Possible mechanisms for Rh-promoted indole formation from vinyl/azidoarenes were examined computationally, and a mechanism is proposed in which the Rh catalyst promotes generation of a nitrene but is not directly involved in cyclization.
Rh(2)(II)-carboxylate complexes were discovered to promote the selective migration of aminomethylenes in β,β-disubstituted styryl azides to form 2,3-disubstituted indoles. Mechanistic data are also presented that suggest that the migration occurs stepwise before diffusion of the iminium ion.
The combination of Mo(CO)6 and 10 mol % of palladium acetate catalyzes the transformation of 2-nitroarenes to 3H-indoles through a tandem cyclization-[1,2] shift reaction of in situ generated nitrosoarenes. Mo(CO)6 appears to have dual roles in this transformation: generate CO and promote C-N bond formation to increase the yield of the N-heterocycle product.
A mechanism study to identify the elements that control the chemoselectivity of metal-catalyzed N-atom transfer reactions of styryl azides is presented. Our studies show that the proclivity of the metal N-aryl nitrene to participate in sp-C-H bond amination or electrocyclization reactions can be controlled by either the substrate or the catalyst. Electrocyclization is favored for mono-β-substituted and sterically noncongested styryl azides, whereas sp-C-H bond amination through an H-atom abstraction-radical recombination mechanism is preferred when a tertiary allylic reaction center is present. Even when a weakened allylic C-H bond is present, our data suggest that the indole is still formed through an electrocyclization instead of a common allyl radical intermediate. The site selectivity of metal N-aryl nitrenes was found to be controlled by the choice of catalyst: Ir(I)-alkene complexes trigger electrocyclization processes while Fe(III) porphyrin complexes catalyze sp-C-H bond amination in substrates where Rh(II) carboxylate catalysts provide both products.
A low-temperature,
protecting-group-free oxidation of 2-substituted
anilines has been developed to generate an electrophilic N-aryl nitrenoid intermediate that can engage in C–NAr bond
formation to construct functionalized N-heterocycles.
The exposure of 2-substituted anilines to PIFA and trifluoroacetic
acid or 10 mol % Sc(OTf)3 triggers nitrenoid formation,
followed by productive and selective C–NAr and C–C bond
formation to yield spirocyclic- or bicyclic 3H-indoles
or benzazepinones. Our experiments demonstrate the breadth of these
oxidative processes, uncover underlying fundamental elements that
control selectivity, and demonstrate how the distinct reactivity patterns
embedded in N-aryl nitrenoid reactive intermediates
can enable access to functionalized 3H-indoles or
benzazepinones.
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