Nitro groups, aromatic with rearrangements

Nitro compounds, in particular aromatic and heterocyclic derivatives, absorb strongly in the near UV. They have properties similar to ketones in their excited state. These compounds are characterized by an unpaired electron in the n0 orbital and thus by a radical character. A typical example of this radical character is the easy intramolecular hydrogen abstraction in nifedipine and related vasodilators (Sch. 7) (18). Another manifestation of the radical character of the nitro group is the rearrangement often observed with nitrated five-membered heterocycles, as in the case of metronidazole (Sch. 8) (19). 

When aromatic nitro compounds are treated with cyanide ion, the nitro group is displaced and a carboxyl group enters with cine substitution (p. 854), always ortho to the displaced group, never meta or para. The scope of this reaction, called the von Richter rearrangement, is variable. As with other nucleophilic aromatic substitutions, the reaction gives best results when electron-withdrawing groups are in ortho and para positions, but yields are low, usually < 20% and never > 50%. 

Nitro group rearrangements have also been postulated during aromatic nitration reactions, particularly those associated with reaction pathways involving ipso attack by the reagent (N02+ or NO2). This topic is more fully discussed elsewhere in this volume and so just a few examples will be presented here. 

When the iridium hydride is reacted with a hydrogen acceptor, simple oxidative addition adducts can be seen for aromatic and vinylic C-H containing substrates. With nitrobenzene, although a thermodynamic preference is seen for an orthometallated chelate product, the kinetic preference is for meta- and para-Gr-W activation, which is then followed by rearrangement to the o/n4n-activated product, which in turn coordinates the nitro group. Hence, chelate assistance is found to have no kinetic benefit for C-H activation in this complex (Equation (22)). 

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