Titanium and Zirconium Complexes with Helical Bis(phenolato) Ligands as Hydroamination Catalysts

Abstract

A racemic bisphenolato (OSSO)-type ligand that contains a trans-1,2-cyclohexanediyl backbone can be obtained in two steps from commercially available starting materials. In situ combination of this ligand with Ti(NMe₂)₄ or Zr(NMe₂)₄ results in the formation of bis(phenolato) complexes that catalyze hydroaminations of alkynes and alkenes.

Key words

alkynes - aminations - amines - homogenous catalysis - titanium

References and Notes

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8

Experimental Procedure: S₂Cl₂ (16.88 g, 125 mmol) was added dropwise to a solution of 2,4-bis(α,α-dimethyl-benzyl)phenol (1, 82.62 g, 250 mmol) and TiCl₄ (0.474 g, 2.5 mmol, 2.0 mol%) in toluene (180 mL) at -5 °C. The resulting mixture was stirred at -5 °C for 30 min and at r.t. for 72 h. Then, the mixture was washed with aq HCl (2 × 250 mL, c = 19%), sat. aq Na₂CO₃ solution and H₂O. The organic layer was dried with MgSO₄ and concentrated under vacuum. Crystallization from MeCN (1800 mL) gave analytically pure disulfide 2 (49.88 g, 69 mmol, 55%). ¹H NMR (300 MHz, CD₂Cl₂): δ = 0.97 (s, 12 H), 1.04 (s, 12 H), 6.49-6.62 (m, 24 H). Anal. Calcd for C₄₈H₅₀O₂S₂ (723.0): C, 79.73; H, 6.97; S, 8.87. Found: C, 79.70; H, 6.93; S, 8.96.

10

Experimental Procedure: Cyclohexene (4.6 g, 56 mmol) and BF₃·OEt₂ (0.5 mL) were added to a solution of disulfide 2 (20 g, 28 mmol) in a mixture of nitromethane (15 mL) and CH₂Cl₂ (15 mL) at -10 °C. The resulting mixture was stirred at -10 °C for 3 h and at r.t. for 72 h. Then, the mixture was washed with sat. aq NaHCO₃ solution. The organic layer was dried with MgSO₄ and concentrated under vacuum. Crystallization from a 4:1-mixture of MeCN and acetone (200 mL) gave analytically pure bis(phenolato) ligand rac-3 (21.68 g, 27 mmol, 96%). ¹H NMR (500 MHz, CD₂Cl₂): δ = 1.02-1.06 (m, 4 H), 1.54-1.58 (m, 2 H), 1.64 (s, 12 H), 1.70 (s, 12 H), 1.70-1.74 (m, 2 H), 2.47-2.51 (m, 2 H), 6.83 (s, 2 H), 7.10-7.14 (m, 6 H), 7.15-7.22 (m, 8 H), 7.25-7.30 (m, 8 H), 7.37 (d, J = 2.1 Hz, 2 H). Anal. Calcd for C₅₄H₆₀O₂S₂ (805.2): C, 80.55; H, 7.51; S, 7.96. Found: C, 80.38; H, 7.51; S, 7.97.

11

Colorless crystal(polyhedron), dimensions 0.42 × 0.18 × 0.10 mm³, crystal system triclinic, space group P1, Z = 2, a = 8.5820(2) Å, b = 16.3662(4) Å, c = 18.3078(5) Å, α = 73.1710(10)°, β = 83.3570(10)°, γ = 77.9890(10)°, V = 2403.18(10) ų, ρ = 1.169 g/cm³, T = 200(2) K, Θ_max = 25.42°, radiation Mo-K_α, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 21011 reflections measured, 8802 unique [R(int) = 0.0883], 4564 observed [I >2σ(I)], intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS¹⁶ based on the Laue symmetry of the reciprocal space, µ = 0.15 mm^-1, T_min = 0.94, T_max = 0.98, structure solved by direct methods and refined against F² with a full-matrix least-squares algorithm using the SHELXTL-PLUS¹⁶ software package, 577 parameters refined, hydrogen atoms were treated using appropriate riding models, except for H1 and H2 at the oxygen atoms, which were refined isotropically, goodness of fit 0.98 for observed reflections, final residual values R1(F) = 0.062, wR(F2) = 0.115 for observed reflections, residual electron density -0.30 eÅ^-3 to 0.24 eÅ^-3. CCDC number 652550 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

