B(C₆F₅)₃-Catalyzed transfer hydrogenation of esters and organic carbonates towards alcohols with ammonia borane

Abstract

Herein, we report an efficient metal-free system for the transfer hydrogenation of esters and carbonates by-passing the otherwise ubiquitous formation of transesterification side-products. The Lewis acid B(C6F5)3 is used as...

Synthesis: Molecular Structure, Thermal-Calorimetric and Computational Analyses, of Three New Amine Borane Adducts

“…However, many other ABAs have been developed in parallel, not necessarily for being used as a hydrogen carrier, thus for various uses in chemistry. Examples are follows: ethylenediamine bisborane as hydrogen carrier [ 5 ]; diisopropylaminoborane for one-pot borylation [ 6 ]; morpholine borane as radical initiator [ 7 ]; pyridine borane as reducing agent [ 8 ]; ammonia borane as hydrogenation reagent [ 9 ]; 1,3,5-( p -aminophenyl)benzene-borane as monomer of borazine-linked polymer [ 10 ]; and, methylamine borane as precursor of boron-carbon-nitrogen layers [ 11 ]. Providing an exhaustive list of the ABAs reported so far and of their potential applications goes beyond the scope of the present article, and for further information, the reader is referred to appropriate review articles [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

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 .…”

“…Following the successful demonstration of the conversion of esters to ethers, we pursued the reduction of esters to alcohols with 1a in the presence of BF 3 −Et 2 O (Table 2, entry 10), which has been recently reported to require elevated temperatures in DCE and B(C 6 F 5 ) 3 as cocatalyst. 39 A selection of esters from the deoxygenation study were subjected to the reduction to alcohols, including aliphatic esters and aromatic esters with both electron-donating and -withdrawing groups. The results are summarized in Scheme 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 .…”

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

“…15 Among many metal-free hydrogenation reactions, those involving frustrated Lewis pairs are a well-known system. 16…”