Mechanism of cyclopropanation. reactions mediated by (5,10,15,20-tetra-p-tolylporphyrinato)osmium(II) complexes

Catalytic systems derived from [Os(TTP)](2) or Fe(TTP) (TTP = 5,10,15,20-tetra-p-tolylporphyrinato) are extremely efficient at converting styrenes and diazo reagents to cyclopropanes in high yields and high stereoselectivity. A number of mechanistic studies have been undertaken to elucidate the catalytic pathway. A mono(carbene) complex, (TTP)Os=CHCO2Et, has been isolated but is not the catalytically active species. An electron-withdrawing ligand trans to the carbene in (TTP)Os=CHCO2Et activates the carbon fragment toward transfer to an olefin. Labeling studies with (TTP)Os=CHX and N2CHY and substrate reactivity profiles are consistent with a trans-osmium(II) bis(carbene) species as the active catalyst.