Sodium borohydride - A versatile reducing agent

Abstract

(A) Enantioselective reduction of prochiral ketones by NaBH4 and chiral amino alcohols led to optically active alcohols. The chiral oxazaborolidine-catalyzed reduction (CBS) of ketones was an important method for secondary chiral alcohol synthesis. Jiang has shown that (S)-α,α- diphenylpyrrolidinemethanol catalyzed the NaBH4 reduction of achiral ketone to afford the chiral alcohol with excellent ee and almost quantitative yield. A similar chiral auxiliary was employed in the reduction of the carbonyl group of α,β-unsaturated ketones. At the chiral interface of an amphiphilic dendrimer, alcohols were obtained in high ee by reduction of the corresponding prochiral ketones. (B) Polymer reagents are useful in NaBH 4 reduction. Quaternary ammonium-functionalized cross-linked polyacrylamide was used as a solid-liquid phase transfer catalyst in the reduction of carbonyl compounds with NaBH4. The resin proved to be efficient in carbonyl reduction and highly regioselective in α,β-unsaturated aldehyde and ketone reduction. A new combination system of Amberlyst-15(H+)-NaBH4-LiCl facilitated the reduction of oximes and hydrazones to the corresponding amines and hydrazines. (C) A highly stereoselective reduction of γ-oxo-α-amino acids by NaBH4 in the presence of a catalytic amount of manganese(II) chloride gives syn-γ-hydroxy-α-amino acids. In most reported cases, the dr is higher than 97:3. (D) A combination of NaBH4 and a catalytic amount of indium(III) chloride in acetonitrile reduces exclusively the α,β carbon-carbon double bond in α,β,γ,δ- unsaturated diaryl ketones, dicarboxylic esters, cyanoesters, and dicyano compounds. The same system reduces selectively the carbon-carbon double bonds in activated conjugated alkenes. (E) A practical procedure for the catalytic reduction of nitriles to Boc amines has been developed; it is neither air- nor moisture-sensitive, and is easily worked up. Nitro compounds, imines, and azides are reduced by Lewis acid-assisted NaBH4 to afford amines. (F) Reduction of the ester group in Baylis-Hillman adducts by NaBH4 is facilitated by the secondary hydroxyl group present at the β-position. This method is efficient for the preparation of substituted propane-1,3-diols. (G) NaBH4 in combination with Lewis acids is effective in the selective deprotection or cleavage of certain protective groups. A room-temperature deprotection method for trityl amines, trityl ethers, and trityl thioethers was reported. An efficient and chemoselective deprotection of prenyl ethers of phenols and alcohols with ZrCl4/NaBH4 in CH 2Cl2 was achieved in high yields. Reagents: 1. TFA, H 2O; 2. HgHal2/TFA, H2O; 3. HgHal 2/TFA, H2O, NaBH4; 4. TFA, H2O, NaBH4; 5. Hg(OAc)2/AcOH, H2O, HgCl 2/MeCN, H2O; 6. NaBH4/MeCN; 7. HgHal 2/MeCN, NaBH4; 8. i. 7 and ii. NaBH4; 9. HCl/MeCN, H2O.