Substituent groups hydrogen atom distribution

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

With benzoic acid (XXXI) the hydrogen in 2-position and the oxygen atoms of the carboxyl group must be regarded as the elec-trophilic/nucleophilic system on the grounds of the charge distribution (Fig.17). Comparison with the phenols in Table XVII shows that the benzene nucleus can contribute the electrophilic as well as the nucleophilic group to the bipolar system. The effect of further substituents depends on type and relative position. 

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