Ring Size Alters Reactivity of Two Dibenzo‑Annulated Electrophiles

This study measured how two related α,β‑unsaturated ketones—one built on a six‑membered dibenzo ring and the other on a five‑membered dibenzo ring—respond to nucleophilic attack. Reaction rates for carbon–carbon bond formation with carbanions were obtained in DMSO at 20°C and analyzed with the Mayr–Patz equation to extract electrophilicity parameters. Quantum chemical calculations supported the experimental ranking, showing differences in transition‑state bond formation and thermodynamic driving force. The phenanthrenone‑derived acceptor was found to be more electrophilic than the acenaphthenone derivative. This places the phenanthrenone derivative near related ortho‑quinone methides on the reactivity scale, while the acenaphthenone derivative aligns with cyclic α,β‑unsaturated lactones with exocyclic methylene groups. The results enable systematic selection of further nucleophilic partners using established reactivity scales.

What the study examined

The work compared two dibenzo‑annulated α,β‑unsaturated ketones that differ in ring size: a six‑membered ring framework derived from 9(10H)‑phenanthrenone and a five‑membered ring framework derived from 1‑acenaphthenone. The goal was to assess how ring size influences the reactivity of the exocyclic electron‑deficient π‑system toward nucleophilic carbon–carbon bond formation.

To quantify reactivity, second‑order rate constants for nucleophilic addition of carbanions were measured in DMSO at 20°C by UV–Vis spectroscopy. Those kinetic data were combined with known nucleophile reactivity parameters and the Mayr–Patz equation to determine electrophilicity values for the two electrophiles.

Key findings

Measured reaction rates and Mayr analysis gave distinct electrophilicity parameters for the two compounds. The phenanthrenone‑derived acceptor was more electrophilic than the acenaphthenone‑derived acceptor according to the calculated parameters.

Quantum chemical calculations supported the experimental ordering. For the phenanthrenone derivative, the calculations indicated a less advanced bond in the transition state together with a larger thermodynamic driving force for forming the adduct than for the acenaphthenone case. The reported electrophilicity values locate the phenanthrenone derivative in a reactivity range similar to structurally related ortho‑quinone methides, while the acenaphthenone derivative is comparable to cyclic α,β‑unsaturated lactones that bear an exocyclic methylene group.

These combined kinetic and computational results provide a basis for choosing additional nucleophilic partners for both electrophiles using established reactivity scales.

Why it matters

Quantitative placement of these two dibenzo‑annulated electrophiles on a reactivity scale gives a clearer picture of how ring size affects the behavior of exocyclic electron‑deficient π‑systems. Such placement helps categorize the compounds relative to known classes of electrophiles, linking experimental kinetics to theoretical descriptions of transition states and thermodynamics.

By relating measured rates to the Mayr scale and supporting the findings with quantum calculations, the study creates a practical framework for predicting which nucleophiles will react with each electrophile and for comparing these systems to other well studied electrophiles in organic chemistry.