Olefin Cyclopropanation Catalyzed by Iridium(III) Porphyrin Complexes

Tetratolylporphyrinato (TTP) iridium complexes were shown
to be
extremely active and robust catalysts for the cyclopropanation of
olefins using diazo compounds as carbene sources. Ir­(TTP)­CH3 (1) catalyzed the cyclopropanation of styrene with
ethyl diazoacetate (EDA) at −78 °C and achieved 4.8 ×
105 turnovers in three successive reagent additions with
no sign of deactivation. High yields and moderate trans selectivities
were attained for electron-rich and sterically unhindered substrates.
A Hammett ρ+ value of −0.23 was determined
by competition experiments with para-substituted styrenes. Furthermore,
competitive cyclopropanation of styrene and styrene-d8 with EDA and 1 demonstrated a moderate
inverse secondary isotope effect of 0.86 ± 0.03. These data are
consistent with a catalytic cycle that proceeds through a metalloporphyrin
carbene intermediate. Carbene transfer to olefin substrates appears
to be rate limiting, as indicated by kinetic studies. Hexavalent iridium
halogenato tetratolylporphyrinato complexes of the form Ir­(TTP)­X­(L),
where X = Cl, Br, I, NCS and L = CO, NMe3 (2–6), and cationic analogues, where X = BF4 and L = CO or vacant site (7, 8), also demonstrated high catalytic cyclopropanation activity.