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

In this paper we report the Cp'(2)Ln/Me2SiCp''(2)Ln-catalyzed (Cp' = eta(5)-Me5C5; Cp'' = eta(5)-Me4C5) hydrogenation of acyclic imines to yield the corresponding amines. At 190 psi of H-2, the observed turnover frequencies (h(-1)) (100:1 substrate:catalyst ratio, Cp'(2)Ln, temperature (degrees 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-(alpha-methylbenzylidene) (methyl)amine, Ln = Sm, 90)0.20; (5) (N-(alpha-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'Sm-2-substrate complex which undergoes either hydrogenolysis/hydrogenation or competing C=N insertion of a second substrate molecule to yield a Cp'Sm-2-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'Sm-2-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'Sm-2-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 H-2 yields S-6-symmetric (Cp'2SmCN)(6), which contains an unusual chairlike 18-membered (SmCN)(6) ring (characterized by X-ray diffraction).