Organolanthanide-catalyzed imine hydrogenation. Scope, selectivity, mechanistic observations, and unusual byproducts

In this paper we report the Cp'₂LnMe₂SiCp''₂Ln-catalyzed (Cp' = η⁵-Me₅C₅; Cp'' = η⁵-Me₄C₅) hydrogenation of acyclic imines to yield the corresponding amines. At 190 psi of H₂, the observed turnover frequencies (h^-1) (100:1 substrate:catalyst ratio, Cp'₂Ln, temperature (°C)) are (1) (N-benzylidene(methyl)amine, Ln = La, 50) 0.03; (Ln = Sm, 90) 1.0; (Ln = Sm + PhSiH₃, 90) 2.2; (Ln = Lu, 90) 0.60; (2) (N-benzylideneaniline, Ln = Sm, 90) 0.10; (3) (N-benzylidene(trimethylsilyl)amine, Ln = Sm, 90) 0.40; (4) (N-(α-methylbenzylidene)(methyl)amine, Ln = Sm, 90) 0.20; (5) (N-(α-methylbenzylidene)(benzyl)amine, Ln = Sm, 90) 0.70. The stoichiometric reaction of N-benzylidene(methyl)amine with Cp'₂SmCH(SiMe₃)₂ or (Cp'₂SmH)₂ yields an orthometalated Cp'₂Sm-substrate complex which undergoes either hydrogenolysis/hydrogenation or competing C=N insertion of a second substrate molecule to yield a Cp'₂Sm-imine-amido complex with a seven-membered chelate ring. The stoichiometric reaction of 2-methyl-1-pyrroline with Cp'₂SmCH(SiMe₃)₂ or (Cp'₂SmH)₂ yields a Cp'₂Sm-imine-amido complex in which two substrate molecules have been coupled to form a six-membered chelate ring (characterized by X-ray diffraction). The stoichiometric reaction of N-benzylidene(trimethylsilyl)amine with (Cp'₂SmH)₂ yields a desilylated Cp'₂Sm-imine-amido complex with a four-membered Sm(NSiMe₃)(CPh)N=CHPh chelate ring (characterized by X-ray diffraction). Additional heating of this product under H₂ yields S₆-symmetric (Cp'₂SmCN)₆, which contains an unusual chairlike 18-membered (SmCN)₆ ring (characterized by X-ray diffraction).