Quantifying the Electrophilicity of 9(10 H )‐Phenanthrenone‐ and 1‐Acenaphthenone‐Derived α, β‐Unsaturated Ketones

ABSTRACT The dibenzo‐annulated six‐membered ring of 9(10 H )‐phenanthrenone‐ and the dibenzo‐annulated five‐membered ring in 1‐acenaphthenone‐derived α, β‐unsaturated ketones were chosen to study the influence of the ring size on the reactivity of the exocyclic electron‐deficient π‐system. The kinetics of nucleophilic addition of carbanions to both Michael acceptors in DMSO at 20°C was monitored by UV‐Vis spectroscopy to determine the second‐order rate constants k 2 of the carbon–carbon bond‐forming reactions. The Mayr–Patz equation, lg k 2 = s N ( N + E ), along with reported reactivity parameters ( N and s N ) of the carbanions were used to calculate the electrophilicity parameter E for both electrophiles. The higher electrophilicity of the phenanthrenone‐derived Michael acceptor was further scrutinized by quantum chemical calculations, which showed a less advanced bond formation in the transition state and a greater thermodynamic driving force for adduct formation for the phenanthrenone ( E = −15.93) than for the acenaphthenone‐derived α, β‐unsaturated ketone ( E = −18.72). Thus, the phenanthrenone derivative is located on the electrophilicity scale in the reactivity range of structurally related ortho ‐quinone methides. The acenaphthenone derivative has similar electrophilicity as cyclic α, β‐unsaturated lactones with an exocyclic methylene group. Further nucleophilic reaction partners for both studied electrophiles can now be systematically selected by using the Mayr reactivity scales.