Catalytic Cycle and the Mechanism of Propylene Epoxidation

The proposed catalytic cycle is shown in Fig. 8.5. A variety of molybdenum compounds may be used as the precatalyst, and Mo(CO)6 is shown as a representative one. Under the strong oxidizing conditions the precatalyst is oxidized to 8.21, a species that has molybdenum in a 6+ oxidation state and a di-MoOf unit. The other ligands are two solvent molecules and hydroxo and/ or alkoxo groups. In the absence of a solvent, positions occupied by S are occupied by /-butanol, the decomposition product of /-butyl hydroperoxide. The important points to note are that molybdenum is in its highest oxidation state (6+), and there are weakly bound solvent molecules. 

In the other mechanistic postulate, coordination by the alkene as in 8.25 followed by insertion into the Mo-O bond to give a metallocyclic species 8.26 is proposed. As shown by Path B, this metallocyclic species is then converted to 8.24 through an intramolecular rearrangement. 

Species like 8.22 with V5+ as the metal ion has been isolated and characterized by X-ray studies (see Section 2.5.3). The reactivity of such complexes with alkenes has considerable similarities to the molybdenum-catalyzed epoxidation reaction. Kinetic studies with these model complexes indicate coordi- 

Unfortunately the macroscopic rate law cannot differentiate between Path A and Path B. The rate law in this and other related epoxidations is found to be 

All the evidence taken together suggest possible coordination by propylene as in 8.25, but conversion of 8.25 directly to 8.24. In other words, there is no evidence for the involvement of a species such as 8.26. In asymmetric epoxi-dation of allylic alcohols, to be discussed in the next chapter, the alkene coordinates to the metal ion through the oxygen lone pair rather than the C-C double bond. 

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