Steric inhibition, of resonance

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

When a hydrogen atom is peri to an azine-nitrogen, there is no steric inhibition of resonance activation as there is in 1-nitronaph-thalene (4-methoxy-dechlorination of its 4-chloro derivative seems to be thereby decelerated only 2-fold in rate). Steric hindrance of nucleophihc substitution by the co-planar peri hydrogen is sometimes... 

De la Mare et al.260 measured the rates of chlorination of biphenyl, a wide range of its methyl derivatives, and anisole in acetic acid at 25 °C. Second-order rate coefficients (104 2) were biphenyl (6.40), 2-methylbiphenyl (3.20), 3-methyl-biphenyl (820), 4-methylbiphenyl (30.0), 2.2 -dimethylbiphenyl (4.40), 3.3 -dimethylbiphenyl (2,630), 4,4 -dimethylbiphenyl (70.0), 2,6 -dimethylbiphenyl (1,130), 3,4,3, 4 -tetramethylbiphenyl (19,300), anisole (12.5 x 104), and these results showed very clearly the effect of steric inhibition of resonance between the phenyl rings through the presence of ortho methyl groups260. Similar (but rather more emphatic) results were obtained262 in chlorination of the /-butyl derivatives for which the corresponding rate coefficients were 2-/-butylbiphenyl (1.0) 4-/-butylbiphenyl (25.7), 2,2 -di-/-butylbiphenyl (1.8), 4,4 -di-/-butylbiphenyl (70.0). 

Wepster, Prog. Siereochem. 1958, 2, 99-156, p. 125. For another example ot this type of steric inhibition of resonance, see Exner Folli Marcaccioli Vivarelli J. Chem. Soc., Perkin Trans. 2 1983, 757. 

The presence of ortho substituents on the C-phenyl ring of the imine also has an accelerating effect.182,331,341 Based on spectral and kinetic evidence, it is suggested that the ortho substituent twists the phenyl ring out of the plane of the C=N bond, thus causing steric inhibition of resonance, which raises the ground-state energy and lowers the activation barrier of the imine.341... 

The concept of steric inhibition of resonance has, however, recently been criticized by Exner, Gal and their coworkers93, who consider that it has often been invoked for molecules in which it is unlikely to occur. These include o-mcthyl benzoic acid. Substantial evidence was assembled in support of this view. The authors suggest that a primary steric effect may be involved, which is greater in the undissociated molecule than in the ion and is therefore acid-strengthening. This idea would presumably be applicable to d.v-crotonic... 

The small accelerating influence of the -phenyl substituent is readily understood to be the consequence of the steric inhibition of resonance. Phenyl-substituted alkylcarbonium ions (20) achieve a high degree of resonance stabilization from the aryl substituent. The ortho hydrogens are easily accommodated in a planar conformation. In the incipient... 

Nitro groups in the 4-position may also participate in the rearrangement. For example, 5-chloro- and 5-methyl-4-nitrobenzofuroxans rearrange to their 7-chloro- and 7-methyl-4-nitro isomers, the ease of the process being attributed to steric inhibition of resonance for the nitro group in the starting materials. Similarly, 4-ethoxycarbonylamino-5,7-dinitroben-zofuroxan equilibrates with its 4,6-dinitro-5-urethane isomer. 

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