Deactivation of Triplet Phenyl Alkyl Ketones by Conjugatively Electron-Withdrawing Substituents

Article abstract of DOI:10.1021/ja00414a048

Para-cyano, -carbomethoxy, and -acyl substituents decrease the triplet reactivity of valerophenone (γ-hydrogen abstraction), whereas comparable meta substituents increase reactivity.Spectroscopic results are presented which indicate that para-(-R) substituents lower ?,?^* triplet energies so much more than n,?^* energies that the lowest triplets become largely ?,?^* in nature.Meta-(-R) substituents do not stabilize ?,?^* triplets enough to invert triplet levels.Both substitution patterns support a largely 1,4-biradical structure for the lowest ?,?^* triplet of acylbenzenes.Ortho substituents show the usual steric anomalies: ortho cyano enhances valerophenone triplet reactivity by stabilizing the n,?^* triplet; ortho carbomethoxy deactivates valerophenone by stabilizing the ?,?^* triplet but not the n,?.^*