IDENTIFICATION OF THE ACTIVE CATALYST IN THE RHODIUM PORPHYRIN-MEDIATED CYCLOPROPANATION OF ALKENES

Iodorhodium porphyrins are extremely active catalysts for the cyclopropanation of alkenes by diazo esters. Mechanistic studies of this reaction have resulted in the spectroscopic characterization of several potential organometallic intermediates in the reaction, including a novel metal diazonium complex resulting from alkylation of the rhodium center by ethyl diazoacetate. This compound is thought to decompose to a metal carbene that subsequently transfers the carbene fragment to the substrate. While our studies have led to a satisfactory general picture of the reaction mechanism, the axial ligation state of the metal under typical catalytic conditions remains unclear. We show here that the predominant active species is an iodoalkylrhodium complex that results from attack of the metal carbene by iodide. Iodination of the carbene is competitive with cyclopropanation even though the substrate is present in a vast excess. The partition ratio (cyclopropanation events/iodination events) is approximately 100:1.