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

• Published in: ChemistrySelect (Weinheim) Vol. 5; no. 46; pp. 14735 - 14740
• Main Authors: Clarisse, Faure, Jamet, Hélène, Belle, Catherine, Moulinet d'Hardemare, Amaury
• Format: Journal Article
• Language: English
• Published: 11.12.2020
• Subjects: Chemoselectivity; Coumarin; Density Functional Calculations; Nickel Boride; Reduction
• ISSN: 2365-6549; 2365-6549
• DOI: 10.1002/slct.202004038

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‐one) are only reduced with 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. he reduction of 3,7‐subsituted coumarins could be reduced by NaBH4 in methanol only when they were substituted with R1=EW‐group. When using Ni2B generated in situ with NiCl2 and NaBH4, reductions occurred with coumarins bearing either R1=EW‐group or R1=ED‐group. The reduction's selectivity of these two reducing agents toward three products, an hydrogenated lactone (a), an ester product (b), and an alcohol product (c) is pointed out.