Naphthalenes direct steric effects

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

These effects occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Examples of systems exhibiting direct steric effects are ortlio-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho (2,2-, 2,1- and 2,3-) and peri (1,8-) substituted naphthalenes, 3,4,5 and 6, respectively. 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

The importance of the steric effect accounts for the spread of the data for lf-N in the substitution reactions. Nitration and non-catalytic chlorination, reactions of modest steric requirements, define points which fall above the arbitrary reference line. Bromination, a reaction of somewhat greater steric requirements, is not accelerated to the extent anticipated on the basis of the results for nitration or chlorination. The benzoylation reaction with large steric requirements is two orders of magnitude slower than the equally selective chlorination reaction. The unusually small ratio for lf-N/2f-N for the acylation reaction is a further indication of the steric effects. Apparently, the direct substitution reactions of naphthalene respond to the retarding steric influence of the peri hydrogen in much the same way as for other ortho substituents. 

The isomerization shown in Scheme 6.32 involves two intermediary dianions and not the direct conversion of the initial anion-radical into the final one. However, what is the driving force for this isomerization The molecule of l,3,6,8-tetra(tcrt-butyl)naphthalene is nonplanar the tert-butyl group and carbon atom at position 1 of the naphthalene skeleton lie off the plane of the molecule. The corresponding anion-radical has the same stereochemical peculiarity (Goldberg 1973). Such bending removes the steric strain but, naturally, decreases the degree of the n-electron delocalization over the neutral molecule. As for the anion-radical, its unpaired electron delocalizes less effectively than the anion-radical of the unsubstituted naphthalene. Bending of the naphthalene skeleton... 

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