A New Asymmetric Tridentate Carbazole Ligand: Its Preparation and Application to Nozaki-Hiyama Allylation

Abstract

The manuscript describes our studies on a newly designed tridentate ligand. The new ligand 1 was successfully synthesized, and it was found that the asymmetric catalysis of Nozaki-Hiyama allylation with ligand 1 affords the product with good enantioselectivity in high yield.

Key words

asymmetric catalysis - carbazole - chromium - allyl­ations - Nozaki-Hiyama reaction - tridentate ligand

References

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  Data for 1: mp 144-145 °C; [α]_D ²¹ +757 (c 0.15, CHCl₃); ¹H NMR (400 MHz, CDCl₃) δ = 12.26 (1 H, s), 8.24 (2 H, d, J = 7.6 Hz), 7.98 (2 H, d, J = 7.6 Hz), 7.37-7.20 (12 H, m), 5.49 (2 H, dd, J = 10.0 Hz, 8.8 Hz), 4.86 (2 H, dd, J = 10.0 Hz, 8.3 Hz), 4.31 (2 H, dd, J = 8.8 Hz, 8.3Hz); ¹³C NMR (100 MHz, CDCl₃) δ = 163.7, 142.2, 139.0, 128.4, 127.1, 126.5, 125.8, 123.7, 123.4, 118.8, 110.1, 73.7, 69.8; IR (KBr) 3368, 1642, 1618, 1604, 1500, 1428, 1327, 1298, 1286, 1210, 1170, 1138, 1062, 1052, 958, 748, 700 cm^-1; FAB-MS [M + H]⁺ calculated for C₃₀H₂₄O₂N₃ : 458.1869, found : 458.1837.

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9

General Procedure: A mixture of ligand (S, S)-1 (26.9 mg, 0.059 mmol), CrCl₂ (7.0 mg, 0.057 mmol), and Mn (85.3 mg, 1.55 mmol) was azeotroped three times with toluene and dried under high vacuum, and was suspended in THF (2 mL). The color of the suspension immediately turned to brown. To the stirred suspension was added triethylamine (0.016 mL, 0.118 mmol), and after 30 min to the resulting mixture was added allylbromide (0.102 mL, 1.18 mmol). After stirring for 30 min, to the stirred mixture were added benzaldehyde (0.060 mL, 0.59 mmol) and TMSCl (0.149 mL, 1.18 mmol) successively at room temperature. After 12 h the color of the reaction mixture turned to reddish-brown. The reaction was quenched with saturated aqueous NaHCO₃ (1 mL), filtered through Celite, and evaporated under vacuum. The crude product was dissolved in THF (5 mL), and the stirred mixture was treated with 2 N HCl (1 mL) for 20 min. The reaction was quenched with adding saturated aqueous NaHCO₃ (3 mL), and the aqueous layer was extracted with CH₂Cl₂ (10 ml × 4). The combined organic layer was dried over Mg₂SO₄, and evaporated. The residue was purified by flash chromatography (hexane/ethyl
acetate = 10:1) to afford the known compound, (S)-1-phenyl-3-buten-1-ol (64.7 mg, 71% ee, 96%): ee was determined by HPLC (254 nm); Daicel Chiral Cell OD-H 0.46 cm φ × 25 cm; hexane/iso-propanol = 19:1; flow rate=0.3 mL/min); retention time: 26.4 min for (R)-1-phen-yl-3-buten-1-ol, 28.7 min for (S)-1-phenyl-3-buten-1-ol.

10

In the absence of CrCl₂ and ligand 1, the allylated products were obtained in 8% yield under the conditions of entry 1 (Table [1] ). Aliphatic aldehydes are surmised to be rather inert to the allylmanganase reagent. Cf. ref. [3]

11

The relationship between the reaction temperature and the solubility of the Cr(II)-ligand 1 complex and/or the related allyl complex was hard to observe under the described reaction condition because insoluble manganese powder was in the flask. Further investigation of the chromium complexes formed in situ is now under investigation.