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

Organolanthanide complexes of the general type Cp'₂LnCH(TMS)₂ (Cp' = η⁵-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)_nCHR'NH₂ to yield the corresponding heterocycles RCH=CHCHNHCH(R')(CH₂)_n-1CH₂ (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 α-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.