Solvent‐Free Epoxidation of Olefins Catalyzed by “[MoO 2 (SAP)]”: A New Mode of tert ‐Butylhydroperoxide Activation

The mononuclear molybdenum complexes [MoO 2 (acac) 2 ] ( 1 , acac=acetylacetonate), [MoO 2 (SAP)(MeOH)] ( 2 ), and dinuclear oxomolybdic complexes [MoO 2 L] 2 [L=salicylideneaminophenolato (SAP, 5 ), salicylideneaminoethanolato (SAE, 6 ), salicylideneaminomethylpropanolato (SAMP, 7 )] have been investigated as (pre)catalysts for the epoxidation of olefins under solvent‐free conditions, using tert ‐butylhydroperoxide (TBHP, 70 % in water) as an oxidant. Complexes 6 and 7 , although active, are limited by ligand hydrolysis during the catalytic process, whereas complexes 2 and 5 are not altered under catalytic conditions and yield essentially the same selectivity and activity, which is not suppressed by excess MeOH. Although these catalysts are less active than 1 , their selectivity is higher (97–98 %). DFT calculations are consistent with the active form of the catalyst being the 5‐coordinate “[MoO 2 (SAP)]”. The oxidant is activated by forming a weak adduct stabilized by a very loose Mo⋅⋅⋅O interaction and a hydrogen bond, predisposing it to the oxygen transfer to external olefin by a mechanism closely related to Bartlett’s epoxidation with peroxyacids.