Exploring Coumarins Reduction: NaBH4/MeOH versus Nickel Boride Generated In Situ

Résumé

The role of reagents NaBH4/MeOH and nickel boride (Ni2B) generated in?situ from NaBH4 and NiCl2, are compared in the reduction process of coumarin and a variety of 3,7-substituted coumarins bearing electro-donating (ED-group) or electro-withdrawing groups (EW-group). Coumarins (chromen-2-ones) are only reduced by Ni2B to the cyclic chromanones. This provides a useful and very simple reduction method for electron-rich coumarins, which are resistant to many other reducing methods. DFT calculations underlined the role of substituents electronic effects in the reactivity. Subsequent methanolysis may open the ring to methyl phenylpropanoate esters and alcohols resulting from their reductions can also be produced.