Chiral Bis-pyridinium Salts as Novel Stereoselective Catalysts for the Metal-Free Diels-Alder Cycloaddition of α,β-Unsaturated Aldehydes

Abstract

The synthesis of enantiomerically pure C ₂-symmetric bis-pyridinium salts was realized through a simple condensation of 2-pyridyl-carboxyaldehydes with a chiral diamine. The catalytic properties of such novel compounds were preliminarily studied and the trifluoracetate salts of chiral bis-imines of 2-picolinaldehydes derived from 1,1′-binaphthyl-2,2′-diamine were shown to catalyze the Diels-Alder reaction between cyclopentadiene and α,β-unsaturated aldehydes. Modest exo stereoselectivities and enantioselectivities of up to 55% were obtained.

chiral imines - Diels-Alder reaction - pyridinium salt - enantioselective catalysis - hydrogen bonding activation

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17

When the reaction was promoted by catalysts generated in situ by mixing the chiral diamine with the required amount of trifluoroacetic acid for 15 min in the reaction solution before adding the reagents, the formation of the product was observed in only trace amounts.

18

Among others, triflic acid and benzoic acid were used; with stronger acids, such as HCl, no reaction was observed.

19

By running the reaction at lower temperature (0 ˚C), the yields were usually depressed without any positive effect on the stereoselectivity.