The effect of replacing carbon by nitrogen in reductions with SmI ₂: Reduction of azobenzene

The reduction of azobenzene by SmI₂ in THF to give hydrazobenzene was investigated. The kinetics are first order in the substrate and first order in SmI₂. The kinetic order in MeOH is ca. 0.56, and in TFE it is ca. 0.2. The fractional order in the proton donors is interpreted as being a result of their acting in two opposing manners. In one the proton donor enhances the reaction by protonation of the radical anion, and in the other it slows the reaction by binding to the lone pair electrons of the nitrogen in the azobenzene. This hampers the fast inner-sphere electron-transfer mode. Experiments conducted in the presence of low concentrations of HMPA show rate enhancement suggesting that the SmI₂, which is partly coordinated to HMPA molecules, has some free sites to bind to the substrate. When more HMPA is added, it prevents the fast inner-sphere mechanism and the rate decreases. In this system, the increase in the reduction potential of SmI₂ caused by HMPA is similar to the rate enhancement by an inner sphere mechanism. In general, the replacement of a skeletal carbon by a nitrogen atom causes a significant rate enhancement.