Reactivity studies of biomimetic catalytic epoxidation of alkenes with tetrabutylammonium periodate in the presence of various manganese porphyrins and nitrogen donors: significant axial ligand π-bonding effects

Abstract

Highly selective epoxidation (>95%) of unfunctionalized alkenes was performed by tetrabutylammonium periodate in the presence of six different phenyl substituted manganese(III) meso-tetraphenylporphyrins [Mn(Por)] and imidazole in CH₂Cl₂. Electron-withdrawing and bulky substituents on the phenyl groups lowered the catalytic activities of the corresponding Mn(Por). Less bulky alkenes with electron -rich double bonds showed greater reactivity in the epoxidation. Co-catalytic activities of four different classes of axial nitrogen donors are compared in the presence of various Mn(Por). In general no direct correlation was found between co-catalytic activities and the pK_a values of the nitrogen donors. Strong σ-donor amines (pK_a = 10.6 to 11.123) and weak π-donor pyridines (pK_a = 5.25 to 6.65) showed comparable co-catalytic activities. Strong π-donor aminopyridines (pK_a = 7 to 9.71) and imidazoles (pK_a = 6.95 to 7.86) are generally much better co-catalysts than pure σ-donors, suggesting the importance of π-bonding interactions of the nitrogenous donors. N–H imidazoles with the possibility for N–H⋯B hydrogen bonding are much more effective co-catalysts than 1-methylimidazole. Methylpyridines and aminopyridines with substituents at the 2 or 2,6 positions showed particularly high co-catalytic activities toward manganese(III) meso-tetrakis(pentafluorophenylporphyrin) acetate. This “unusual” observation suggests the occurrence of some attractive hydrogen bonding interactions between ortho-fluorines of the manganese porphyrin and the substituents of these donors. A simple qualitative molecular orbital diagram is presented to describe the possible π-interactions of (d_xz, d_yz) metal orbitals with axially coordinated ImH and periodate in six-coordinate Mn(Por)(ImH)(IO₄), as the active oxidizing species. A catalytic cycle is postulated in which the intermediate involves complexation of an alkene to the coordinated periodate, and attempts have been made to visualize the orbital interactions for this intermediate species.