Bridge head radicals

Certain radicals have rigid molecular structures with fixed bond angles and dihedral angles. These are known as bridge head radicals and have pyramidal structure. This has been supported on the basis of physical and chemical evidences. 

The FE-NBMO and HMO-NBMO coefficients of it radicals corresponding to a- and 0-substitutions in naphthalene are derived in Chart I to serve as examples. In the case of a-substitution, the FEMO model offers two possibilities for treating the effective potential at a bridge head atom adjacent to a substitution site, that is, either as a joint or a nonjoint atom. In the first case, an appendage of the free-electron pathway is considered to be pointing toward the a-position. In this chapter, these types of atoms in PAH fragments are taken to be joint positions. 

These aforementioned 5-(l,2,4-triazolidin-l-yl)uracils (77CB1716) are easily oxidized by NBS to urazolyl radicals (77CB1699), which attack each other head-to-tail to form tetrazocane diradicals that then recombine in the final step to afford bridged 1,2,5,6-tetrazocanes with uracil and urazolo bridges (X-ray). This is a decisively simple approach to a rather complex heterocyclic molecule in one step (85CB436) (Scheme 160). 

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