Organolanthanide-catalyzed intramolecular hydroamination/cyclization of aminoalkynes

This contribution reports the efficient and regiospecific Cp'(2)LnCH(SiMe(3))(2) (Ln = La, Nd, Sm, Lu; Cp' = eta(5)-Me(5)C(5))- and Me(2)SiCp(2)('')LnCH(SiMe(3))(2) (Ln = Nd, Sm; Cp('') = eta(5)-Me(4)C(5))-catalyzed hydroamination/cyclization of aliphatic and aromatic aminoalkynes of the formula RC=C(CH2)(n)NH2 to yield the corresponding cyclic imines RCH(2)C=N(CH2)(n-1)CH2, where R, n, N-t h(-1) (degrees C) = Ph, 3, 77 (21 degrees C); Ph, 3, 2830 (60 degrees C); Me, 3, 96 (21 degrees C); CH2=CMeCH(2), 3, 20 (21 degrees C); H, 3, 580 (21 degrees C); Ph, 4, 4 (21 degrees C), Ph, 4, 328 (60 degrees C); Ph, 5, 0.11 (60 degrees C); and SiMe(3), 3, > 7600 (21 degrees C), and of aliphatic secondary amino-alkynes of the formula RC=C(CH2)(3)NHR(1) to generate the corresponding cyclic enamines RCH=CNR(1)(CH2)(2)CH2 where R, R(1), N-t h(-1) (degrees C) = SiMe(3), CH2=CHCH2, 56 (21 degrees C); H, CH2=CHCH2, 27 (21 degrees C); SiMe(3), CH2=CH(CH2)(3), 129 (21 degrees C); and H, CH2=CH(CH2)(3), 47 (21 degrees 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'(2)LnCH(SiMe(3))(2) and more open Me(2)SiCp(2)('')LnCH(SiMe(3))(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.