Stereoselectivity, Different Oxidation States, and Multiple Spin States in the Cyclopropanation of Olefins Catalyzed by Fe-Porphyrin Complexes

The mechanism of the cyclopropanation of alkenes with diazocompounds catalyzed by ClFeIII-porphyrin, [C1Fe(II)-porphyrin](-), and Fe-II-porphyrin has been investigated by density functional theory calculations. The obtained results indicate that the only viable catalyst is the Fe-II-porphyrin, whose catalytic cycle involves a stepwise multistate (singlet, triplet, and quintet spin states) mechanism. The triplet Fe-II-porphyrin interacts with diazomethane leading to the formation of the axial and bridged Fe-II-carbene complexes. The former type is favored at the singlet state, the latter is the most stable at higher states, and both conformations are accessible at the experimental conditions. The second key step of the reaction consists of the (2 + 1) cycloaddition of the axial Fe-II-carbene complex to yield the corresponding cyclopropane. Additionally, we have explored the possibility of catalyzing the reaction via a double incorporation of the carbene moiety into the metal coordinating sphere. The obtained results indicate that the formation of the biscarbene Fe-II-porphyrin intermediates and generation of the (2 + 1) cycloadduct present small energetic barriers. Consequently, both complexes, monocarbene and biscarbene, result to be crucial for a complete understanding of the mechanism and kinetics in the cyclopropanation of olefins catalyzed by Fe-II-porphyrin. Several minimum energy crossing points ensure the kinetic and thermodynamic feasibility of the reaction. Additionally, the reported mechanism is compatible with the observed trans selectivity.