Mechanisms in manganese oxidation catalysis with 1,4,7-triazacyclononane based ligands

Activation of alkyl hydroperoxides by manganese complexes of tmtacn for initiation of radical polymerisation of alkenes

“…15,23 This catalyst activation step is not achieved with alkyl hydroperoxides, however, as they are not able to reduce 1. Indeed in the oxidation of benzyl alcohol with tBuOOH reported by Zondervan et al, 16,24,30 activity was only observed when preactivation of 1 was carried out with H 2 O 2 . Later, de Boer showed that tBuOOH could be used without catalyst preactivation, with the already reduced (relative to 1) complex 2a, albeit with much lower extent of oxidation of alkene than with H 2 O 2 as oxidant.…”

“…Complex 1 has been applied widely with H 2 O 2 in the epoxidation and dihydroxylation of alkenes, alcohol, aldehyde, and C-H oxidations. 5,16 The reduction of 1 and subsequent ligand exchange reactions to form, e.g., 2b and 2a, are key to its catalytic activity in these reactions. The reduction is autocatalytic and the lag periods observed in oxidations with H 2 O 2 are due to waiting for reduction of 1 (Fig.…”

“…23,37 Hence, it is expected that changes to the redox state of 1 and 2b can be monitored in real-time by UV/vis spectroscopy. It should be noted that although EPR spectroscopy at 77 K is generally useful in the study of manganese complexes, the complexes 1, 2b and 2a are EPR silent, and the EPR signals expected for Mn(II) complexes of tmtacn, 16 are overwhelmed by the organic radicals present in cured resins. 14…”

“…[18][19][20] Amongst the various classes of manganese complexes, those of the triazacyclononane family, and especially N,N′,N″-trimethyl-1,4,7,-triazacyclononane (tmtacn), have received considerable attention for activation of H 2 O 2 over the last three decades. 16,21,22 Manganese complexes of tmtacn show a diverse oxidation state dependent coordination chemistry, with isolated monoand dinuclear complexes ranging from the Mn(II) to Mn(IV) oxidation states (Fig. 1).…”

“…2,13 Manganese complexes of alkyl amine and alkyl pyridyl ligands have proven to be a diverse and robust family of catalysts for the activation of peracids and H 2 O 2 , especially towards alkene, alcohol, aldehyde and alkane C-H oxidation. [5][6][7]16 For fine chemical production, most academic efforts have focused on oxidations in CH 3 CN and water, with a particular focus on activation of peracids and H 2 O 2 to generate reactive high valent complexes that engage in hydrogen atom abstraction and oxygen atom transfer reactions. The activation of alkyl (hydro)peroxides, especially by manganese complexes, 17 has received much less attention in part due to their reduced atom economy and tendency to form alkoxy radicals.…”