Selective reductions. 39. Partial reduction of carboxylic acids with thexylchloroborane-methyl sulfide. A direct and simple aldehyde synthesis

Low‐Pressure Hydrogenation of Arenecarboxylic Acids to Aryl Aldehydes

“…[1] It is usually performed using stoichiometric amounts of metal-or metal hydride-based reducing agents. [2,3,4] However, most protocols involve an additional reaction step for the preparation of activated carboxylic acid derivatives. Also, unwanted side reactions, such as the overreduction to alcohols and the reduction of other functional groups, are notoriously hard to suppress.…”

“…However, a systematic investigation of various phosphine ligands revealed that with a more electronrich triarylphosphine, moderate yields are achieved even at this reduced pressure (entries 2 and 3). Sterically demanding, electron-rich trialkylphosphines such as PCy 3 and some of Buchwalds dicyclohexylbiarylphosphines gave even better results (entries 4-9). Full conversion of the carboxylic acid and high yields of the aldehyde were finally achieved with dicyclohexylphenylphosphine, a ligand seldom used in palladium catalysis (entry 10).…”

Thirty Six Years of Research on the Selective Reduction and Hydroboration

“…Soon after, the reagent is found to be an excellent reducing agent for the direct conversion of carboxylic acids to the corresponding aldehydes. 11,12 The slow addition of 10% excess reagent (2.2 equiv) to one equivalent of compound in methylene chloride at 0 o C for the hydrogen evolution stage, followed by reduction at room temperature, is optimum for the reduction to aldehyde. Generally, the yields of aromatic series are lower than those in the aliphatic series.…”

“…However, in the case of aliphatic compounds, it appears that the yields of aldehydes are almost quantitative, with no observable dependence on the structure of the acid. Even the aliphatic dicarboxylic acids are reduced to the corresponding dialdehydes in essentially quantitative yields 12,13 (Eq. 8).…”

“…The procedure for direct partial reduction involves the utilizing thexylhalo-or thexylalkoxyboranes-methyl sulfide (ThxBHX·SMe 2 , X=Cl, Br, OR), 12,13,26 as illustrated in Scheme 3 and 4. Especially, the ability for converting only aromatic carboxylic acids to the corresponding aldehydes selectively in the presence of aliphatic acids intact with thexylhalo-and thexylalkoxyboranes is noteworthy.…”