Highly Efficient System for Reduction of Carboxylic Acids and Their Derivatives to Alcohols by HfCl₄/KBH₄

Abstract

HfCl₄/KBH₄ was found to be a facile, efficient, convenient, and chemoselective system for the reduction of carboxylic acids and their derivatives to the corresponding alcohols under mild conditions. HfCl₄/NaBH₄ was also utilized to reduce the same carboxylic acids and their derivatives, and it was found that the reducing ability of HfCl₄/NaBH₄ was similar to that of HfCl₄/KBH₄. The action of HfCl₄/KBH₄ on other types of substrates, such as benzyl chloride, peracid, epoxide, ketone, amide, imine, pyridine-N-oxide, and nitrile, was investigated, too. In addition, some competitive reductions of styrene oxide in the presence of carboxylic acid, carboxylic ester, nitrile, and amide were achieved.

Keywords:

• Alcohols
• carboxylic acid derivatives
• carboxylic acids
• HfCl₄/KBH₄
• reduction

Notes

^a Dry THF was added to the mixture of HfCl₄ and KBH₄ under nitrogen and stirred for 3 h at room temperature, and then a solution of 1 in dry THF was added dropwise and heated to the reaction temperature (according to typical experimental procedure 1).

^b Isolated yield; all products were identified by ¹H NMR and mass spectrometry.

^c The preparation of reducing agent was not under nitrogen.

^a Dry THF was added to the mixture of HfCl₄ and KBH₄ under nitrogen and stirred for 3 h at room temperature, and then a solution of substrate in dry THF was added dropwise and heated to the reaction temperature (according to typical experimental procedure 1).

^b Isolated yield; all products were characterized by ¹H NMR and mass spectrometry.

^a Reaction was performed with a 0.8/1.0/4.0 molar ratio of substrate/HfCl₄/NaBH₄ (according to typical experimental procedure 2).

^b Isolated yield; all products were identified by ¹H NMR and mass spectrometry.

^a Dry THF was added to the mixture of HfCl₄ and KBH₄ under nitrogen and stirred for 3 h at room temperature, and then a solution of substrate in dry THF was added dropwise and heated to the reaction temperature. (according to typical experimental procedure 1).

^b Isolated yield; all products were characterized by ¹H NMR and mass spectrometry or by comparison with authentic samples.

^c Yield of ¹H NMR.

^d Determined by ¹H NMR and comparison with authentic samples.