The reduction of organic halides and diazonium salts with sodium borohydride

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Virginia Polytechnic Institute and State University


Sodium borohydride in aqueous dimethylsulfoxide has been shown to be a good reducing agent for converting certain activated aromatic halo-compounds to their corresponding dehalogenated products. The order for ease of removal of the halogen is I>Br>Cl. The activating groups are those that are strongly electron-withdrawing. The reactivity for activating aryl halides substituted with groups such as -NO₂, -CF₃, -F, -Cl, -Br, and -I is in the order ortho > meta > para, thus indicating that the activating effect has its origins in inductive rather than resonance effects.

The removal of halogen appears to be occurring via a displacement on-halogen. This has been demonstrated in certain cases by using deuterium oxide instead of water in the reaction. When this is done, the halogen is replaced by deuterium instead of hydrogen. This indicates that the halogen leaves without its bonding electrons, thus leaving a carbanionic site on the aromatic ring. The carbanion is subsequently quenched by a proton from the water.

This reaction has also been shown to be applicable to aromatic systems other than benzene.

On-halogen type displacement by the hydride ion also occurs on certain polyhalogenated alkanes with the formation of a guasi-carbanionic intermediate which can be quenched by a proton from water, undergo alpha elimination, or undergo beta elimination. The alpha elimination occurs when a good leaving group is not present on the beta carbon. Attempts at trapping the carbene type intermediate from alpha eliminations were unsuccessful, evidently because of the presence of the water in the reaction medium. In two cases eliminations have occurred through a benzene system to generate para-xylylenes.

Diazonium salts were also shown to undergo direct reduction with sodium borohydride, thus providing a new route for the deamination of aromatic amines.