Sterically inhibited

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... 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

The influence of steric constraints on the nucleophilic substitution of Cl- by a nucleophile is not very important and does not much influence the yield even in the presence of two ortho-methyl groups. Steric inhibition was only found when... 

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). 

Monomers 24 and 25 behave differently when exposed to catalyst 14, shown in Fig. 8.15. Divinyltetramethyldisiloxane 24 is found to be metathesis inactive due to similar steric inhibitions experienced with divinyldimethylsilane. Monomer 25 is synthesized with one additional methylene spacer unit between the silicon atom and the olefin moiety, which then is reacted with Schrock s [Mo] catalyst. Here, metathesis occurs quite readily, exclusively forming a seven-membered cychc molecule (26) instead of polymer. The formation of the cyclic product can be explained by tire Tliorpe-Ingold effect.15... 

In these solvents at sufficiently low Br2 concentration (< 10-3 m) the kinetics are first order both in the olefin and in Br2 and the main solvent effect consists of an electrophilic solvation of the departing Br ion. A nucleophilic assistance by hydroxylic solvents has also been recognized recently (ref. 26) (Scheme 10). So far, return during the olefin bromination in methanol had been admitted only for alkylideneadamantanes, and was ascribed to steric inhibition to nucleophilic attack at carbons of the bromonium ion (ref. 26). 

Oae and Khim measured the rates of hydrolysis of chlorophenyl phenyl sulfoxides and sulfones with hydroxide ion in aqueous DMSO at 158 °C. Both SOPh and S02Ph were found to activate the nucleophilic substitution from ortho- and para-positions, but the effect of SOjPh was considerably larger than that of SOPh. The results were interpreted in terms of 7t(pd) conjugation in the intermediate complexes. In a later paper it was shown that the introduction of a methyl group ortho to SOPh or S02Ph slightly retards the above and related reactions but this was attributed to the inductive effect of Me rather than steric inhibition of 7t(pd) conjugation (Section III.A.l). 

That this difference is not due to differing electron availability at the nitrogen atom in the two cases is confirmed by the fact that the two amines differ very little in their strengths as bases (cf p. 72) the uptake of a proton constituting very much less of a steric obstacle than the uptake of the relatively bulky BMe3. Esterification and ester hydrolysis are other reactions particularly susceptible to steric inhibition (cf. p. 241). 

It should be emphasised that such steric inhibition is only an extreme case, and any factors which disturb or inhibit a particular orientation of the reactants with respect to each other, short of preventing their close approach, can also profoundly affect the rate of reactions a state of affairs that is often encountered in reactions in biological systems. 

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

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