Mannich-type Reaction Catalyzed by HBF₄ in Water: Effect of the Loading of Surfactant

Abstract

The HBF₄ (0.1 equiv)-catalyzed Mannich-type reactions of ketene silyl acetals with aldimines proceeded smoothly in water in the coexistence of as low as 1 mol% of SDS. Furthermore, the Mannich-type reaction also took place in water in the absence of SDS by means of 0.3 equiv of HBF₄ to afford the corresponding β-amino esters in high yields.

Key words

Brønsted acid - surfactant - Mannich-type reactions - Schiff bases - water

References

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10

We optimized the loading of SDS by use of a silyl enol ether as a substrate in ref. [8]

12

General Experimental Procedure for Protocol A (entry 3 of Table 1). Aq solution of HBF₄ (8 µL, 0.0080 mmol, 1.3598 mol/L) was added to a mixture of N-benzylidene-p-anisidine(1) (17 mg, 0.0805 mmol), ketene silyl acetal(2a) (48 µL, 0.241 mmol), SDS (8 µL, 0.0080 mmol, 0.10027 mol/L solution) in H₂O (0.5 mL) at r.t. After being stirred at the temperature for 0.5 h, the reaction was quenched by addition of sat. NaHCO₃ and CH₂Cl₂. The aq layer was extracted with CH₂Cl₂ and the combined organic layers were washed with brine, dried over anhyd Na₂SO₄, and concentrated to dryness. Purification of the crude mixture by preparative TLC (SiO₂, hexane:ethyl acetate = 10:1, v/v) gave 3a in 94% yield.

13

Because H₃PO₄ also worked equally as a Brønsted acid for the present Mannich-type reaction, presence of the fluoride ion, which might be generated from HBF₄, is not essential for the Mannich-type reaction. Authors are grateful to a referee for pointing out the issue.

14

General Experimental Procedure for Protocol B (entry 5 of Table 1). Aq solution of HBF₄ (48 µL, 0.0480 mmol, 1.0 mol/L) was added to a mixture of N-benzylidene-p-anisidine(1) (33.6 mg, 0.159 mmol), ketene silyl acetal (2a) (99 µL, 0.477 mmol) in H₂O (1.0 mL) at r.t. After being stirred at the temperature for 0.5 h, the reaction was quenched by addition of sat. NaHCO₃ and CH₂Cl₂. The aq layer was extracted with CH₂Cl₂ and the combined organic layers were washed with brine, dried over anhyd Na₂SO₄, and concentrated to dryness. Purification of the crude mixture by preparative TLC (SiO₂, hexane:ethyl acetate = 10:1, v/v) gave 3a in 81% yield.