Mechanism of C-H Amination using Hypervalent Iodine Reagents

Mechanism of C—H Amination using Hypervalent Iodine Reagents 

Further evidence to exclude the triplet radical pathway includes the use of cyclopropyl substrates, which serve as a radical clock. In all cases, the reaction proceeds with no indication of ring fragmentation. The nature of the transition state of the C—H insertion step has been analyzed, via a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes. A negative q value of 0.73 is obtained for the intermolecular reaction with trichloroethylsulfamate [71]. Such data indicate that there is a small, but significant, preference for electron-rich substrates, thus the resonance does contribute to the stabilization of a partial positive charge at the insertion carbon in the transition state. A kinetic isotope value of 1.9 is observed for competitive intramolecular C—H amination with a deuterated substrate (Eq. (5.21)). 

The development of in situ procedures to prepare iminoiodinanes from amine precursors and hypervalent iodine reagents has had a significant impact on the progress made towards C—H amination reactions. However, there are some drawbacks associated with the use of hypervalent iodine reagents, in particular, their cost 

The cyclization of N-tosyloxycarbamates is also stereospedfic and enantiopure substrates produce the corresponding oxazolidinone without racemization (Eq. (5.23)). 

Davies has recently developed a chiral rhodium dimer derived from adamantane glycine, Rh2[(S)-tcptad]4 [75]. He demonstrated that this was the most efficient catalyst for the enantioselective synthesis of oxazolidinones via C—H amination, using N-tosyloxycarbamates as nitrene precursors (Eq. (5.24)). Enantioselectivities up to 82% are observed in this case. 

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