Lewis Base Organocatalyzed Enantioselective Hydrosilylation of 1,4-Benzoxazines

Abstract

A chiral Lewis base organocatalyzed enantioselective hydrosilylation of 1,4-benzoxazines is presented. The reactions ­afforded various enantioenriched 3-substituted dihydro-2H-1,4-benzoxazines with high yields (up to 98%) in moderate enantioselectivities (up to 87% ee).

Key words

asymmetric catalysis - enantioselectivity - heterocycles - hydrosilylation - imines

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• 9 Enantioselective Hydrosilylation of Benzoxazine; General Procedure: Trichlorosilane (51 μL, 0.5 mmol, 2.0 equiv) dissolved in THF (0.15 mL) was added to a stirred solution of 1,4-benzoxazine 2 (0.25 mmol) and catalyst (0.025 mmol) in anhydrous THF (2 mL) at 0 °C. The mixture was stirred at the same temperature for 12 h, then quenched with sat. aq NaHCO₃ and extracted with EtOAc. The combined extract was washed with brine and dried over anhydrous Na₂SO₄ and the solvents were evaporated. Purification by column chromatography (silica gel; hexane–EtOAc) afforded pure product 3. The ee values were determined by using established HPLC techniques with chiral stationary phases
• 10 3-(Furan-2-yl)-3,4-dihydro-2H-benzo[b][1,4]oxazine (3l): Yield: 90%; light-yellow oil; 87% ee. ¹H NMR (300 MHz, CDCl₃): δ = 7.39 (d, J = 0.9 Hz, 1 H), 6.84–6.78 (m, 2 H), 6.72–6.65 (m, 2 H), 6.36 (dd, J = 1.8, 3.2 Hz, 1 H), 6.30 (d, J = 3.2 Hz, 1 H), 4.63–4.62 (m, 1 H), 4.43–4.38 (m, 1 H), 4.23 (dd, J = 7.0, 10.6 Hz, 1 H), 4.10 (br s, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 152.5, 143.4, 142.2, 132.6, 121.5, 119.2, 116.6, 115.6, 110.4, 106.7, 67.8, 48.3. HPLC (OD-H column; n-hexane–2-propanol, 80:20; flow rate = 1.0 mL/min): t _R = 8.08 (minor), 9.12 (major) min; [α]_D ²⁰ +32.6 (c = 0.4, CHCl₃). HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₂H₁₁NO₂ + H⁺: 202.0863; found: 202.0865