The Influence of Heterocyclic Additives

The increased rate of epoxidation observed in the presence of added pyridine has been studied by Espenson and Wang and was to a certain degree explained as an accelerated formation of peroxorhenium species in the presence of pyridine [71]. Stabilization of the rhenium catalyst through pyridine coordination was also 

A major improvement regarding epoxidation of terminal alkenes was achieved upon replacing pyridine (pJCa=5.4) with its less basic analog 3-cyanopyridine (pKa = 

The second major discovery for the use of MTO as an epoxidation catalyst came in 1996, when Sharpless and coworkers reported on the use of sub-stoichiometric amounts of pyridine as co-catalysts in the system [61]. The switch of solvent from tert-butanol to dichloromethane, and the introduction of 12 mol% of pyridine allowed for the synthesis of even very sensitive epoxides using aqueous hydrogen peroxide as the terminal oxidant. A significant rate-acceleration was also observed for the epoxidation reaction performed in the presence of pyridine. This discovery was the first example of an efficient MTO-based system for epoxidation under neutral-to-basic conditions. Under these conditions the detrimental acid-induced decomposition of the epoxide is effectively avoided. Employing this novel system, a variety of 

The increased rate of epoxidation observed using pyridine as an additive has been studied by Espenson and Wang and was to a certain degree explained as an accelerated formation of peroxorhenium species in the presence of pyridine [62]. A stabilization of the rhenium-catalyst through pyridine coordination was also detected, although the excess of pyridine required in the protocol unfortunately led to increased catalyst deactivation. As can be seen above, MTO is stable under acidic conditions but at high pH an accelerated decomposition of the catalyst into perrhenate and methanol occurs. The Bronsted basicity of pyridine leads to increased amounts of HO2 which speeds up the formation of the peroxo-complexes and the decomposi- 

Yield % (reaction time h) [a] Anhydrous H202 in MluOH. [b] Aqueous H202 (30%), [c] Pyridine and 3-cyanopyridine (6 mol% of each). 

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