Diarylborinic Acid Derivatives as a Catalytic Iminium Ion Generator in the Mannich-Type Reaction Using Secondary Amines, Aldehydes, and Ketene Silyl Acetals

Yusuke Tanaka; Tomoaki Hasui; Michinori Suginome

doi:10.1055/s-2008-1072724

LETTER

Diarylborinic Acid Derivatives as a Catalytic Iminium Ion Generator in the Mannich-Type Reaction Using Secondary Amines, Aldehydes, and Ketene Silyl Acetals

Yusuke Tanaka, Tomoaki Hasui, Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
Fax: +81(75)3832722; e-Mail: suginome@sbchem.kyoto-u.ac.jp;

Abstract

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Abstract

Reactions of secondary amines, aldehydes, and ketene silyl acetals were efficiently promoted by catalytic amounts of diarylborinic acid esters, Ar₂B(OR), affording β-amino esters selectively with no formation of the corresponding β-hydroxy esters.

Key words

boron-based catalyst - β-amino acids - iminium ion intermediate - ketene silyl acetal - nucleophilic additions

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References

References and Notes

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General Procedure for the Ph ₂ BOMe-Catalyzed Mannich-Type Reaction
To a solution of Ph₂BOMe (3.9 mg, 0.02 mmol) in DMSO (1.0 mL) were added amine 3 (0.4 mmol), aldehyde 2 (0.4-0.6 mmol), and ketene silyl acetal 4 (0.8 mmol). The mixture was stirred for 12 h at 60 °C, quenched with ice water, and extracted with EtOAc. The organic layer was washed with H₂O, and aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na₂SO₄ and evaporated under vacuum. The crude 5 was purified by silica gel column chromatography. The imine-containing products 8 and 10 could not be purified by column chromatography, but were isolated by bulb-to-bulb distillation.

The imine-amine conjugate 7 exists as the cyclized aminal form, which may be in equilibrium with the imine form shown in the scheme.