Heteropoly Acid Supported on Silica Gel as Catalyst for the Asymmetric Transfer Allylation of Aromatic Aldehydes under Solvent-Free Conditions

Abstract

A new convenient method for the asymmetric transfer allylation of aromatic aldehydes was developed. The reaction gave the best results using a chiral allyl donor molecule derived from (–)-menthone in the presence of heteropoly acids supported on silica gel under solvent-free conditions, and the desired homoallylic alcohol derivatives were obtained in good yields with good to high enantioselectivity.

Key words

asymmetric transfer allylation - heteropoly acid - silica gel support - solvent-free conditions - homoallylic alcohols

• References and Notes

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• 6 Phosphomolybdic acid (PMA, H₃PMo₁₂O₄₀·nH₂O) supported on silica gel (10 wt%) was prepared as described in the literature.¹⁶ PMA (1.0 g) was dissolved in 5.0 mL of MeOH, and to this mixture was added SiO₂ (9.0 g, Kanto Silica Gel 60N, spherical, neutral, 63–210 mm). After sufficient shaking, the mixture was dried at 50 °C (1 mmHg, 1 h). The resulting olive-green powder was used for the subsequent experiments.
• 7 The acid strength of HPAs in AcOH decreases in the order: H₆P₂W₁₈O₆₂·nH₂O (pK ₁ = 4.39) > H₃PMo₁₂O₄₀·nH₂O (pK ₁ = 4.68) > H₄SiW₁₂O₄₀·24H₂O (pK ₁ = 4.87). See: Timofeeva MN. Appl. Catal., A 2003; 256: 19 ; the current price of PMA·nH₂O is ¥2500/25 g (TCI)
• 8 Stereochemically pure sample was used throughout the experiments after a small amount (ca. 7%) of the corresponding stereoisomer was removed by silica gel column chromatography (hexane–EtOAc, 9:1).

In contrast, use of the analogue derived from (+)-camphor was found to be unsatisfactory for the present purpose due to significant decomposition of this donor substrate. See:
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• 10 Excess allyl donor molecule 4 could be easily recovered during purification of the final product 3 by silica gel column chromatography.
• 11 General Procedure for the Asymmetric Transfer Allylation of Aromatic Aldehydes To a solution of aldehyde 1 (0.5 mmol) in anhydrous CH₂Cl₂ (1.5 mL) was added chiral allyl donor 4 (196 mg, 1.0 mmol) followed by PMA on SiO₂ (10 wt%, 455 mg), and the mixture was dried in vacuo (10 mmHg) at r.t. The resulting dark blue-colored mixture was allowed to stand at r.t. until the reaction was complete (monitored by TLC). After removal of the solid acid catalyst by filtration, the reaction mixture was rinsed thoroughly with CH₂Cl₂, and the organic layer was concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane–EtOAc) to afford the desired product 3 in a pure form, and its ee was determined by chiral HPLC analysis. The results are summarized in Scheme 2. (S)-1-(4-Nitrophenyl)-3-buten-1-ol [(S)-3a] Orange oil; R_f = 0.15 (hexane–EtOAc, 4:1). [α]_D ²⁸ –52.43 (c 1.00, CHCl₃, 88% ee) {lit.¹⁷ (R)-enantiomer: [α]_D ²⁴ +65.87 (c 1.07, CHCl₃, 98% ee)}. FTIR (KBr): ν = 3404, 1605, 1520, 1347 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 2.42 (dt, J = 14.0, 8.0 Hz, 1 H), 2.48 (br, 1 H), 2.51 (dt, J = 14.0, 6.5 Hz, 1 H), 4.82 (dd, J = 8.0, 5.0 Hz, 1 H), 5.14 (d, J = 17.0 Hz, 1 H), 5.15 (d, J = 9.5 Hz, 1 H), 5.74 (ddt, J = 17.0, 9.5, 7.0 Hz, 1 H), 7.49 and 8.15 (AA′BB′ system, 4 H). ¹³C NMR (125.8 MHz, CDCl₃): δ = 43.87, 72.10, 119.63, 123.60 (2×), 126.52 (2×), 133.17, 147.16, 151.09. The ee of the product was determined by chiral HPLC analysis with a Chiralpak AD-H column (0.46 × 25 cm, hexane–2-PrOH (98:2), flow rate 1.0 mL min^–1, λ = 254 nm): t _R (minor) = 30.72 min and t _R (major) = 32.52 min.
• 12 The major byproducts formed in these reactions were aldol adducts caused by the condensation of an excess of aryl aldehydes 1 with the liberated (–)-menthone.
• 13 Due to this evidence, we did not try to extend this reaction to electron-rich aromatic aldehydes or aliphatic aldehyde homologues.
• 14a Unfortunately, with respect to the formation of 3i, no significant changes in ee were observed at lower temperatures (0 °C, 12 h, 77%, 64% ee).
• 14b See also ref. 9b,d and: Loh T.-P, Hu Q.-Y, Chok Y.-K, Tan K.-T. Tetrahedron Lett. 2001; 42: 9277
• 15 We briefly checked the possibility of equilibrium racemization in this system. For example, we observed that the optical purity of (S)-3a (88% ee) was dramatically decreased by exposure to PMA on SiO₂ (10 wt%) in the presence of 1 equiv of 1a at r.t. under solvent-free conditions: 58% ee (20 min), 52% ee (40 min), 48% ee (60 min), and 46% ee (80 min). Without 1a, no detectable change in either the chemical (100%) or optical yield (88% ee) was observed even after the reaction mixture was allowed to stand for 24 h at r.t. See also ref. 3.
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• 17 Jain P, Antilla JC. J. Am. Chem. Soc. 2010; 132: 11884