Peri-hydrogen

The steeply increasing steric compression shown when passing from one to two adjacent methyl groups is dramatically illustrated in the case of peri-hydrogens. Annelation of abenzo ring has the usual rate-increasing effect (see Section IV, C, 4), i.e., on going from 4-chloro-... 

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... 

The first quantitative study of the reaction was carried out with anthracene-9-carboxylic acid (which possesses the necessary steric requirement by virtue of the peri-hydrogen atoms, and is very reactive at the 9 position towards electrophilic substitution). Schenkel632 found that the decarboxylation rate was increased in the presence of acid, and the first-order rate coefficients (believed to be in sec-1) are given in Table 205. It was subsequently concluded that in the absence of acid,... 

Even more pronounced steric effects have been observed for the free radical alkylation of protonated N-heterocyclic bases by the procedure of Minisci69, b d. Quinoline is attacked selectively in the 2- and 4-position by nucleophilic alkyl radicals in sulfuric acid. The largest radicals, t.-butyl, react exclusively in the 2-position because of steric hindrance by the peri-hydrogen when attack occurs at the 4-position. 

The dimethylamino derivative 27 a was found to give no higher quantum yield in chemiluminescence than 27. This may be due to a steric hindrance of the dimethylamino group by the neighboring peri-hydrogen atom. However, 8-dimethylamino-naphthalene-2.3-dicar-boxylic acid hydrazide 97> 28 where a similar steric effect should be expected, is far more efficient than the parent compound (naphthalene 2.3-dicarboxylic hydrazide), the ratio of their quantum yields being about 40 1. 

Based on the Chapman mechanism, Pitts (1983) proposed that 6-N02-BaP, with two peri hydrogens and the N02 out of plane, should be less stable photo-chemically than the 1- and 3-NOz isomers with only one peri hydrogen. This proved to be the case. Thus, in solution-phase irradiations of these isomers, 6-NOz-BaP decomposed rapidly whereas the 1- and 3-isomers were much more stable (Zielinska, 1985). A key question then was whether or not these results could be extrapolated to give their relative photodecomposition rates when irradiated as particle-bound species on the surfaces of primary combustion products and ambient aerosols (vide infra see also Feilberg and Nielsen, 1999b). 

A peri hydrogen bonding enables juglone to partake in a Diels-Alder reaction in a regioselective fashion [176]. The H-bonded carbonyl group becomes more highly polarized which in turn causes its p-carbon more acceptor-like. 

The report made in connection with a reaction between 1-dimethylaminoisoquinoline and Mel at 20° invites speculation. Quater-nization of the annular nitrogen atom results.94 This contrasts with reaction at the side chain of both 2-dimethylaminopyridine97 102 and 2-dimethylaminoquinoline94 under similar conditions. Perhaps the neighboring peri-hydrogen causes the dimethylamino group to adopt a conformation that minimizes its steric effect and thereby allows the isoquinoline to react at the annular position. 

C to rt produced exclusively trans-p-lactams in good yields, (Scheme 32). The domestic microwave irradiation on this type of substrates was utilized with good success. The effect of a peri hydrogen was found to be significant in controlling the stereochemistry of the resulting p-lactams. 

To investigate the reaction in more detail, isomeric naphthalenyl and anthrace-nyl imines were employed in the Staudinger reaction. The 1-substituted compounds (21 and 23) produced only the trans isomers (22 and 24). However, the 2-substituted compounds (42 and 45) produced a mixture of cis (44 and 47) and trans isomers (43 and 46) in a ratio of 1 1 (Scheme 8). The reason for this observation is not clear. The peri hydrogen in 21 and 23 was closer to the imine bond than it was in 42 and 45. Further research is necessary to define the role of the peri hydrogen in this type of reaction. 

Coplanarity is made impossible by the close approach of the neighboring peri hydrogen atoms (H-l, H-l and H-8, H-87H-9, H-9 ), and these compounds have two principal routes to minimize the steric strain based on the mechanisms mentioned in Section I. One way, twisting, is to rotate the two halves of the molecule about the formal double bond... 

The log lf-N values (Fig. 33) are considerably more scattered from an arbitrary correlation line than the data for the equally reactive 2-position of fluorene. This behavior is not unexpected. The significance of the steric effect of the peri hydrogen is well known. The rates... 

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. 

Fig. 10.3 The nitro group in (a) 9-nitroanthracene is peri to two hydrogen atoms while in (b) 1-nitropyrene is peri to one hydrogen atom. Steric forces resulting from the peri hydrogen atoms force the nitro group to be out-of-plane relative to the aromatic moiety...

The equilibrium composition was calculated for phenanthridine and a number of other azaaromatics, but no simple relationship existed between these figures and the corresponding pKa values and no quantitative significance could be attached to the presence or absence of peri-hydrogen atoms,235 which have a controlling influence in other reactions of phenanthridine vide infra). (Infrared studies on hydrogen bonding are considered below.)... 

The relative rates of N-alkylation by methyl iodide of 7 and 8 relative to 6 suggest a steric effect of a peri hydrogen a little larger than that of the comparable 2-methyl [Eq. (2)]. Later a more precise estimation, using a Bronsted equation to estimate the electronic effect of the benzo, shows that the... 

The higher barrier in pyrido(3,2-d)pyrimidine 95b was mainly attributed to the difference in steric requirements of the peri hydrogen (95a) and the... 

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