Activation of the C–C Bond Using Brønsted Acid NaHSO₄/SiO₂: Synthesis of Triarylmethanes from 3-Benzhydryl Pentane-2,4-dione and Aromatic Compounds

Abstract

A novel and efficient method for the activation of the C(sp³)–C(sp³) bond was developed. The C(sp³)–C(sp³) bond in 3-benzhydryl pentane-2,4-dione was easily cleaved in the presence of NaHSO₄/SiO₂ to form benzhydryl cation which reacted with aromatic compounds at 110 °C for five to six hours to afford triarylmethanes in good yields. This reaction using Brønsted acids was applicable for the alkylation of electron-rich aromatics containing alkyl halides as a side chain.

Key words

C–C bond activation - C–C bond formation - solid acid - alkylation - heterogeneous reaction

• References and Notes

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• 11 Melting points were determined on Büchi Melting Point B-540. NMR spectra were recorded on a JEOL ECX 400 spectrometer. Tetramethylsilane (δ = 0 ppm) was used as an internal standard for ¹H NMR and CDCl₃ (δ = 77.0 ppm) for ¹³C NMR spectroscopy. MS analyses were performed on a SHIMAZU GCMS-QP2010 or on a JMS-GCmate (JEOL). Elemental analyses were performed on a J Science Lab. micro corder JM-10. Preparation of NaHSO₄/SiO₂ SiO₂ [Wakogel C-200 (Wako Pure Chemical Ind. Ltd.), 10 g] was added to a solution of NaHSO₄·H₂O (30 mmol, 4.14 g) in distilled H₂O, and the mixture was stirred at r.t. for 0.5 h. H₂O was removed by rotary evaporator under reduced pressure, and the resulting reagent was dried in vacuo (1.33 mbar) at 120 °C for 5 h. Typical Procedure A mixture of diketones 1 (1.0 mmol), aromatics 2 (3.0 mmol), and NaHSO₄/SiO₂ (2.1 mmol/g, 1.00 g) in chlorobenzene (5 mL) was stirred at 110 °C for 6 h, and then the used supported reagent was removed by filtration. The filtrate was evaporated to leave crude product, which was purified by glass tube oven and column chromatography (hexane–EtOAc) to obtain 3. 2-Benzhydryl-4-chlorophenol (3an) Oil. ¹H NMR (400 MHz, CDCl₃): δ = 4.78 (s, 1 H), 5.67 (s, 1 H), 6.68 (dd, J = 7.1, 2.5 Hz, 1 H), 6.77 (d, J = 2.5 Hz, 1 H), 7.06–7.12 (m, 5 H), 7.21–7.32 (m, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 50.73, 117.3, 125.7, 126.9, 127.7, 128.7, 129.2, 130.2, 132.2, 141.6, 152.0. HRMS (EI): m/z [M⁺] calcd for C₁₉H₁₅OCl: 294.0811; found: 294.0809. 2-Benzhydryl-4-nitrophenol (3ao) Yellow solid; mp 254–257 °C (hexane). ¹H NMR (400 MHz, CDCl₃): δ = 5.75 (s, 1 H), 6.89 (d, J = 8.9, 1 H), 7.12–7.14 (m, 4 H), 7.27–7.37 (m, 7 H), 7.77 (d, J = 2.8 Hz, 1 H), 8.07 (dd, J = 8.9, 2.8 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 50.6, 116.1, 124.4, 126.5, 127.3, 128.9, 129.2, 131.5, 141.0, 141.7, 159.2. HRMS (EI): m/z [M⁺] calcd for C₁₉H₁₅NO₃: 305.1052; found: 305.1054. Anal. Calcd for C₁₉H₁₅NO₃: C, 74.74; H, 4.95; N, 4.59. Found: C, 74.89; H, 4.83; N, 4.51. 