Phosphoric Acid Diesters as Efficient Catalysts for Hydroaminations of Nonactivated Alkenes and an Application to Asymmetric Hydroaminations

Abstract

Catalytic amounts of phosphoric acid diesters enabled highly efficient intramolecular hydroaminations of nonactivated alkenes. With an enantiomerically enriched Brønsted acid catalyst an unprecedented metal-free asymmetric hydroamination of a nonactivated alkene was accomplished.

Key words

alkenes - amines - asymmetric catalysis - Brønsted acids - phosphates

References and Notes

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• 16 Representative Procedure: Synthesis of 2a (Table 3, Entry 1) A solution of 1a (162 mg, 0.495 mmol) and 3e (42.8 mg, 0.050 mmol) in dry Cl₂HCCHCl₂ (1.0 mL) was stirred under N₂ for 23 h at 130 °C. At ambient temperature sat. aq NaHCO₃ (25 mL) and Et₂O (50 mL) were added. The separated aqueous phase was extracted with Et₂O (2 × 50 mL). The combined organic layers were dried over Na₂SO₄ and concentrated in vacuo. The remaining residue was purified by column chromatography on silica gel (n-pentane-Et₂O, 15:1) to yield 2a (158 mg, 97%) as a light yellow solid (mp 71.2-72.4 °C). ¹H NMR (300 MHz, CDCl₃): δ = 7.42-7.37 (m, 15 H), 4.12 (d, J = 13.3 Hz, 1 H), 3.70 (d, J = 10.0 Hz, 1 H), 3.30 (d, J = 13.3 Hz, 1 H), 2.99-2.83 (m, 3 H), 2.25 (dd, J = 12.2, 7.2 Hz, 1 H), 1.21 (d, J = 6.1 Hz, 3 H). ¹³C NMR (75 MHz, DEPT, CDCl₃): δ = 150.5 (C_q), 148.6 (C_q), 139.9 (C_q), 128.6 (CH), 128.2 (CH), 128.1 (CH), 127.8 (CH), 127.4 (CH), 127.2 (CH), 126.8 (CH), 125.8 (CH), 125.4 (CH), 66.4 (CH₂), 59.7 (CH), 58.0 (CH₂), 52.5 (C_q), 47.9 (CH₂), 19.5 (CH₃). IR (ATR): 3061, 3029, 2960, 2924, 2788, 1491, 1445, 1373, 730, 695 cm^-1. MS (EI): m/z (relative intensity) = 327 (18) [M⁺], 312 (75), 147 (100), 91 (64), 56 (98). HRMS (EI): m/z calcd for C₂₄H₂₅N 327.1987; found: 327.2002. The spectral data are in accordance with those reported in the literature: Bender CF. Widenhoefer RA. J. Am. Chem. Soc.  2005,  127:  1070 

15

Acetone, benzene, MTBE, cyclohexane, and CCl₄ gave only traces of the desired product.

17

Under these reaction conditions, there was no indication for catalyst decomposition, and primary aminoalkenes reacted less efficiently.

18

Enantiomerically enriched catalyst 3a provided, under otherwise identical reaction conditions, only racemic product.