Deciphering Intermediates and Additives Effect on the Reduction of Nitrobenzene by SmI₂

Product distribution in the reaction of nitrobenzene (NB) with SmI₂ shows a strong dependence on the number of equivalents of SmI₂ used. Phenylhydroxylamine (PHA), azoxybenzene (AZOB) and azobenzene (AZB) are the main products when the number of equivalents of SmI₂ is up to 4 equivalents. When 6 equivalents are used, aniline (AN) yield is higher and with 8 equivalents, the yield of aniline is 95 %. Using various techniques including chemical labelling, it was shown that none of the above compounds is an intermediate on the path of nitrobenzene to aniline. The suggested mechanism involves the formation of nitrosobenzene which is converted to an intermediate that could generate phenylhydroxylamine. Yet, in the presence of a large amount of SmI₂ it is converted directly to aniline, circumventing the production of phenylhydroxylamine. The ratio of aniline production to the condensed products (AZOB and AZB) was shown to be inversely proportional to the ability of the additives used to coordinate to SmI₂. The observed order was: MeOH>Water>ethylene glycol>HMPA. Trapping of proposed intermediate with benzaldehyde gives synthetically useful products such as nitrone and Imine.