Metal Nitrenes from Iminoiodinanes

Rhodium dimer complexes are the most widely used catalysts to perform C—H aminations. Other metal complexes that catalyze metal nitrene C—H insertion reactions, include metalloporphyrins [12, 39], silver [21, 40], copper [22], palladium [41] and gold [42] complexes. 

A number of reviews have appeared recently on C—H insertion of metal nitrenes [18-20], some of them specifically dedicated to rhodium catalysts [16, 24, 43]. 

This chapter vhll focus only on the most recent aspect of rhodium nitrene chemistry to perform C—H amination of alkanes, which has not been covered in previous reviews. The literature has been surveyed from 2001 to September 2008. 

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High yields are obtained with iminoiodinane reagents, however, they cannot be stored and must be used immediately [15]. Moreover, they are tedious to purify, as they form polymeric structures and are insoluble in most organic solvents. Besides tosyl- and nosyl-substituted iminoiodinanes, others are almost impossible to isolate in pure form. It is to overcome these problems that one-pot processes in which the iminoiodinane species is prepared in situ rather than being isolated, have been developed. Che first demonstrated such a reaction for an intermolecular C H insertion (Eq. (5.3)) [31, 32]. The reactive metal nitrene species is generated from a mixture of the requisite amine with diacetoxyiodobenzene, in the presence of manganese porphyrin catalyst. 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

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