1-(4-Methoxyphenylethyl)-3,4-dimethoxybenzene (3ch) Colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.59 (d, J = 6.8 Hz, 3 H), 3.78 (s, 3 H), 3.82 (s, 3 H), 3.85 (s, 3 H), 4.05 (q, J = 6.8 Hz, 1 H), 6.70 (d, J = 1.8 Hz, 1 H), 6.75–6.85 (m, 4 H), 7.11–7.14 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 22.3, 43.5, 55.2, 55.8, 55.8, 110.9, 111.0, 113.7, 119.1, 128.4, 138.8, 139.4, 147.2, 148.7, 157.7. HRMS (EI): m/z [M⁺] calcd for C₁₇H₂₀O₃: 272.1412; found: 272.1413. 1-(4-Methoxybenzhydryl)-3,4-dimethoxybenzene (3gh) Colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 3.75 (s, 3 H), 3.78 (s, 3 H), 3.84 (s, 3 H), 5.45 (s, 1 H), 6.59 (dd, J = 8.2, 2.0 Hz, 1 H), 6.66 (d, J = 2.0 Hz, 1 H), 6.77 (d, J = 8.2 Hz, 1 H), 6.80–6.84 (m, 2 H), 7.00–7.01 (m, 2 H), 7.10–7.12 (m, 2 H), 7.17–7.30 (m, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 55.2, 55.5, 55.7, 55.8, 110.7, 112.6, 113.6, 121.3, 126.1, 128.2, 129.2, 130.2, 136.2, 136.8, 144.4, 147.3, 148.6, 157.9. HRMS (EI): m/z [M⁺] calcd for C₂₂H₂₂O₃: 334.1569; found: 334.1568. 2-Benzhydryl-4-(2-chloroethyl)phenol (3as) Oil. ¹H NMR (400 MHz, CDCl₃): δ = 2.88 (t, J = 7.6 Hz, 2 H), 3.56 (t, J = 7.6 Hz, 2 H), 4.61 (s, 1 H), 5.71 (s, 1 H), 6.64 (d, J = 2.3 Hz, 1 H), 6.73 (d, J = 8.2 Hz, 1 H), 6.98 (dd, J = 8.2, 2.3 Hz, 1 H), 7.10–7.13 (m, 4 H), 7.22–7.32 (m, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 38.4, 45.3, 50.8, 116.2, 126.7, 128.1, 128.6, 129.3, 130.3, 130.5, 130.8, 142.3, 152.3. HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₉OCl: 322.1124; found: 322.1126. 2-Benzhydryl-4-(2-bromoethyl)phenol (3at) Oil. ¹H NMR (400 MHz, CDCl₃): δ = 2.96 (t, J = 7.8 Hz, 2 H), 3.41 (t, J = 7.8 Hz, 2 H), 4.70 (s, 1 H), 5.71 (s, 1 H), 6.63 (d, J = 1.8 Hz, 1 H), 6.71 (d, J = 8.0 Hz, 1 H), 6.96 (dd, J = 8.0, 1.8 Hz, 1 H), 7.12 (d, J = 7.3 Hz, 4 H), 7.21–7.31 (m, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 33.5, 38.6, 50.7, 116.2, 126.7, 128.0, 128.5, 129.3, 130.5, 130.7, 131.2, 142.3, 152.3. HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₉OBr: 366.0619; found: 366.0618. 2-Benzhydryl-4-(2-bromopropyl)phenol (3au) White solid; mp 88–91 °C (hexane). ¹H NMR (400 MHz, CDCl₃): δ = 1.59 (d, J = 6.4 Hz, 3 H), 2.90 (dd, J = 14.2, 7.0 Hz, 1 H), 3.05 (dd, J = 14.2, 7.0 Hz, 1 H), 3.70 (s, 3 H), 4.10–4.19 (m, 1 H), 5.91 (s, 1 H), 6.67 (d, J = 2.3 Hz, 1 H), 6.81 (d, J = 8.2 Hz, 1 H), 7.04 (dd, J = 8.2, 2.3 Hz, 1 H), 7.07–7.10 (m, 4 H), 7.17–7.28 (m, 7 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.4, 46.6, 49.4, 51.4, 55.6, 110.5, 126.0, 128.1, 29.4, 130.1, 131.3, 132.6, 143.7, 143.8, 156.0. HRMS (EI): m/z [M⁺] calcd for C₂₃H₂₃OBr: 394.0932; found: 394.0932. Anal. Calcd for C₂₃H₂₃BrO: C, 69.88; H, 5.86. Found: C, 69.75; H, 5.70.