Organolanthanide-catalyzed intramolecular hydroamination/cyclization of aminoalkynes

This contribution reports the efficient and regiospecific Cp'₂LnCH(SiMe₃)₂ (Ln = La, Nd, Sm, Lu; Cp' = η⁵-Me₅C₅)- and Me₂SiCp''₂LnCH(SiMe₃)₂ (Ln = Nd, Sm; Cp'' = η⁵-Me₄C₅)-catalyzed hydroamination/cyclization of aliphatic and aromatic aminoalkynes of the formula RC≡C(CH₂)(n)NH₂ to yield the corresponding cyclic imines RCH₂C=N(CH₂)(n-1)CH₂, where R, n, N(t) h^-1 (°C) = Ph, 3, 77 (21°C); Ph, 3, 2830 (60°C); Me, 3, 96 (21°C); CH₂=CMeCH₂, 3, 20 (21°C); H, 3, 580 (21°C); Ph, 4, 4 (21°C); Ph, 4, 328 (60°C); Ph, 5, 0.11 (60°C); and SiMe₃, 3, >7600 (21°C), and of aliphatic secondary amino-alkynes of the formula RC≡C(CH₂)₃NHR₁ to generate the corresponding cyclic enamines RCH=CNR₁(CH₂)₂CH₂ where R, R₁, N(t) h^-1 (°C) = SiMe₃, CH₂=CHCH₂, 56 (21°C); H, CH₂=CHCH₂, 27 (21°C); SiMe₃, CH₂=CH(CH₂)₃, 129 (21°C); and H, CH₂=CH(CH₂)₃, 47 (21°C). Kinetic and mechanistic evidence is presented arguing that the turnover-limiting step is an intramolecular alkyne insertion into the Ln-N bond followed by rapid protonolysis of the resulting Ln-C bond. The use of larger metal ionic radius Cp'₂LnCH(SiMe₃)₂ and more open Me₂SiCp''₂LnCH(SiMe₃)₂ complexes as the precatalysts results in a decrease in the rate of hydroamination/cyclization, arguing that the steric demands in the -C≡C- insertive transition state are relaxed compared to those of the analogous aminoolefin hydroamination/cyclization.