Aromatic compounds ortho,para-directing groups

Sometimes the orientation of two substituents m an aromatic compound precludes Its straightforward synthesis m Chloroethylbenzene for example has two ortho para directing groups m a meta relationship and so can t be prepared either from chloroben zene or ethylbenzene In cases such as this we couple electrophilic aromatic substitution with functional group manipulation to produce the desired compound... 

Donor groups therefore direct attack of the electrophile to the ortho and para positions and are known as ortholpara directors. Conversely, aromatic compounds containing electron acceptor groups are attacked at the meta position since this is the least electron-deficient site. Such groups are called meta directors. Not all substituents fit exactly into this picture halogens are deactivating but direct attack to the ortho and para positions. 

The synthesis of URB597 was extremely challenging, more so for the positions of its substituents than for their structure. Both groups are located in meta positions relative to the bond between the rings. Problem 39 revealed that phenyl rings as substituents are ortho, para directing. Therefore electrophilic aromatic substitution reactions performed on biphenyl cannot serve as an entry to the preparation of this important compound. 

This simple transformation is extremely important because it turns the mefo-directing nitro group into an ortho, para-directing amino group (although as you saw on p. 483, the amino group may need taming to make its reactivity useful). The sequence of nitration—reduction allows us to introduce a useful NH J equivalent into an aromatic molecule, and can let us make otherwise difficult-to-form mefa-substituted amino compounds. 

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