2,4-Bis[(2,6-diisopropylphenyl)imino]-3-methylpentan-3-ol

Abstract

The compound 2,4-bis[(2,6-diisopropylphenyl)imino]-3-methylpentan-3-ol was synthesized with a yield of approximately 80% by the reaction of 2,4-bis(2,6-diisopropylphenylimino)pentan-3-one with trimethylaluminum, which was followed by hydrolysis with an aqueous NaOH solution. A chemoselective addition to the C=O bond occurred in this reaction. The structure was confirmed by X-ray crystallography. This new compound was also fully characterized by ¹H, ¹³C-NMR spectroscopy and ¹H-¹³C HSQC spectroscopy, mass spectrometry and elemental analysis.

1. Introduction

β-Diketiminates (Scheme 1, I) or β-diimines (the tautomers of β-diketiminates, Scheme 1, II) are well known as the spectator ligands to stabilize metallic complexes. The steric and electronic properties of β-diketiminates or β-diimines can be easily modified with the appropriate substituents. Thus, these ligands have been widely applied in the metal-catalyzed organic reactions and polymerizations [1,2,3,4]. However, the synthesis of β-diimines with a functional group at the α-site remains scarce (Scheme 1, III–V) [5,6,7,8]. One notable example is that the introduction of the carbonyl functionality at the α-site (α-keto-β-diimine, Scheme 1, V) enables an improvement in the control of nickel-catalyzed polymerization [8]. Furthermore, the reaction of the α-keto-β-diimine nickel complex with trimethylaluminum (TMA) results in the formation of binuclear organometallic species (Scheme 1, VI) [9]. On the basis of these works, we decided to synthesize the α-hydroxyl-β-diimine by the reduction of the C=O double bond of α-keto-β-diimine. Since the reduction of C=N double bonds may also occur in this reaction, selective reduction of only the C=O double bond is the main difficulty that we had to consider [10]. In this contribution, the first available preparation and characterization of 2,4-bis[(2,6-diisopropylphenyl)imino]-3-methylpentan-3-ol were described.

2. Results and Discussion

Previously, the selective reductive addition of the α-imino ketones with the organoaluminium regents has been reported [11,12,13]. As a result, trimethylaluminum (TMA) was chosen as a reducing agent to obtain the desired product. As shown in Scheme 2, treating 2,4-bis(2,6-diisopropylphenylimino)pentan-3-one with 1.3 or 3 equiv of AlMe₃ at the reflux temperature for 10 h created the aluminum complex in situ, before the reaction mixture was carefully hydrolyzed with an aqueous NaOH solution. According to the reported complex VI (Scheme 1) [8], the intermediate aluminum salt is deemed to be N,O-bound with a tetrahedral arrangement (Scheme 2). The desired product was obtained in high yields of up to 79.3%. To our surprise, the selective reduction of C=O double bonds still occurs even in the presence of an excess proportion of AlMe₃. This was the first reported compound, which was fully characterized by ¹H, ¹³C-NMR spectroscopy and ¹H-¹³C HSQC spectroscopy, mass spectrometry (Figures S1–S4), elemental analysis and X-ray crystallography (Table S1). In CDCl₃, the characteristic peak in the ¹H-NMR for the products is at approximately δ = 6.27 ppm, which corresponds to the proton of the hydroxyl group. The chemical shift at 80.63 ppm in the ¹³C-NMR represents the signals for the C-OH of the compound. The chemical shifts of other protons and carbons were further assigned by ¹H, ¹³C-NMR spectroscopy and the ¹H-¹³C HSQC spectroscopy (see Materials and Methods).

The recrystallization of this compound in the ethanol solution at −20 °C created single crystals that were suitable for X-ray diffraction. The molecular structure of the product is shown in Figure 1. It is clear that the tertiary carbon is located on the hydroxyl group rather than N(1) or/and N(2), which further confirms that the selective reductive addition is carried out at the C=O double bonds. No intermolecular interactions were observed in this structure.

