Iron-catalyzed carbene and carbene radical cascade reactions: Mechanistic study and synthetic applications

Transition metal-catalyzed carbene transfer reactions are some of the most widely used methods that facilitate a range of otherwise inaccessible chemistry. However, these advantages generally promoted by precious metal catalysts, using inexpensive and less toxic iron complexes as catalysts is under development. Moreover, surprisingly little is known about the mechanistic aspects, in particular the structures of these intermediates. Herein, we report an iron-catalyzed cascade reaction of alkyne-tethered diazo compounds, offering an efficient approach for the synthesis of carbocyclic molecules with structural diversity and flexibility under mild conditions. Control experiments and density functional theory calculations unambiguously reveal two distinct reaction pathways catalyzed by either Fe(II) or Fe(III) porphyrin complexes, which involves carbene and carbene radical intermediates, respectively. The structure of the key vinyl iron carbene intermediate has been determined by X-ray diffraction. The synthetic utility has been demonstrated by gram-scale preparation and synthesis of poly-substituted arenes via a streamlined one-pot process.