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.
A new reactivity pattern of Rh(II)-N-arylnitrenes was discovered that facilitates the synthesis of medium-sized N-heterocycles from ortho-cyclobutanol-substituted aryl azides. The key ring-expansion step of the catalytic cycle is both chemoselective and stereospecific. Our mechanistic experiments implicate the formation of a rhodium N-arylnitrene catalytic intermediate and reveal that sp C-H bond amination of this electrophilic species is competitive with the ring-expansion process.
The development of the first intermolecular
Rh2(II)-catalyzed
aziridination of olefins using anilines as nonactivated N atom precursors
and an iodine(III) reagent as the stoichiometric oxidant is reported.
This reaction requires the transfer of an N-aryl
nitrene fragment from the iminoiodinane intermediate to a Rh2(II) carboxylate catalyst; in the absence of a catalyst only diaryldiazene
formation was observed. This N-aryl aziridination
is general and can be successfully realized by using as little as
1 equiv of the olefin. Di-, tri-, and tetrasubstituted cyclic or acylic
olefins can be employed as substrates, and a range of aniline and
heteroarylamine N atom precursors are tolerated. The Rh2(II)-catalyzed N atom transfer to the olefin is stereospecific as
well as chemo- and diastereoselective to produce the N-aryl aziridine as the only amination product. Because the chemistry
of nonactivated N-aryl aziridines is underexplored,
the reactivity of N-aryl aziridines was explored
toward a range of nucleophiles to stereoselectively access privileged
1,2-stereodiads unavailable from epoxides, and removal of the N-2,4-dinitrophenyl group was demonstrated to show that
functionalized primary amines can be constructed.
Recent catalytic methods to construct medium-sized lactams and partially saturated benzazepines and their derivatives are surveyed. The review is divided into the following sections:1 Introduction2 Non-Transition-Metal-Catalyzed Reactions2.1 Beckmann Rearrangement2.2 Non-Beckmann Rearrangement Reactions2.3 Multicomponent reactions3 Transition-Metal-Catalyzed Reactions3.1 Gold-Catalyzed Reactions to Access Medium-Sized N-Heterocycles3.2 Reactions Involving a Metal η3-Complex Catalytic Intermediate3.3 Transition-Metal-Catalyzed Reactions of Strained Cycloalkanes4 Conclusions
The development of the first intermolecular Rh<sub>2</sub>(II)-catalyzed aziridination of di-, tri-, or tetraubstituted olefins using aryl- or heteroaryl amines as nonactivated N-atom precursors and an iodine(III) reagent as the stoichiometric oxidant is reported. The Rh<sub>2</sub>(II)-catalyzed N-atom transfer to the olefin is stereospecific, chemo- and diastereoselective to produce the <i>N</i>-aryl aziridine as the only amination product.
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