AZOMETHINE YLIDE GENERATION VIA THE DIPOLE CASCADE

A series of N-acyl 2-diazo-3-oxobutanoates, when treated with a catalytic quantity of a rhodium(II) carboxylate, were found to afford substituted pyrroles derived from an azomethine ylide intermediate. The initial reaction involves generation of the expected carbonyl ylide dipole by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the neighboring amide group. The primary cycloadduct undergoes a subsequent rearrangement-fragmentation reaction to give the pyrrole derivative. When the alpha-position of the alpha-diazoketone was blocked with two methyl groups, the rhodium(II) -catalyzed cycloaddition with dimethyl acetylenedicarboxylate led to the carbonyl ylide cycloadduct in high yield. MNDO calculations show that the cyclic carbonyl ylide derived from ethyl 2-diazo-4-dibenzoylamino-3-oxobutyrate is 3.3 kcal lower in its heat of formation than the corresponding azomethine ylide. This thermodynamic difference in stability of dipoles nicely accounts for why the carbonyl ylide cycloadduct is the major product from the rhodium(II) catalyzed reaction of ethyl 2-diazo-4-dibenzoylamino-3-oxobutyrate.