Mechanism of the rhodium porphyrin-catalyzed cyclopropanation of alkenes.
The rhodium porphyrin-catalyzed cyclopropanation of alkenes by ethyl diazoacetate (EDA) is representative of a number of metal-mediated cyclopropanation reactions used widely in organic synthesis. The active intermediate in these reactions is thought to be a metal carbene complex, but evidence for the involvement of metal-olefin pi complexes has also been presented. Low-temperature infrared and nuclear magnetic resonance spectroscopies have been used to characterize a rhodium porphyrin-diazoalkyl adduct that results from the stoichiometric condensation of the catalyst and EDA. Optical spectroscopy suggests that this complex is the dominant steady-state species in the catalytic reaction. This compound decomposes thermally to provide cyclopropanes in the presence of styrene, suggesting that the carbene is indeed the active intermediate. Metal-alkene pi complexes have also been detected spectroscopically. Kinetic studies suggest that they mediate the rate of carbene formation from the diazoalkyl complex but are not attacked directly by EDA.
Maxwell
J L
JL
Brown
K C
KC
Bartley
D W
DW
Kodadek
T
T