Understanding the Selectivity of a Moderate Oxidation Catalyst:  Hydrogen Abstraction by a Fully Characterized, Activated Catalyst, the Robust Dihydroxo Manganese(IV) Complex of a Bridged Cyclam

Abstract

Distinctive mechanistic pathways account for the remarkable selectivity of a molecularly designed and robust manganese catalyst, Mn(Me 2 EBC)Cl 2, whose activated form has been isolated and thoroughly characterized. In keeping with the existence of two activated catalytic functional groups, Mn IV (OH) 2 2+ (BDE OH = 83.0 kcal/mol) and Mn IV (O)OH + (BDE OH = 84.3 kcal/mol), two distinctive Polanyi correlations are observed, the latter being over 10-fold more rapid despite their similarity. Remarkably, only relatively weak C−H bonds (≤∼82 kcal/mol) suffer hydrogen abstraction by either group, with the stoppage occurring at substrate BDE CH ≥ catalyst BDE OH . Earlier studies established a selective Lewis acid mechanism as the dominant oxygen atom insertion pathway for this catalyst. These two pathways (hydrogen abstraction and oxygen atom transfer) dominate the oxidation reactions catalyzed by these systems, accounting for the moderate oxidizing power and associated selectivity of Mn(Me 2 EBC)Cl 2 in H 2 O 2 oxidations.