Facile Alkylation of Cobalt(III) Porphyrins by Organosilicon Compounds

(σ-Alkyl)cobalt(III) porphyrins were easily synthesized and isolated as stable crystalline compounds by the reaction of halogenocobalt(III) porphyrins with silyl enol ethers and ketene silyl acetals. The yields depended on the solvent (CH2Cl2 > MeOH), the substituent of the porphyrin periphery (meso-tetraphenylporphyrin > octaethylporphyrin), and the axial halogeno ligand of cobalt(III) porphyrins (F ≥ Cl ≫ Br ≈ I ≈ ClO4). 1-((Trimethylsilyl)oxy)-1,3-butadiene afforded (σ-4-oxobut-2-enyl)cobalt(III) porphyrins, which showed very broad 1H NMR signals at 25 °C. The temperature-dependent dynamic behavior of this complex was explained in terms of the rotation around the C(α)−C(β) bond but not of the σ−π rearrangement in the 4-oxobut-2-enyl ligand.