Silver-Catalyzed Nitrene Transfer Reactions

Department of Chemistry, University of Chicago, Chicago, Illinois 

Nitrogen is a prevalent element in naturally occurring molecules, and chemical formation of new carbon-nitrogen bonds has broad applications in both industry and academic research. Despite more recent advances in carbon-nitrogen bond formation, new facile methods are still sought after. Great advances have been made since the late 1990s, particularly in the development of the efficient hydroamination of 

Silver in Organic Chemistry Edited by Michael Harmata Copyright 2010 John Wiley Sons, Inc. 

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are 

In addition to rhodium and ruthenium, silver catalysts have also been investigated, and this chapter discusses these silver-based nitrene transfer reactions.2 1 In discussing the work chronologically, we hope that the readers can get an idea of the evolution of thinking in the research process. 

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Zraras-aziridine products were still detected from r/.v-olefin substrates, and sometimes as the predominant product. Current results on silver-catalyzed nitrene transfer reactions, indicate that silver probably can interact with iminoiodanes to generate a silver nitrene precursor. This precursor can lead to reactions via either a concerted metal nitrene or a stepwise radical pathway, depending on the substrate and reaction conditions (Scheme 6.8). 

The silyl group-transfer reaction, or the transfer of a silylene or a silylenoid intermediate to an unsaturated C—C bond, is analogous to nitrene and carbene transfers (136). Fewer methods were developed for the silylenoid transfer this is likely due to the difficulty of handling the substrates and products (137). Woerpl and co-workers (138) discovered several silver-catalyzed silylene-transfer reactions, which greatly enriched silylene-transfer chemistry and its applications. 

Subsequently, Schomaker and coworkers have developed the first example of ligand-controlled and tunable silver-catalyzed C(sp )-H amination method (Scheme 9.31) [37]. Simple silver catalysts supported by common nitrogenated ligands were used to tune a nitrene transfer reaction between two different types of C-H bonds while silver catalysts supported by Bubipy appear to prefer amination of the most electron-rich C-H bond, silver supported by a tpa ligand is more sensitive to the steric environment around the C-H bond, as well as the bond dissociation energy. 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

In 2005, Cho and Bolm (152) used the previously discussed [Ag2(f-Bu3tpy)2] system to catalyze the imination of sulfoxides. Good to excellent yields could be achieved with various sulfoximines under mild conditions. If a chiral sulfoxide is employed, the corresponding sulfoximine can be prepared after oxidation and deprotection with retention of ee. This reaction further highlights the versatility of the disilver(I) system in catalytic nitrene-transfer chemistry (111,112). The silver-bathophenanthroline system was not employed in this chemistry, however, it may give interesting results as well (Fig. 38) (120). 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

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