Rhodium(II)-catalyzed equilibration of push-pull carbonyl and ammonium ylides. A computationally based understanding of the reaction pathway

alpha-Diazo esters containing an amido group in the gamma-position have bee n found to undergo a rhodium(II)-catalyzed transformation, producing five-m embered ammonium or carbonyl ylides depending on the reaction conditions us ed. In the absence of an external dipolarophile, ammonium ylides are the ex clusive products formed. In most cases these ylides cannot be isolated as t hey readily undergo sigmatropic rearrangement or fragmentation reactions. I n the presence of typical dipolarophiles such as DMAD or N-phenylmaleimide, cycloaddition products derived from cyclic carbonyl ylide dipoles are form ed as the major products. The rhodium carbenoid intermediate generated in t hese reactions can either attack the lone pair of electrons on the amide ni trogen (ammonium ylide formation) or the lone pair of electrons on the carb onyl oxygen (carbonyl ylide formation). The experimental observations refle ct a catalyst-promoted system of equilibria with a clear-cut thermodynamic bias. To examine the underlying mechanism in detail, density functional the ory (DFT) calculations were performed on all plausible intermediates, inclu ding the full dirhodium tetracarboxylate functionality. A semiquantitative energy manifold is developed that rationalizes the empirical observations a nd provides a detailed picture of the role of the dirhodium(II) catalyst.