Solvent-Free Epoxidation of Olefins Catalyzed by “[MoO2(SAP)]”: A New Mode of TBHP Activation

What is it about?

The mononuclear molybdenum complexes [MoO2(acac)2] (1, acac=acetylacetonate), [MoO2(SAP)(MeOH)] (2), and dinuclear oxomolybdic complexes [MoO2L]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 “[MoO2(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

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Why is it important?

We have shown here that [MoO2(SAP)]2 is able to act as a precatalyst in a solvent-free process using aqueous TBHP. Contrary to suggestions that “μ-oxo dimeric complexes lead to irreversible catalyst deactivation”, the μ-oxo dimeric SAP derivative is a stable, long-shelf-life, convenient, reversible source of 5-coordinate “[MoO2(SAP)]” with the same activity as that provided by the methanol adduct. In agreement with previous related studies, which focused on the use of alcohol adducts in organic solvents, this system shows high activity and selectivity towards the epoxidation product. The computational investigation highlights a unique feature of this “5-coordinate” catalytic system, which can activate the TBHP oxidant through a simple adduct formation, benefitting from the combined action of a hydrogen bond and a weak Mo⋅⋅⋅Oβ interaction. The ring strain in the resulting 5-membered cycle at the level of the transition state helps reduce the O atom transfer barrier. The resulting mechanism (cycle A of Scheme 3) is closely related to the classical Bartlett’s mechanism for the stoichiometric epoxidation with peracids. The results of this study point towards the power of 5-coordinate [MoO2L] structures in efficient epoxidation catalysis. It is also conceivable that some of the catalytic systems based on pseudo-6-coordinate [MoO2L] complexes, for which cycles B and C have been proposed, might operate through cycle A after prior partial decoordination of the chelating ligand.