Diimides relative reactivities

The reduction of dienes by diimide depends on the nature of the substitution of the diene. Several studies of relative reactivity have been carried out and they indicated that an increasing degree of alkyl substitution on the double bond results in decreasing reactivity82. In the case of allenes, the reduction of the less substituted allenic double bonds and the formation of the thermodynamically less stable cis olefin can be explained by the steric control of the approach of the diimide (equation 23)83. 

TABLE 1. Relative reactivities" in the reduction of dienes by diimide... 

The relative reactivity of alkenes toward reduction by diimide depends on the degree of substitution. Increasing alkyl substitution results in decreasing reactivity, and strained alkenes exhibit higher reactivity than nonstrained compounds 187... 

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

Table 1 Relative Reactivities of Substituted Alkenes Toward Reduction with Diimide ...

Table 2 Relative Reactivities of Unsaturated Acids Toward Reduction with Diimide " ...

It was shown that conjugated dienes are more reactive than monoenes in their reduction by diimide ". According to the data of Table 1, conjugation increases the relative reactivity in reduction of dienes (fcjei) compared with the reduction of monoenes, but the more substituted double bond is less reactive. 

---