Copper(II) Complexes of Pyridyl-Appended Diazacycloalkanes:  Synthesis, Characterization, and Application to Catalytic Olefin Aziridination

Abstract

As part of an ongoing effort to rationally design new copper catalysts for olefin aziridination, a family of copper(II) complexes derived from new tetradentate macrocyclic ligands are synthesized, characterized both in the solid state and in solution, and screened for catalytic nitrene transfer reactivity with a representative set of olefins. The pyridylmethyl-appended diazacycloalkane ligands L6(py)2, L7(py)2, and L8(py)2 are prepared by alkylation of the appropriate diazacycloalkane (piperazine, homopiperazine, or diazacyclooctane) with picolyl chloride in the presence of triethylamine. The ligands are metalated with Cu(ClO4)(2).6H2O to provide the complexes [(L6(py)2)Cu(OClO3)]ClO4 (1), [(L7(py)2)Cu(OClO3)]ClO4 (2), and [(L8(py)2)Cu](ClO4)2 (3), which, after metathesis with NH4PF6 in CH3CN, afford [(L6(py)2)Cu(CH3CN)](PF6)2 (4), [(L7(py)2)Cu(CH3CN)](PF6)2 (5), and [(L8(py)2)Cu](PF6)2 (6). All six complexes are characterized by X-ray crystallography, which reveals that complexes supported by L6(py)2 and L7(py)2 (1, 2, 4, 5) adopt square-pyramidal geometries, while complexes 3 and 6, ligated by L8(py)2 feature tetracoordinate, distorted-square-planar copper ions. Tetragonal geometries in solution and d(x2 - y2), ground states are confirmed for the complexes by a combination of UV-visible and EPR spectroscopies. The divergent flexibility of the three supporting ligands influences the Cu(II)/Cu(I) redox potentials within the family, such that the complexes supported by the larger ligands L7(py)2 and L8(py)2 (5 and 6) exhibit quasi-reversible electron transfer processes (E1/2 approximately -0.2 V vs Ag/AgCl), while the complex supported by L6(py)2 (4), which imposes a rigid tetragonal geometry upon the central copper(II) ion, is irreversibly reduced in CH3CN solution. Complexes 4-6 are efficient catalysts (in 5 mol % amounts) for the aziridination of styrene with the iodinane PhINTs (in 80-90% yields vs PhINTs), while only 4 exhibits significant catalytic nitrene transfer reactivity with 1-hexene and cyclooctene.

Catalytic Aziridination of Styrene with Copper Complexes of Substituted 3,7‐Diazabicyclo[3.3.1]nonanones

“…[19] The ligands discussed in those studies involve two tertiary amine and one or two pyridine donors, leading to a planar geometry of the putative nitrene intermediate with the copper-nitrene bond inplane; the reduced reactivity of the tetradentate ligand complexes was believed to be due to a pre-equilibrium, involving copper-pyridine bond breaking. [19,20] This is in agreement with the above mentioned theoretical studies which indicate that the formation of the nitrene intermediate is the rate determining step of the catalytic cycle. Note however that the DFT calculations were done with different and only bidentate ligands.…”

“…This is as expected from theoreti- Figure 1. ORTEP [34] [20] ( Table 1. Selected bond lengths (Å ) and angles (deg) of the molecular cations of the copper() complexes Figure 1).…”

New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination: The Effect of Ligand Steric Hindrance on Reactivity

“…The 2-phenyl-1-tosylaziridine obtained was characterized by 1 H NMR spectroscopy and the data were identical with those reported in the literature. [13] Cyclooctene: Aziridinations were performed as described above by stirring mixtures of PhINTs (0.3-0.4 mmol), the olefin (10 to 5 equiv. vs. PhINTs), and the copper catalyst (5 mol-% vs. PhINTs) in 2 mL of anhydrous CH 3 CN.…”

“…They also reported aziridination of olefins catalyzed by a series of square-based copper(II) complexes in which tetradentate pyridyl-appended diazacycloalkane ligands are meridionally coordinated. [13] In these complexes the axial coordination site(s) are either vacant or occupied by a readily displaced solvent or counterion, which facilitates the aziridination with the nitrene source PhINTs. The reactivity of the copper(II) complexes is significantly enhanced for aziridination of styrene when the ligand denticity is lowered from tetradentate to tridentate.…”

“…As the highly oxidized coppernitrene species are suggested [10] to be stabilized by electronrich and sterically hindered ligands, sterically hindering NMe and quinolyl moieties were incorporated into the tridentate N 3 ligands (Scheme 2) chosen for the present study. The ring system of the ligand would provide resistance [13] towards oxidative degradation of the catalyst by the strongly oxidizing nitrene transfer agent PhINTs. The coordinative unsaturation as well as steric crowding around copper(II) in the present complexes is expected to lead to improved selectivity, reaction rates, and broad substrate tolerance.…”