Organolanthanide-Catalyzed Intramolecular Hydroamination/Cyclization of Aminoalkynes
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Abstract
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)nNH2 to yield the corresponding cyclic imines RCH2CN(CH2)n-1CH2, where R, n, Nt h-1 (°C) = Ph, 3, 77 (21 °C); Ph, 3, 2830 (60 °C); Me, 3, 96 (21 °C); CH2CMeCH2, 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 RCHCNR1(CH2)2CH2 where R, R1, Nt h-1 (°C) = SiMe3, CH2CHCH2, 56 (21 °C); H, CH2CHCH2, 27 (21 °C); SiMe3, CH2CH(CH2)3, 129 (21 °C); and H, CH2CH(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.
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