Nitrenoid compounds

Representative examples of ring syntheses involving carbenoid (Table 6) or nitrenoid (Table 7) intermediates are given. In many cases, the free carbene or nitrene is probably not involved, and the distinction between insertion and addition reactions given in the tables is not always clear cut. Such reactions are particularly useful for the preparation of tricyclic compounds. 

One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

Animations in this area involve anions of both it-excessive and it-deficient heterocycles, which are generated from the halo compounds or by direct metalation. Most of the animating reagents seem to be applicable except that phenylthiomethyl azide (32) fails with the 2-lithium or 2-copper derivatives of furan, thiophene, N-methylpyrrolc, and (V-methylindole.274 Similarly, chloramine and O-methylhydroxylaminc, but not phenyl azide, fail to aminate 2-lithio-1-methylimidazole 66 and the MeN(Li)OMe nitrenoid does not react with 2-lithiothiophene.97 The reactions that appear to be most widely applicable to heterocyclic carbanions are shown in Eqs. 73,100,101 74,316 and 75.358... 

Direct preparation of an aziridine from an alkene is possible by reaction of the alkene with a nitrene or metal nitrenoid species. Nitrenes can be generated thermally or photochemically from azides, although their reaction with alkenes to give aziridines is often low yielding and is complicated by side reactions. Oxidation of iV-amino-phthalimide or related hydrazine compounds (e.g. with Pb(OAc>4 or by electrolysis) and reaction with an alkene has found some generality. The metal-catalysed reaction of nitrenes with alkenes has received considerable study. A variety of metal catalysts can be used, with copper(II) salts being the most popular. For example, styrene was converted to its A-tosyl aziridine 72 by reaction with [A-(tosyl)imino]phenyliodinane (PhI=NTs) and copper(II) triflate (5.75). ... 

Nevertheless, there still exist some obstacles to be overcome. Generally, the diazo compounds as carbene precursors require a slow addition technique to avoid potential side reactions such as dimerization. For C—H bond insertion by metal nitrenoids, and metal oxo species, only a handful of substrates can lead to products in satisfactory yield and enantioselectivity. Under these circumstances, albeit in limited cases, new carbene precursors are still in great demand to make the reaetion more operative. Since Rh catalysts are not so eheap, reduetion in the eatalyst loading, recovery of eatalyst, simplified syn-thetie routes for the eatalyst, and other cheap and efficient metal catalysts might be the potential solutions to solve this issue. Furthermore, C—H bond insertion by metal nitrenoids, and metal oxo species are largely unexplored. 

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