Mechanism of copper(I)-catalyzed cyclopropanation: a DFT study calibrated with copper(I) alkene complexes

The complete catalytic cycle of copper-catalyzed cyclopropanation is investigated by density functional model calculations. The study focuses on obtaining accurate relative stabilities of the ligand–metal cores of all catalytic intermediates and transition states. The copper iminophosphanamide [H2P(NH2)2κ2N]Cu serves as model fragment, since three copper catalyst key intermediates with an iminophosphanamide ligand have been isolated or detected. The electronic structure of the active intermediates, copper(I) carbenes of the d10-ML3 type, and their reactivity towards ethene and vinyl alcohol is investigated. The reliability of the computed data is confirmed by their comparison with experimental data of [t-Bu2P(NSiMe3)2κ2N]Cu alkene and alkyne complexes, using relative ligand binding strengths and alkene ligand rotation barriers. The uncharged nature of the model complexes minimizes solvation artifacts and thus renders the evaluation of associative and dissociative ligand exchange pathways possible. The associative alkene ligand exchange with diazoalkane to a κN,κO-diazoalkane complex, subsequent intramolecular rearrangement to a κC-complex and N2 extrusion are identified on the most probable pathway to the electrophilic copper(I) carbene complexes of the Fischer type. They are predicted to react with alkenes to labile cupracyclobutanes with a planar N2CuIIIC2 core, which undergo facile reductive elimination of a cyclopropane derivative.