Mechanistic investigation of intramolecular aminoalkene and aminoalkyne hydroamination/cyclization catalyzed by highly electrophilic, tetravalent constrained geometry 4d and 5f complexes. Evidence for an M-N σ-bonded insertive pathway

A mechanistic study of intramolecular hydroamination/cyclization catalyzed by tetravalent organoactinide and organozirconium complexes is presented. A series of selectively substituted constrained geometry complexes, (CGC)M(NR ₂)Cl (CGC = [Me₂Si(η⁵-Me₄C ₅)(^tBuN)]^2-; M = Th, 1-Cl; U, 2-Cl; R = SiMe₃; M = Zr, R = Me, 3-Cl) and (CGC)An(NMe₂)OAr (An = Th, 1-OAr; An = U, 2-OAr), has been prepared via in situ protodeamination (complexes 1-2) or salt metathesis (3-Cl) in high purity and excellent yield and is found to be active precatalysts for intramolecular primary and secondary aminoalkyne and aminoalkene hydroamination/cyclization. Substrate reactivity trends, rate laws, and activation parameters for cyclizations mediated by these complexes are virtually identical to those of more conventional (CGC)-MR ₂ (M = Th, R = NMe₂, 1; M = U, R = NMe₂, 2; M = Zr, R = Me, 3), (Me₂SiCp″₂)UBn₂ (Cp″ = η⁵-Me₄C₅; Bn = CH ₂Ph, 4), Cp′₂AnR₂ (Cp′ = η⁵-Me₅C₅; R = CH₂SiMe ₃; An = Th, 5, U, 6), and analogous organolanthanide complexes. Deuterium KIEs measured at 25°C in C₆D₆ for aminoalkene D₂NCH₂C(CH₃)₂-CH ₂CHCH₂ (11-d₂) with precatalysts 2 and 2-Cl indicate that k_H/k_D = 3.3(5) and 2.6(4), respectively. Together, the data provide strong evidence in these systems for turnover-limiting C-C insertion into an M-N(H)R σ-bond in the transition state. Related complexes (Me₂SiCp″₂)U(Bn)(Cl) (4-Cl) and Cp′₂An(R)-(Cl) (R = CH₂(SiMe₃); An = Th, 5-Cl; An = U, 6-Cl) are also found to be effective precatalysts for this transformation. Additional arguments supporting M-N(H)R intermediates vs M=NR intermediates are presented.