12

Experimental Procedure: TiCl₄ (95 mg, 0.5 mmol) was added to a solution of rac-3 (402 mg, 0.5 mmol) in toluene (3 mL) at r.t. After the mixture had been stirred for 3 h at r.t., all volatiles (toluene, HCl) were removed under reduced pressure. The residue was washed with hexane. Crystallization from toluene gave complex rac-4 (456 mg, 0.49 mmol, 99%) as red-brown crystals. ¹H NMR (500 MHz, CD₂Cl₂): δ = 0.64-0.72 (m, 2 H), 0.87-0.93 (m, 2 H), 1.46-1.48 (m, 2 H), 1.65 (s, 6 H), 1.68-1.69 (m, 2 H), 1.70 (s, 6 H), 1.73 (s, 6 H), 1.79-1.81 (m, 2 H), 1.85 (s, 6 H), 6.80 (d, J = 2.2 Hz, 2 H), 7.12-7.32 (m, 20 H), 7.56 (d, J = 2.1 Hz, 2 H). In order to avoid the formation of solid byproducts the reaction was performed in the absence of a base.

13

Red-brown crystal(polyhedron), dimensions 0.20 × 0.16 × 0.05 mm³, crystal system monoclinic, space group C2/c, Z = 4, a = 28.177(3) Å, b = 11.1949(13) Å, c = 19.892(2) Å, β = 105.392(3)°, V = 6049.5(12) ų, ρ = 1.215 g/cm³, T = 200(2) K, Θ_max = 28.40°, radiation Mo-K_α, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 31295 reflections measured, 7545 unique [R(int) = 0.0531], 5283 observed [I >2σ(I)], intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS¹⁶ based on the Laue symmetry of the reciprocal space, µ = 0.34 mm^-1, T_min = 0.93, T_max = 0.98, structure solved by direct methods and refined against F² with a full-matrix least-squares algorithm using the SHELXTL-PLUS¹⁶ software package, 488 parameters refined, hydrogen atoms were treated using appropriate riding models, goodness of fit 1.03 for observed reflections, final residual values R1(F) = 0.057, wR(F²) = 0.117 for observed reflections, residual electron density -0.20 eÅ^-3 to 0.38 eÅ^-3. CCDC number 652551 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

15

General Procedure: A Schlenk tube equipped with a Teflon stopcock and a magnetic stirring bar was charged with Ti(NMe₂)₄ (27 mg, 0.12 mmol, 5.0 mol%), rac-3 (97 mg, 0.12 mmol, 5.0 mol%) and toluene (1.0 mL). After this mixture had been stirred for 30 min at r.t., the alkyne (2.40 mmol) and the amine (2.64 mmol) were added and the resulting mixture was heated to 105 °C for 24 h. Then the mixture was cooled to r.t. and a mixture of NaBH₃CN (302 mg, 4.80 mmol) and ZnCl₂ (326 mg, 2.40 mmol) in MeOH (10 mL) was added. After this mixture had been stirred at 25 °C for 20 h, CH₂Cl₂ (50 mL) and sat. Na₂CO₃ solution (20 mL) were added. The resulting mixture was filtered and the solid residue was washed with CH₂Cl₂ (50 mL). After extraction, the organic layer was separated. The aqueous layer was extracted with CH₂Cl₂ (6 × 50 mL). The combined organic layers were dried with Na₂SO₄. After concentration under vacuum, the residue was purified by flash chromatog-raphy (SiO₂). All compounds were identified by comparison of the obtained ¹H and ¹³C NMR spectra with those reported in the literature.¹⁷