Cooperative Metal–Ligand Hydroamination Catalysis
Supported by C–H Activation in Cyclam Zr(IV) Complexes
Posted on 2018-09-21 - 17:48
Complexes of the type (R2Cyclam)ZrCl2 (where R = CH2C(H)CH2 (All), CH2C(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 ((CHC(H)CH2)2Cyclam)Zr, 14, ((CHC(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.
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Alves, Luis G.; Madeira, Filipe; Munhá, Rui F.; Maulide, Nuno; Veiros, Luis F.; Martins, Ana M. (2018). Cooperative Metal–Ligand Hydroamination Catalysis
Supported by C–H Activation in Cyclam Zr(IV) Complexes. ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.8b02396
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AUTHORS (6)
LA
Luis G. Alves
FM
Filipe Madeira
RM
Rui F. Munhá
NM
Nuno Maulide
LV
Luis F. Veiros
AM
Ana M. Martins
KEYWORDS
C 3 H 3 N 2tetraanionic hexacoordinated ligandsCH 2 Phligand side armGrignard reagentsbondComplexes 14species 16O t Bu4.5 hbenzophenone hydrazonecyclam ligandcyclam ringNHNCPh 2Zrproton exchangedisplay metalated allylprotic substratesamido ligandTOFPhCH 2mechanismcomplexdianionic tetracoordinated cyclam-based ligandsconversion valuesCyclamalkyl derivatives