Transition-metal-catalyzed hydroamination indoles

The cationic imidazolium rhodium complex (56) has been found to catalyze the intramolecular hydroamination of alkynes in refluxing THF. In the case of 2-ethynylaniline, indole is formed in 100% yield over 9h at 55 °C (Scheme 38).173 One of the earliest examples of late transition metal-catalyzed hydroamination involved the use of the iridium(I) complex [Ir(PEt3)2(C2H4)Cl] as... 

Recently, transition metal-catalyzed hydroamination reaction of alkynes with hydrazines was used to generate aryl hydrazones, highly reactive and versatile intermediates in a subsequent metal-catalyzed Fischer indole synthesis. Thus, Odom first reported a 3 + 2 synthesis of 1,7-fused, di-, and tri-substituted indoles 285 featuring the Ti(IV)-catalyzed hydroamination of various alkynes 283 with... 

Tokunaga and Wakatsuki developed a 3 + 2 transition metal-catalyzed route to multisubstituted indoles 277 which relies on the Bischler indolization reaction (Scheme 9.96) [253]. The key 2-arylaminoketone intermediates 276 and 276 were accessed via tandem Ru(0)-catalyzed hydroamination-isomerization of pro-pargylic alcohols 275 with substituted anilines 274 under solvent-free conditions. 

Among the first hydroaminations of alkynes to be published were cyclizations catalyzed by palladium complexes to form indoles. Hydroaminations of alkynes catalyzed by low-valent, late transition metal complexes occur with a narrower scope than the reactions catalyzed by lanathanide complexes. More recently, examples of the hydroaminations of alkynes catalyzed by rhodium complexes have been reported. 

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