Dinitrobenzenes steric effects

Fig. 14.1. Steric effects in the selective hydrogenation of 1 -substituted-2,4-dinitrobenzenes.

A summary of ring activation would not be complete without mention of steric effects. Since nucleophilic attack occurs in a plane perpendicular to that of the aromatic ring, substitutions are not usually very sensitive to the bulk of orf/io-substituents. However, steric hindrance by an ortho-methyl substituent may be observed as the steric requirements of the nucleophile increase. Thus, the introduction of a 6-methyl group in l-chloro-2,4-dinitrobenzene causes rate decreases by factors of 14, 22, and 276 in reactions with methoxide, aniline, and piperidine, respectively. The first two figures are attributable to the electronic effect of the methyl substituent, but with piperidine, an additional steric effect is apparent [50]. 

Reactions involving amine nucleophiles have played an important part in the study of aromatic substitutions, partly because the observation of base catalysis in these reactions provides evidence for the two-step nature of the mechanism. Early work [1 ] on the reactions of aliphatic amines with l-chloro-2,4-dinitrobenzene in ethanol, where nucleophilic attack is rate limiting, showed the importance of steric effects so that, for example, wo-propylamine is roughly 10 times less reactive than n-propylamine. The general pathway for these reactions is shown in Scheme 6.10. Here, the base B may be excess of the nucleophile or added bases such as DABCO or hydroxide ions. The... 

For imsymmetrically substituted /H-dinitrobenzenes (l-X-2,4-dinitrobenzenes) the first reduction occurs either at the 2-nitro group (X = Cl, Br, I) or at the 4-nitro group (X = COOH, CH3, C2H5) [122]. These differences may be caused by inductive effects and by steric inhibition of coplanarity with the benzene ring of the nitro group or of other substituents. In 2,4-dinitrochlorobenzene the inductive effect from the chlorine on the 2-nitro group probably is responsible for its easier reduction in the first four-electron reduction 2-hydroxylamino-4-nitrochlorobenzene is formed in the second four-electron step, 2,4-dihydroxylaminochlorobenzene and in the last four-electron reduction, 2,4-di-aminochlorobenzene. 

---