Selective Reduction of Aromatic Nitriles to Aldehydes by Lithium Diisobutylpiperidinohydroaluminate (LDBPA)

Abstract

Partial reduction of nitriles into aldehyde is one of the most important and highly desirable means in organic synthesis, and a large number of reducing agents for this have been reported. Among them, a commercially available diisobutylaluminium hydride (DIBAH) is commonly used for the preparation of aldehydes from both aromatic and aliphatic nitriles, although the reagent provides moderate yields (48-90%). It has been also reported that other several reducing agents such as potassium 9-sec-amyl-9-boratacyclo[3.3.1]nonane (K-9-sec-Am-9-BBNH), lithium tris(dihexylamino)aluminium hydride (LTDHA) and lithium N,N'-dimethylethylenediaminoaluminium hydride (LDMEDAH) reduce chemoselectively nitriles to aldehydes. We have recently reported that diisobutylpiperidinohydroaluminate (LDBPA, 1) can effectively reduce esters to aldehydes in moderate to good yields at 0 C. As a part of our research program directed toward the discovery of new reducing agents through the simple modification of commercial DIBAH (Scheme 1), we applied this reagent for the partial reduction of nitriles to aldehydes. We first examined partial reduction of benzonitrile and capronitrile with 1 in THF at 0 C. The reduction of benzonitrile provided benzaldehyde in quantitative yield in 1 h. Using the same methodology, the partial reduction of other aromatic and aliphatic nitriles to the corresponding aldehydes was carried out. As shown in Table 1, aromatic benzonitriles, such as 1-cyanonaphthalene, chlorobenzonitriles, toluonitriles and methoxybenzonitriles were smoothly reduced to the corresponding aldehydes in quantitative yields (entries 1-11). Among them, the reduction of tolunitriles and methoxybenzonitriles required longer reaction time than other nitriles (entries 7-11). This may be