Intramolecular hydrophosphination/cyclization of phosphinoalkenes and phosphinoalkynes catalyzed by organolanthanides: Scope, selectivity, and mechanism

Organolanthanide complexes of the general type Cp′₂LnE(TMS)₂ (Cp′ = η^/5-Me₅C₅; Ln = La, Sm, Y, Lu; E = CH, N; TMS = SiMe₃) serve as effective precatalysts for the rapid intramolecular hydrophosphination/cyclization of the phosphinoalkenes and phosphinoalkynes RHP(CH₂)_nCH=CH₂ (R = Ph, H; n = 3, 4) and H₂P(CH₂)_nC≡C-Ph (n = 3, 4) to afford the corresponding heterocycles CH₃CH(CH₂)_nPR and Ph(H)C=C(CH₂)_nPH, respectively. Kinetic and mechanistic data for these processes exhibit parallels to, as well as distinct differences from, organolanthanide-mediated intramolecular hydroamination/cyclizations. The turnover-limiting step of the present catalytic cycle is insertion of the carbon-carbon unsaturation into the Ln-P bond, followed by rapid protonolysis of the resulting Ln-C linkage. The rate law is first-order in [catalyst] and zero-order in [substrate] over approximately one half-life, with inhibition by heterocyclic product intruding at higher conversions. The catalyst resting state is likely a lanthanocene phosphine-phosphido complex, and dimeric [Cp′₂YP(H)Ph]₂ was isolated and cystallographically characterized. Lanthanide identity and ancillary ligand structure effects on rate and selectivity vary with substrate unsaturation: larger metal ions and more open ligand systems lead to higher turnover frequencies for phosphinoalkynes, and intermediate-sized metal ions with Cp′₂ ligands lead to maximum turnover frequencies for phosphinoalkenes. Diastereoselectivity patterns also vary with substrate, lanthanide ion, and ancillary ligands. Similarities and differences in hydrophosphination vis-à-vis analogous organolanthanide-mediated hydroamination are enumerated.