Divergent Functionalization of N-Alkyl-2-alkenylanilines: Efficient Synthesis of Substituted Indoles and Quinolines

Iron-Catalyzed Oxidative Cyclization of 2-Amino Styrenes with Alcohols and Methyl Arenes for the Synthesis of Polysubstituted Quinolines

Herein, we present the iron-catalyzed oxidative cyclization of alcohol/methyl arene with 2-amino styrene to synthesize polysubstituted quinoline. Low-oxidation level substrates such as alcohols and methyl arenes are converted to aldehydes in the presence of an iron catalyst and di-t-butyl peroxide. Then, the quinoline scaffold is synthesized through imine condensation/radical cyclization/oxidative aromatization. Our protocol showed a broad substrate scope, and various functionalization and fluorescence applications of quinoline products demonstrated its synthetic ability.

Rhodium-Catalyzed Annulation of N-Acetoxyacetanilide with Substituted Alkynes: Conversion of Nitroarenes to Substituted Indoles

A general and efficient rhodium-catalyzed redox-neutral annulation of N-acetoxyacetanilides, readily accessible from nitroarenes, with alkynes has been accomplished for the synthesis of substituted indole derivatives. A wide range of substituted 2,3-diarylindoles were achieved from various substituted N-acetoxyacetanilides and symmetrical/unsymmetrical alkynes in good to excellent yields. The developed method was successfully integrated with the synthesis of N-acetoxyacetanilides for the efficient one-pot synthesis of indoles from nitroarenes. The important features are the introduction of N-acetoxyacetamide as a new directing group, redox-neutral annulation, an additive-free approach, wide functional group tolerance, an intramolecular version, and a one-pot reaction of nitroarenes. The method was further extended to the synthesis of potent higher analogues of indole, viz., pyrrolo[3,2-f]indoles and dibenzo[a,c]carbazoles. In addition, a plausible mechanism was proposed based on the isolation and stoichiometric study of a potential aryl-Rh intermediate.

Cobalt-catalyzed multisubstituted allylation of the chelation-assisted C-H bond of (hetero)arenes with cyclopropenes

Cyclopropenes are highly strained three-membered carbocycles, which offer unique reactivity in organic synthesis. Herein, Cp*CoIII-catalyzed ring-opening isomerization of cyclopropenes to cobalt vinylcarbene has been utilized for the synthesis of multisubstituted allylarenes via directing group-assisted functionalization of C-H bonds of arenes and heteroarenes. Employing this methodology, various substituents can be introduced at all three carbons of the allyl moiety with high selectivity. The important highlights are excellent functional group tolerance, multisubstituted allylation, high selectivity, gram scale synthesis, removable directing group, and synthesis of cyclopenta[b]indoles. In addition, a potential cobaltocycle intermediate was identified and a plausible mechanism is also proposed.

Rhodium-Catalyzed Annulation of ortho-Alkenyl Anilides with Alkynes: Formation of Unexpected Naphthalene Adducts

o-Alkenyl N-triflylanilides underwent rhodium(III)-catalyzed oxidative annulations with alkynes to produce different types of naphthylamides in a process which involves the cleavage of two C-H bonds. Remarkably, besides formal dehydrogenative (4C+2C) cycloadducts, the reaction also produces variable amounts of isomeric naphthylamides, whose formation requires a formal migration of the alkenyl moiety from the ortho to the meta position of the anilide. The annulation reaction can be efficiently carried out in the absence of external oxidants, such as Cu(OAc)₂ .