Meta-allowing substituents

This selective deactivation leaves the meta positions the most reactive, and meta substitution is seen in the products. Meta-directors, often called meta-allowing substituents, deactivate the meta position less than the ortho and para positions, allowing meta substitution. 

Draw resonance forms for the sigma complexes resulting from electrophilic attack on substituted aromatic rings. Explain which substituents are activating and which are deactivating, and show why they are ortho, para-directing, or meta-allowing. Problems 17-47,48, 54, 57, 50, and 64... 

Although there are several reports in the literature on boron-mediated amide formations, the boron reagents had to be used in stoichiometric amounts.1-4-5-6-7-8-9 Recently, Yamamoto et al. presented the first truly catalytic method allowing for a direct amide formation from free carboxylic acids and amines as the reaction partners.10-1112 Best results were obtained by using phenylboronic acids bearing electron withdrawing substituents in the meta- and/or para-positions such as 3,4,5-trifluorophenylboronic acid or 3,5-bis(trifluoromethyl)boronic acid as the catalysts. 

Multiple regression on j and og-type parameters employs the dual substituent-parameter equation, which may be written as in equation 891. (The combining of the k and k° terms implies that there is no intercept term allowed, and k° is now the actual value for the parent system, cf below.) For any given reaction series the equation is applied to meta- and para-substituents separately, and so values of pi and pr characteristic both of reaction and of substituent position are obtained. The various op-type scales are linearly related to each other only approximately. In any given application the scale which gives the best correlation must be found92. 

If the electrophile attacks the benzene ring at a position ortho or para to a + / substituent (i.e., to one electron-donating by inductive effect), the activated complex will be similar to 81 or 82, respectively. Resonance structures 81c and 82c are of particularly low energy because in these the positive charge is localized on the carbon that bears the + / group. Attack at the meta position does not allow such a resonance structure to be drawn. 

The elimination of one unknown is accomplished by the assumption that om Me = o-m-Me (— 0.069). Ample evidence is available to substantiate this assignment. Experimentally, the meta reactivities even in electron-deficient reactions are adequately correlated by the Hammett parameters, as illustrated for aromatic nitration (Fig. 3) and solvolysis (Fig. 10). Presumably this is the consequence of the unimportance of resonance interactions between the meta substituents and the reaction site. Solution of the equation (25) with Me as —0.069 yields cr+Me as —0.280. The assignment of a value to oJ.Me allows the extension of the treatment to all other substituents through relations (23) and (24). 

Nitration puts in a substituent para to the alkyl group, which, after reduction, becomes a powerful ortho director so that the bromine is directed meta to the original alkyl group (Chapter 22). Removal of the amino group by reduction allows the preparation of meta bromo alkyl benzenes that cannot be made directly. 

In situation (ii), for instance betaines of type 10, 22,24,26, and 28, the basicity of the benzimidazole anion is that of a classical benzimidazolate perturbed by the substituent at position 2. The pK values have been determined for betaines 10 and related compounds (87BSF604), allowing the determination of the cr meta value for the 2,4,6-triphenylpyridinium substituent close to that of the nitro group, o-m=0.74). The... 

For investigations of intramolecular charge-transfer interactions, donor- and acceptor-substituted cyclophanes are particularly well suited, since they allow for wide variation of substitutents. Interplanar-distance and orientation of aromatic planes can be tuned by choosing appropriate bridge lengths. Synjanti conformational preferences have an additional influence on charge-transfer interactions. Severals studies of this kind were undertaken by Staab et al. 17,34-36]. In usually good yields they synthesized methoxy-sutetituted [2.2]meta- and metaparacyclo-phanes with bromo-, cyano-, nitro-, or ester-substituents. In the case of methoxy-substituted substrates, often a few percent of quinoid by-products do form, e.g., 40, 41, and 44, which have been isolated in some cases. 

Another probe technique that has been used is to compare the effects of trifluoromethyl, at the meta and para positions, in both phenol and benzoic acid [29]. Only in the case where the substituent is in the para position in phenol is it directly conjugated with the ionising centre and therefore allowing a resonance effect to be important. Values of pK for the phenols led to the following substituent parameters o-(p-CF3) = +0.54 and o-(ot-CF3) = +0.43, the ratio o-(p-CF3)/o-(w-CF3) being 1.25, and this is essentially... 

The results confirm that It is the state of the arene which reacts with ground state alkene but do not allow any conclusion to be made about the intermediacy of an exciplex or any other species along the reaction path. Also in the same laboratory, the products of the meta photo-addition of cyclopentene with alkyl and alkoxy benzenes have been determined and the effect of increasing size of the alkyl or alkoxy group examined. The major product isolated is (80). although (81) and other isomers become more important for alkyl benzenes as the substituent s size is increased. The results are consistent with a polar mechanism involving the formation of (82) where the substituent stabilises the positive charge and steric interactions are avoided by the endo orientation of the cyclopentane. Also consistent with a polar intermediate are the meta adducts... 

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