a proton must be removed from the a-carbon atom in order to generate the azomethine ylide. When the a-position of the pyrrolidine ring was blocked by a benzyl group, formation of the azomethine ylide dipole could not occur. In fact, treatment of diazoketone 186 with rhodium(II) acetate in the presence of dimethyl acetylenedicarboxylate afforded only the carbonyl ylide-derived cycloadduct 187 in 95% yield [117]. 

The Rh(II)-catalyzed reaction of pyridone 96 with DMAD was also found to give cycloadducts derived from an intermediate azomethine ylide. The initial reaction involves generation of the expected carbonyl ylide by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the amide group. A subsequent proton shift generates the thermodynamically more stable azomethine ylide, which is trapped by DMAD. This is an example of subsequent formation of ylides of two types, a phenomenon termed a dipole cascade (93JOC1144). 

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