Sodium borohydride - A versatile reducing agent

(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.