In situ silane activation enables catalytic reduction of carboxylic acids

Abstract

We describe a catalytic system for the conversion of carboxylic acids into alcohols using substoichiometric zinc acetate and N-methyl morpholine, in combination with phenylsilane as the nominal terminal reductant. Reaction...

A Safer Reduction of Carboxylic Acids with Titanium Catalysis

“…The reduction of aryl acids with BMS is reportedly improved by their conversion to the acyloxyborane with the addition of trimethyl borate . The alternatives for acid reduction include direct hydrogenation, stoichiometric or catalytic use of metal hydrides, additive-induced metal borohydride reductions, − silane reductions, , single-electron transfer using SmI 2 , , and others. Their drawbacks, such as high pressure and temperature requirements, − the reagent cost and safety concerns, ,,, including the explosion risk, and the undesirable esterification of acids with the product alcohols justify the continued search for further improved methodologies for acid reduction.…”

Facile, green, and functional group-tolerant reductions of carboxylic acids…in, or with, water

“…Aromatic carboxylic acids containing heterocycles like thiophene, benzofuran, indazole, pyridine, benzodioxole, etc., ( products 30, 32, 33, 35, 37, 38) were well tolerated. Electron-withdrawing groups such as nitrile, nitro, ester, and a p-tolylsulfonyl group on the aromatic ring seemed to perform better (entries 26,24,36,40) as compared to the previous cases wherein there was no reaction with moieties containing electron-withdrawing groups. It is interesting to note that the reduction of the intermediate thioester to the corresponding alcohol achieves full conversion (by TLC and crude NMR).…”

“…While there are numerous examples of hydrosilylations of esters 21 and amides, 22 reductions of free carboxylic acids oftentimes resort to large excesses of the silane reagent and rely on noble metals (e.g., Ru, 23 Rh 24 and Ir 25 ). Recently, base metals such as Zn 26 and Mn 27 have been found to reduce acids to alcohols employing a silane reductant. To realize these double reductions, a green technology was envisioned that avoids transition metalbased reagents, takes place efficiently in an aqueous medium under mild conditions, and is very tolerant of functional groups present in the starting acid (Fig.…”

“…, Ru, 23 Rh 24 and Ir 25 ). Recently, base metals such as Zn 26 and Mn 27 have been found to reduce acids to alcohols employing a silane reductant. To realize these double reductions, a green technology was envisioned that avoids transition metal-based reagents, takes place efficiently in an aqueous medium under mild conditions, and is very tolerant of functional groups present in the starting acid (Fig.…”

Facile, green, and functional group-tolerant reductions of carboxylic acids…in water

“…While there are numerous examples of hydrosilylation of esters 21 and amides, 22 reductions of free carboxylic acids tend to require a large excess of the silane reagent and use noble metals (e.g., Ru, 23 Rh 24 and Ir 25 ). Recently, base metals such as Zn 26 and Mn 27 have been found to reduce acids to alcohols employing a silane reductant. For accomplishing these reductions, our goal was to develop a green technology that avoids all transition metal-based reagents, takes place efficiently in an aqueous medium under mild conditions, and is very tolerant of functional groups present in the starting acid.…”