Mechanism group 4 metal imido species

Scheme n.17 Proposed mechanism for the hydroamination/cyclization of primary aminoalkenes involving group 4 metal imido species. 

The mechanism of the group 4 metal-catalyzed hydroamination of alkynes (Scheme 4) and allenes has been thoroughly investigated in detailed kinetic and mechanistic [41 6] as well as computational studies [47, 48]. The catalytically active species is believed to be a metal imido complex, which undergoes a reversible, rate-determining [2 + 2]-cycloaddition with an alkyne, respective allene, to yield an azametallacyclobutene species. Subsequent protonolysis leads to the imine hydroamination product. Isolation and characterization of intermediate azacyclobutene species, which are catalytically competent themselves, is a strong support of the mechanism [41,43,49-51]. Although no direct experimental [52, 53]... 

Hydrazinediido complexes have been identified as active species in the titanium-catalyzed hydrohydrazination [12] and iminohydrazination of alkynes and the subsequent transformation of the hydrazones into indoles or tryptamine derivatives (Scheme 13.5) [13]. The postulated reaction mechanism of the group 4 metal-catalyzed reaction is based on the extensively studied mechanistic scheme for hydroaminations of alkynes, in which imido complexes are the key active species. 

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