Trichlorosilane: a versatile reagent for nitroalkanes transformations

Nitroalkanes have proven to be a valuable and versatile class of compounds in organic synthesis. Transformations that permit the interconversion of nitro groups into other functionalities are therefore of primary importance as they potentially broaden the utility of nitroalkanes as intermediates in synthesis.1 Our group has recently reported an unprecedented metal-free protocol for the reduction of nitro derivatives into amines based on the use of trichlorosilane (HSiCl3)2 an inexpensive and readily commercially available compound produced as a waste by the silicon industry. Specifically, it was observed that nitro compounds could be reduced to the corresponding amines when reacted in the presence of HSiCl3 and a tertiary amine under mild reaction conditions. A systematic screening of substrates revealed that this reduction protocol is applicable to both aryl and aliphatic nitro compounds, and was successfully employed in the total synthesis of complex molecules.3 However, depending on the nature of the aliphatic substrate, the corresponding cyano derivative could be observed as a substantial reaction byproduct. Herein we report our efforts for the optimization of this protocol that resulted in the chemoselective, divergent transformation of aliphatic nitro compounds into the corresponding amine or cyano derivative by fine tuning of the reaction conditions. Specifically, by using 8 equiv. of triethylamine (TEA) or diisopropylethylamine (DIPEA) at –20 °C, it is possible to obtain phenylacetonitrile starting from phenylnitroethane, with up to 80% conversion. On the other hand, using 3.5 equivalents of a sterically hindered base, such as pentamethylpiperidine (PMP), leads to the formation of phenylethylamine in up to 98% conversion.