Tuning the selectivity of [Mn^V(TAML)(O)]^- towards allylic oxidation using an axial ligand and a Brønsted acid - a DFT study

The catalytic activity and selectivity of [Mn^V(TAML)(O)]^- (1) towards allylic oxidation of cyclohexenol were investigated using DFT methods. Complex 1 is stable in the low-spin state (S = 0), while reactivity proceeds through the high-spin state (S = 1), requiring +11.3 kcal mol^-1 to attain the reactive spin state. Mechanistic studies reveal that allylic C-H activation (ΔG^‡ = +17.4 kcal mol^-1) is favoured over O-H activation (ΔG^‡ = +23.2 kcal mol^-1) by +5.8 kcal mol^-1, both yielding thermodynamically identical products (ΔG_R.E = -56.4 kcal mol^-1). The competing epoxidation pathway has a comparable barrier (ΔG^‡ = +19.6 kcal mol^-1; ΔG_R.E = -23.4 kcal mol^-1), indicating poor selectivity in 1. Introduction of an axial ligand -OCN^- (1a) changes the active oxidant to Mn^IV-O˙, eliminates the spin-state energy gap, and greatly enhances reactivity, lowering the barriers for C-H activation (ΔG^‡ = +3.2 kcal mol^-1; ΔG_R.E = -81.5 kcal mol^-1) and epoxidation (ΔG^‡ = +6.7 kcal mol^-1; ΔG_R.E = -50.8 kcal mol^-1). Furthermore, incorporation of the HOTf moiety generates Mn^IV-OH as the active oxidant (1b), where the allylic C-H activation occurs with ΔG^‡ = +8.7 kcal mol^-1 (ΔG_R.E = -45.3 kcal mol^-1), while epoxidation becomes highly unfavourable (ΔG^‡ = +42.8 kcal mol^-1; ΔG_R.E = -5.0 kcal mol^-1), demonstrating selective cyclohexenone formation with 50% kinetic enhancement.