Mechanistic studies for dirhodium-catalyzed ring expansion reactions

The mechanisms for dirhodium-catalyzed ring expansion reactions of azide tethered methylenecyclopropanes and their analogues were systematically investigated by DFT calculations. The calculation results indicate that the dirhodium catalyst is essential for generating a reactive Rh2–nitrene intermediate having radical character; however, it is not essential for controlling final product selectivities. For substrates involving a three-membered ring, the experimentally obtained C–N bond formation product is a thermodynamically favored product. In contrast, for substrates having a larger ring, the kinetically favored product is the main product. The DFT calculations presented here account for previous experimental findings, and throw light on other dirhodium-catalyzed reactions involving nitrene or carbene intermediates.