Peri repulsion

The thermochemical database for substituted naphthalenes is far less extensive than for substituted benzenes and is reproduced in Table 5. For a given R, there are two isomeric monosubstituted naphthalenes, the 1-R and the 2-R. The latter might be expected to be more stable because it lacks the 1.8-or peri-repulsion found in the former between the substituent and a small, but unavoidably nearby, hydrogen. For a methyl substituent, the difference between the enthalpies of formation of 1- and 2-me-thylnaphthalene, 113.5 1.7 and 110.7 1.7 kJ/mol, respectively, reveals a very small 2.8 2.4 kJ/mol peri effect. For identical substituents, there are the following naphthalene isomers 1.2-. 1.3-. 1.4-. 1,5-, 1.6-. 1.7-. 1.8-, 2,3-, 2.6-. and 2.7-isomers. There is no substituent for which all 10 isomers have been thermochemically characterized. Furthermore, there is only one substituent for which there are enthalpy of formation data for the isomer of greatest interest— 1,8-dimethylnaphthalene. Equation 20. analogous to eq 18, is now recast for mono- and disubstituted naphthalenes ... 

Extensions of the enamine alkylation to a-tetralones have also been used (245-248), but product yields were lower, presumably due to steric crowding in a transition state where generation of an imonium salt gives rise to a repulsion between a methylene group on nitrogen and a peri aromatic proton. 

In contrast to the HMO method, the repulsion forces of the electrons are taken into account when carrying out the SCF method, so that it may be expected that the results of these calculations will give better results for the positive aromatic ions than do the HMO calculations. If the pif -values are plotted against the localization energies obtained by this method, one obtains excellent conformity with the requisite linearity, as shown in Fig. 27. The figures for peri-condensed aromatic... 

This [3 + 2] addition approach has been extended successfully to indole derivatives. The ready availability of the precursor tosylanilides provides a novel and useful complement to the current approach (94MI3). However, me(a-substituted tosylanilide yields a mixture of two regioisomeric indoles 107a and 107b in different ratios depending on the nature of substitutents (Scheme 30). Furthermore, the reaction of o-methyltosylamide with 102 does not produce any detectable amount of the desired indole, plausibly due to the repulsive peri-type steric interactions as indicated in 108, which would be unavoidable as the carbene approaches the CH bond. 

The JV-amino derivatives 269 underwent oxidative ring expansion to tri-azine 270. Analogous pyrazolo[3,4-c]pyridines, however, gave only intractable products. The latter system lacked the peri interaction necessary for repulsion of nucleophilic attack at the 4-position of the triazine ring.157... 

In fact, the reaction proceeded as expected [23]. Thus, by heating the l-[o-(l-alkynyl)phenyl]cyclopropanol complexes 36-Co2(CO)6 in refluxing 2-propanol, 2,3-dihydro-1-naphthalenone derivatives 37 are obtained as a mixture of (Aland (Z)-isomers in moderate yield accompanied by a substantial amount of an ethyl ketone derivative formed by ring opening of the cyclopropanol moiety. Furthermore, when an analogous naphthyl derivative 38 was employed, the reaction proceeded cleanly and the 2,3-dihydrophenanthren-l-one derivative 39 was obtained in 83 % yield (Scheme 17). The obvious difference in reactivity between phenyl and naphthyl derivatives is probably due to the presence of hydrogen at the peri position of the latter. To avoid steric repulsion between the alkyne-Co2(CO)6 moiety and this hydrogen, the molecule adopts a conforma-... 

Azine approach. The parent cation and substituted derivatives are available by acid-catalyzed cyclization of 2-/3-oxoalkylthiopyridines (401) using an acid such as sulfuric, phosphoric or PPA. Chloro or nitro substituents in the pyridine ring do not seriously interfere (66JHC27). The cyclization of 3-hydroxypyridine analogues (402) is also at the nitrogen to yield the thiazole derivatives. The cyclization, however, is sensitive to the peri interaction between 3- and 5-substituents. In 3,5-dimethyl derivatives (403 R2 = R3 = Me) the steric repulsion is apparent by the unusually low field signals for the methyl protons <81H(15)1349>. 

The peri derivatives, 1-aminoacridine, 8-aminoquinoline, and 8-aminoquinazoline, are anomalous. This correlation for derivatives in the adjoining ring is free from the field effect or charge repulsion which can contribute to the relatively low base-strengthening in 2-aminoazines. The lack of interaction of a 1-aza moiety with the... 

It has been suggested that substituent preference for the axial position in 9,10-dihydroheteroanthracene derivatives is related to repulsions with hydrogens peri to the substituent. If heteroatoms with longer C—M bonds are present the substituents will extend further away from the m -hydrogens and repulsive interactions should decrease. Thus organometallic heterocycles may not exhibit a substituent preference. 

As the hydrogens of the peri positions in 1 are replaced by larger moieties, the repulsion energy increases and AG inv decreases relative to 1 (Table 1) [57, 62]. The order of barrier heights of some 2,3-disubstituted corannulenes determined experimentally follows oxygen (35, 9.9 kcal/mol) > phenyl (32, 9.4 kcal/mol) > bromomethyl (34, 9.1 kcal/mol), and aU these examples exhibit lower barriers than other non-peri disubstituted corannulene derivatives, such as 36 (9.9 kcal/mol) and 37 (10.4 kcal/mol). The lower barriers found for the peri-substituted compounds compared to that found for the same substituents in isolated positions shows the special contribution from peri X/X repulsion. Substitution at the peri positions as well as the 1,6-positions leads to a further reduction in the barrier, for example compound 38 (8.7 kcal/mol). From an assumption of additivity in steric bulk, one can assess the steric size of a peri substituent as being roughly OR < Ph=Cl < Me. 

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