Phenyl ring, aromatic stabilization

Elimination to give a double bond conjugated with an aromatic ring is especially favorable. This presumably reflects both the increased acidity of the proton a to the phenyl ring and the stabilizing effect of the developing conjugation at the transition state. Amine oxides... 

The radical is formed by the loss of an electron from the aromatic ring to give an oxidized tyrosine species, in which the unpaired electron is delocalized over the phenyl ring. The radical species survives almost indefinitely when the protein is stored at —70 °C. This stability is suggested to result from the location of the tyrosine group in a pocket in the protein. The radical is not formed in the apoprotein. 

Aromatic groups on the ring carbons stabilize strongly the C-phenylated group 14 metalloles, as in the case with phospholes109, but the reactivity of the dienic system (i.e. Diels-Alder cycloadditions, formation of transition metal complexes) decreases3,4. 

In the family of phosphonates (RO)2R PO, the aryl derivatives were more stable than the related alkyl compounds, and the benefit was higher than the effect observed from alkylphosphate to alkylphosphonate (96). The same tendency has been observed with dithiophosphinic acids (RO)R PSSH namely, aromatic ligands were more resistant to hydrolysis and radiolysis than aliphatic compounds (49, 61). It was noted that the introduction of chlorine into the phenyl rings reinforced the radiolytic stability of the extractant (49, 61). 

On the other hand, photogenerated closed-ring isomers of diarylethenes with pyrrole, indole, or phenyl rings, which have rather high aromatic stabilization energy, are thermally unstable.1221 The photogenerated, blue, closed-ring isomer of l,2-bis(2-cyano-l,5-dimethyl-4-pyrrolyl)perfluorocyclopentene 11a disappeared in 37 s (= t1/2 ) at 25 °C. 

Regarding the closed-ring isomers, the difference in behavior between those diarylethenes with furan, thiophene, or thiazole rings and those with pyrrole, indole, or phenyl rings agrees well with the theoretical prediction that the thermal stability depends on the aromatic stabilization energy of the aryl group.1201... 

Benzil 20 can be made by the oxidation of benzoin2 21 (chapter 23) and it combines with 19 in one step under base catalysis3 without the need to isolate 18. The problem with these compounds is that 16 has only four rc-electrons delocalised round the ring and is anti-aromatic. Clearly four phenyl groups help stability but 17 exists as deep purple crystals showing an unusually small gap between the populated and unpopulated orbitals. 

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