3. Materials and Methods

3.1. General Information

The compound 2,4-bis(2,6-diisopropylphenylimino)pentan-3-one was synthesized according to the previously reported method [7]. All other reagents were purchased from commercial sources and used without purification. The ¹H-NMR spectra were captured in 5-mm NMR tubes at 298 K on Bruker DPX 500 (¹H = 500.13 MHz) spectrometers (Bruker, Karlsruhe, Germany) using TMS as an internal standard and CDCl₃ as the solvent. The ¹³C-NMR spectra were referenced to the residual solvent (CHCl₃, 77.16 ppm) for chloroform-d₁. The elemental analysis was performed by the Analytical Center of the University of Science and Technology of China. A mass spectrum of the compound was recorded on a Thermo LTQ Orbitrap XL (ESI+, Thermo Fisher Scientific, Waltham, MA, USA). The X-ray diffraction data were collected at 298(2) K on a Bruker Smart CCD area detector (Bruker, Karlsruhe, Germany) with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structures were solved by direct methods, before further refinement with full-matrix least-squares on F² was obtained with the SHELXL program package [14,15].

3.2. Synthesis of 2,4-Bis[(2,6-diisopropylphenyl)imino]-3-methylpentan-3-ol

Under a nitrogen atmosphere, trimethylaluminum (TMA) (1.3 equiv, 6.0 mmol, 3.8 mL, 1.6 M in toluene) was injected slowly through a syringe into a solution of 2,4-bis(2,6-diisopropylphenylimino)pentan-3-one (2.0 g, 4.6 mmol) in 45 mL of toluene at room temperature. The reaction was heated to reflux for 10 h. After the solution was cooled to the ambient temperature, the reaction mixture was carefully hydrolyzed with 5% aqueous NaOH solution in an ice bath. After extraction with ethyl acetate, the organic layer was dried with anhydrous MgSO₄ and filtered. The solvent was removed under reduced pressure. The crude product was isolated as a viscous oil. After recrystallization from ethanol at −20 °C, the purified white crystals can be obtained with a yield of 78.7% (1.62 g). When 3 equiv of TMA was employed, the same product was obtained with a yield of 79.3% (1.64 g). The ¹H-NMR (400 MHz, CDCl₃) obtained δ 7.18–7.05 (m, 6H, aryl-H), 6.27 (s, 1H, OH), 2.73 (heptet, J = 6.8 Hz, 2H, CH(CH₃)₂), 2.66 (heptet, J = 6.8 Hz, 2H, CH(CH₃)₂), 1.85 (s, 6H, N=C(CH₃)), 1.82 (s, 3H, CH₃-C-OH), 1.16 (two overlapping doublets, 24H, CH(CH₃)₂). The ¹³C-NMR (126 MHz, CDCl₃) obtained δ 172.93 (CH₃-C=N), 144.94 (N-C), 136.38 (aryl-C), 136.16 (aryl-C), 124.14 (aryl-C), 123.22 (aryl-C), 123.19 (aryl-C) 80.63 (CH₃-C-OH), 28.20 (CH(CH₃)₂), 24.56 (CH₃-C-OH), 23.53 (CH(CH₃)₂), 23.43 (CH(CH₃)₂), 23.08 (CH(CH₃)₂), 22.83 (CH(CH₃)₂), 16.01 (CH₃-C=N). ESI-MS (m/z): calcd. for C₃₀H₄₅N₂O: 449.3532, found: 449.3521 [M + H]⁺. Anal. Calcd. for C₃₀H₄₄N₂O: C, 80.31; H, 9.88; N, 6.24; Found: C, 80.25; H, 9.89; N, 6.27.

The Crystal Data for C₃₀H₄₄N₂O (M = 448.67 g/mol) are: triclinic, space group P-1, a = 8.9713(8) Å, b = 13.5010(11) Å, c = 13.5357(12) Å, α = 109.324(3) °, β = 99.633(2)°, γ = 104.420(2)°, V =1441.3(2) ų, Z = 2, T = 298(2) K, μ(MoKα) = 0.062 mm⁻¹, Dcalc = 1.034 g/cm³, 7285 reflections measured (−8 ≤ h ≤ 10, −16 ≤ h ≤ 16, −16 ≤ 1 ≤ 15) and 5007 unique (Rint = 0.0281), which were used in all calculations. The final R1 was 0.0839 (I > 2σ(I)) and the wR2 was 0.2452 (all data). The molecule is disordered, the hydroxyl group and methyl group on C3 are disordered over the two sites with occupancies of 0.51 (2): 0.49(2), while the isopropyl group attached to C8 is disordered over the two sites with occupancies of 0.60 (4): 0.40 (4).