Cyclopropanation, Epoxidation, and Aziridination of Alkenes Cu, Rh, Mn, Ti

Ugly facts, unfortunately, sometimes invalidate a beautiful mechanism. The Jacobsen epoxidation sometimes proceeds with loss of configurational purity of acyclic alkenes. This feature of the reaction can be explained by invoking radicals. Homolysis of the Mn C bond in the manganaoxetane intermediate would give a Mn(III) 1,4-diradical complex, and attack of the alkyl radical on O with displacement of Mn(II) would give the epoxide and regenerate the catalyst. 

There still exists a problem with the mechanism The square planar geometry of the salen ligand is unlikely to allow both a Mn-O and a Mn-C bond on the same face of the complex. An alternative mechanism that does not involve a Mn-C bond can be proposed. Electron transfer from the alkene to the Mn(IV) oxo complex might give a Mn(III) radical anion and an organic radical cation. Addition of the oxide to the cation would then give the same Mn(IU) 1,4-dirad- 

The O atom source can be replaced with a source of NR such as PhI=NTs, and in this case, an aziridination reaction is catalyzed. The catalyst is usually a Cu(II) complex. When the complex is chiral, asymmetric aziridination can be achieved. 

Problem 6.6. Draw a reasonable mechanism for the following reaction. You may find it useful to draw PhI=NTs in an alternative resonance form. 

The Sharpless epoxidation proceeds by a completely different mechanism from these reactions. This asymmetric O-transfer reaction uses catalytic amounts of Ti(0-t-Pr)4 and (+)- or ( )-diethyl tartrate to catalyze the epoxidation of allylic alcohols by f-BuOOH. The enantioselectivities are usually very good. 

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