Intramolecular hydroamination cyclization of aminoallenes catalyzed by organolanthanide complexes. Scope and mechanistic aspects

Organolanthanide complexes of the general type Cp'(2)LnCH(TMS)(2) (Cp' = eta(5)-Me5C5; Ln = La, Sm, Y, Lu; TMS = SiMe3) serve as effective precatalysts for the rapid, regioselective, and highly diastereoselective intramolecular hydroamination/cyclization (IHC) of aminoallenes having the general formula RCH=C=CH(CH2)(n)CHR'NH2 to yield the corresponding heterocycles RCH=CHCHNHCH(R')(CH2)(n-1)CH2 (R = CH3, n-C3H7, n-C5H11; R'= H, CH3, n-C4H9, CH2=CHCH2CH2; n = 2, 3). The mono- and disubstituted pyrrolidines and piperidines produced bear an cx-alkenyl functionality available for further synthetic manipulation. Kinetic and mechanistic data parallel organolanthanide-mediated intramolecular aminoalkene and aminoalkyne hydroamination/cyclization, implying turnover-limiting allene insertion into the Ln-N bond followed by rapid protonolysis of the resulting Ln-C bond. The reaction rate is zero-order in [aminoallene] and first-order in [catalyst] over 3 or more half-lives. Hydroamination/cyclization of monosubstituted aminoallenes (R = H; R' = H, CH3; n = 1, 2) is less regioselective, with tetrahydropyridines being the predominant products.