Calcium Metal in Liquid Ammonia for Selective Reduction of Organic Compounds

Selective reduction of functional groups by use of dissolving calcium metal was performed, and the results are in comparison with those by lithium and sodium metals. Reduction with 2.0−2.2 equiv of calcium led chromone ( 1 ) to 4-chromanone ( 2, 75% yield), coumarin ( 4 ) to dihydrocoumarin ( 5, 76% yield) and 3-(2-hydroxyphenyl)-1-propanol ( 6, 6% yield), α-tetralone ( 7 ) to 1,2,3,4-tetrahydro-1-naphthol ( 8, 81% yield), and β-methoxystyrene ( 12 ) to methyl phenethyl ether ( 13, 85% yield). Performance of reductions on the same substrates by use of 4.0 equiv of lithium or sodium metal gave over-reduced products. Reduction of indole-3-carboxaldehyde ( 15 ) to 3-indolemethanol ( 16 ), 1-acetylnaphthalene ( 17 ) to 1-acetyl-3,4-dihydronaphthalene ( 18 ), and trans -4-phenyl-3-buten-2-one ( 19 ) to 4-phenyl-2-butanone ( 20 ) were accomplished by use of calcium, lithium, and sodium metals, among which calcium gave the highest yields (80−90%). Being a milder reducing agent than lithium and sodium, calcium metal in liquid ammonia offered a better selectivity.