Reduction of esters to alcohols and iodides using aminodiborane (μ-NH₂B₂H₅): scope and mechanistic investigations

Abstract

Herein, we report an efficient methodology for the reduction of esters to alcohols and iodides using in situ generated aminodiborane from iodine and ammonia borane.

Ammonia-Borane Dependent Transfer Hydrogenation of Carboxylic Esters to Primary Alcohols

“…(4-Aminophenyl)methanol (from 19). The general procedure of hydrogenation of ester using ammonia borane was followed with 0.5 mmol (75.6 mg) of Methyl 4-aminobenzoate (19) for 20 h to afford a…”

“…(4-Aminophenyl)methanol (from 19). The general procedure of hydrogenation of ester using ammonia borane was followed with 0.5 mmol (75.6 mg) of Methyl 4-aminobenzoate (19) 94% (57.9 mg) isolated yield of desired product (4-Aminophenyl)methanol as yellow-black solid. The product was purified by column chromatography (DCM/MeOH = 95:5) by using 60−120 mesh silica gel.…”

“…Quite recently, B(C 6 F 5 ) 3 17 and BF 3 −Et 2 O, 18 including in situ generated aminodiborane from I 2 and AB protocols, 19 were reported for the reduction of esters to alcohols (Figure 1C). Nevertheless, the complex substrate scope and widespread practical applications are limited, and in some cases, an excess of Lewis acid (150 mol %) and I 2 (50 mol %) 19 is used in the reaction. Thus, strategies permitting a broad functional group tolerance and an extensive substrate scope while providing excellent activity and selectivity remain a formidable challenge in ester reduction chemistry.…”

Catalyst- and Stoichiometry-Dependent Deoxygenative Reduction of Esters to Ethers or Alcohols with Borane–Ammonia

“…A great deal of work has been performed exploring BH 3 −NH 3 as a hydrogen source, 18,19 but it is also gaining prominence as a useful reducing agent. 20,21 Using appropriate catalysts and activators, it has been applied for the reduction of carbonyls, 22,23 esters, 24,25 amides, 26,27 and a variety of other functional groups. 28,29 As part of our program on borane−amines for organic synthesis, we recently reported efficient room temperature (RT) reductions of ketones 30 and carboxylic acids 31 to alcohols with BH 3 −NH 3 using catalytic (10 mol %) TiCl 4 .…”

“…Borane–ammonia (BH 3 –NH 3 , 1a ) is an easily prepared, air and moisture stable alternative to borane–tetrahydrofuran (BTHF) and borane–dimethyl sulfide (BMS). A great deal of work has been performed exploring BH 3 –NH 3 as a hydrogen source, , but it is also gaining prominence as a useful reducing agent. , Using appropriate catalysts and activators, it has been applied for the reduction of carbonyls, , esters, , amides, , and a variety of other functional groups. , As part of our program on borane–amines for organic synthesis, we recently reported efficient room temperature (RT) reductions of ketones and carboxylic acids to alcohols with BH 3 –NH 3 using catalytic (10 mol %) TiCl 4 . During these projects, ketones or acids were selectively reduced in the presence of esters, which is not surprising given the fact that esters are poor substrates for reduction with BTHF or BMS .…”