Catalytic C-H insertions using iron(III) porphyrin complexes

Fe(HI), Cu(II), and Ag(II) porphyrin complexes are active catalysts for benzylic and ring C-H insertions by carbene fragments transferred from methyl diazomalonate, 2. Temperatures above 100 C are required, and yields greater than 70% have been achieved. C-H insertions with cyclohexane and tetrahydrofuran are catalyzed at a lower temperature of 60 C with 60% yields when para-substituted methyl 2-phenyidiazoacetates, 15a-d, are used as carbene sources. The rate for Fe(TPP)CI-catalyzed insertion into the C-H bond of cyclohexane was found to be first-order in the concentration of methyl 2-(p-chlorophenyl)diazoacetate, p-Cl-MPDA, indicating that formation of a carbene complex is the rate-determining step. Competition reactions for cyclohexane insertion with para-substituted methyl 2-phenyldiazoacetates correlated linearly with sigma(+) Hammett parameters With a p value of +1.11 +/- 0.05 when Fe(TPP)Cl was used as a catalyst, demonstrating that electron-donating para-substituents on the phenyl group of the methyl 2-aryldiazoacetates enhanced reactivity. These data are consistent with the involvement of an electrophilic iron-carbene complex in the catalytic cycle. A mechanistic model for the iron-mediated C-H insertion reactions is proposed.