By Group 4 and 5 Metal Catalysts

Dlsubstltuted NHCs and Their Metal Complexes Substituents at positions 4 and 5 of the imidazolium ring by fusion with other heterocyclic rings or by simple alkyl groups modify the electronic and steric properties and may therefore exert a strong influence on the reactivity of the carbene catalyst. 

As discussed in Sect. 5, the intermolecular hydroamination of alkynes catalyzed by group 4 metal complexes is a well-documented process. The less challenging intramolecular transformation can be achieved efficiently with various titanium-based catalysts [51, 125-130]. The cyclization proceeds analogously to the rare earth metal-catalyzed process with exclusive ej o-selectivity and often requires elevated temperatures. However, the homoleptic titanium tetraamide Ti(NMe2)4 catalyzes the cyclization of both terminal and internal aminoalkynes at room temperature (7) [126, 127]. 

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