Substitution, electrophilic Mills-Nixon effect

Since Mills and Nixon studied in their original paper hydroxy derivates of fused aromatic systems like /3-hydroxyindan [1], we examined a series of model compounds possessing OH group attached at the /3-position [14], It was found that the hydroxy group exerted overwhelming influence on the electrophilic susceptibility of the aromatic carbons by substantial activation of its ortho positions. However, the free /3 position is the most active one because of the Mills-Nixon effect, which amplifies the OH inductive effect. Hence the selectivity in the electrophilic substitution reactions is governed once again by the MN effect [14]. 

It will be noted that Friedel-Crafts substitution on (264a) is successful, but reaction with bromine takes place by addition rather than by electrophilic substitution. It has been suggested that this may be an example of the Mills-Nixon effect , which has received little attention in recent years. Stated simply, the effect is one of reduction in aromatic character due to the strain imposed by annelation. Additional support for the Mills-Nixon effect comes from e.s.r. studies of radical anions of a series of strained compounds, including naphthalenocyclobutene. Large spin-density perturbations were observed which were correlated with calculations supporting the above effect. ... 

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