Atom Transfer of Oxo and Imido Groups to Olefins

The mechanism of the transfer of an oxygen atom in a metal-oxo complex to an olefin to form an epoxide includes a vast literature, which has been summarized in books on biomimetic oxidations. Thus, it is impossible to even touch on most of the work in a few paragraphs. As a result, this section focuses on examples of well-defined metal-oxo complexes that react with olefins to form epoxides. 

The mechanisms of the epoxidations of olefins have been studied intensively.3 3 -376-378 In general, the reactions of metal-oxo complexes with olefins to form epoxides do not involve intermediates containing metal-carbon bonds. The 0x0 group tends to act as an electrophile and interact with the HOMO of the olefin during the transfer of the 0x0 to the olefin. After this initial interaction, the epoxide may form by a non-radical concerted process or by a stepwise process involving radical or cationic intermediates.  

The epoxidation of olefins does not appear at this time to involve radical intermediates in most systems, but epoxidation through two different spin states of the same complex has gained acceptance. In some cases, side products imply that intermediates are present, but these intermediates have been concluded by Bruice to be carbocationic instead of radical. The carbocation would allow for Z/E isomerization and formation of oxidation products other than epoxides. Studies on the epoxidations of an olefin attached to 

The aziridination of olefins has also been studied, but fewer complexes catalyze this reaction as efficiently as iron and manganese complexes catalyze the epoxida-tion of olefins. Nevertheless, the aziridinations of olefins catalyzed by copper, ruthenium, and rhodium complexes have been reported. The source of nitrogen is usually [N-(p-toluenesulfonyl)imino]phenyliodinane (PhI=NTs) or a precursor to a related iodoarylimine. The aziridine is likely generated from these copper- and rhodium-catalyzed reactions by an outer-sphere process in which the olefin interacts with the LUMO of the complex, which is located at the nitrogen. This mechanism is more likely to be followed by these catalysts than a [2-t-2] process, followed by reductive elimination. 

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