Metal-Free Reduction of Aromatic Nitro Compounds to Aromatic Amines with B2(OH)4

Abstract

Chemoselective reduction of nitro groups is important for organic synthesis, in particular synthesis of medicine and fine chemicals. A variety of strategies have been developed to achieve the selective reduction of nitroarenes to their corresponding amines. In general, this transformation has been performed mainly by transition-metal-based methods but this approach has some limitation including the use of a precious metal, inflammable gas and harsh conditions such as high temperature, pressure, long reaction times. Recently, for the reduction of nitroarenes, the methods by utilizing the reducing ability of BBA under transition-metal-free conditions were developed. Although reported protocols do not require any transition-metal-catalysts and elaborated experimental tools, they have still drawbacks, such as a lack of selectivity, high temperature and using a large excess of base. Therefore, the development of method with the high chemoselective and mild conditions for the reduction of nitro compounds is desired.

In this study, we developed transition-metal-free and a highly selective reduction of aromatic nitro compounds to aromatic amines. Commercially available diboron source, BBA, and 4,4’-bipyridine as an organocatalyst were used. This reduction was proceeded at room temperature under air conditions. Under mild conditions, nitroarenes with reducible functional group such as carbonyl, vinyl, ethynyl and halide were converted to corresponding anilines in 5 min with excellent selectivity. Our new noble method shows much enhanced chemoselective reduction without the use of a transition-metal-catalyst compared to other reduction of nitroarenes using BBA.