Cooperative Metal-Ligand Hydroamination Catalysis Supported by C-H Activation in Cyclam Zr(IV) Complexes.

Abstract

Complexes of the type (R2Cyclam)ZrCl2 (where R = CH2═C(H)CH2 (All), CH2═C(Me)CH2 (MeAll), and PhCH2 (Bn)) react with suitable Grignard reagents to produce the corresponding alkyl derivatives (R2Cyclam)ZrR'2 (R' = Me, CH2Ph). Thermally induced double metalation of the pending arms of the cyclam ligand led to the formation of complexes ((CH═C(H)CH2)2Cyclam)Zr, 14, ((CH═C(Me)CH2)2Cyclam)Zr, 15, or ((C6H4CH2)2Cyclam)Zr, 16. These reactions proceed through C(sp2)-H bond activation and R'H elimination and convert the original dianionic tetracoordinated cyclam-based ligands in tetraanionic hexacoordinated ligands that establish two new Zr-C bonds. The cleavage of the Zr-C bonds may be readily achieved by treatment of the bis( ortho-metalated) species 16 with protic substrates ( tert-butanol, phenol, thiophenol, aniline, benzophenone hydrazone, pyrazole, and N, N'-diphenylhydrazine), to give rise to (Bn2Cyclam)ZrX2 complexes (X = OtBu, OPh, SPh, NHPh, NHNCPh2, C3H3N2, N, N'-PhNNPh). In catalytic conditions, complexes (All2Cyclam)Zr(NMe2)2, 14, 15, or 16 convert 2,2-diphenyl-pent-4-enylamine to 2-methyl-4,4-diphenylpyrrolidine with 100% selectivity and conversion values varying between 61 and 88% in 4.5 h, at 115 °C. Complexes 14 and 15, which display metalated allyl and methallyl pending groups on the cyclam ring, are the most active species (1.7 < TOF < 2.0 h-1). The mechanism of this reaction was studied by density functional theory that revealed two competitive paths, one proceeding through an imido intermediate and another that occurs via an amido ligand. Both cases represent cooperative mechanisms with active participation of the cyclam, as proton exchange between the coordinated substrate and the ligand side arm with reversible C-H activation is a crucial feature of the mechanism.