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

Overview

Kinetic and computational quantification of electrophilicity for two dibenzo‑annulated α,β‑unsaturated ketones derived from 9(10H)‑phenanthrenone (six‑membered annulation) and 1‑acenaphthenone (five‑membered annulation). The study probes how ring size alters the reactivity of the exocyclic electron‑deficient π‑system by determining second‑order rate constants for carbanion additions in DMSO at 20 °C and by deriving Mayr electrophilicity parameters (E). Quantum chemical calculations were employed to interpret transition‑state structure and thermodynamic driving force differences that underlie the observed kinetic behavior.

Methods and approach

Kinetic monitoring: UV–Vis spectroscopy was used to follow the nucleophilic addition of characterized carbanions to both Michael acceptors in DMSO at 20 °C, enabling determination of second‑order rate constants k2 for the carbon–carbon bond‑forming steps. Mayr analysis: The Mayr–Patz linear free‑energy relationship (lg k2 = sN(N + E)) was applied using literature nucleophile parameters (N and sN) to extract the electrophilicity parameter E for each electrophile. Computational analysis: Quantum chemical calculations were performed to characterize transition‑state geometries and to estimate relative thermodynamic driving forces for adduct formation, providing mechanistic interpretation of the kinetic and Mayr‑derived electrophilicity differences.

Results

Measured second‑order rate constants for carbanion additions allowed extraction of Mayr E parameters that distinguish the two electrophiles: the phenanthrenone‑derived Michael acceptor displays higher electrophilicity (E = −15.93) than the acenaphthenone‑derived α,β‑unsaturated ketone (E = −18.72). Quantum chemical calculations indicate the phenanthrenone derivative has a less advanced C•••C bond formation in the transition state and a larger thermodynamic driving force for adduct formation relative to the acenaphthenone derivative, consistent with its greater Mayr electrophilicity. Placement on established Mayr‑type scales situates the phenanthrenone derivative in the reactivity range of structurally related ortho‑quinone methides, whereas the acenaphthenone derivative aligns with cyclic α,β‑unsaturated lactones bearing exocyclic methylene groups.

Implications

Quantified Mayr E parameters enable rational selection of nucleophiles for further synthetic applications involving these two electrophiles by mapping their reactivity onto established electrophilicity scales. The combined kinetic and quantum chemical evidence links ring annulation size to both transition‑state structure and thermodynamic driving force, providing a mechanistic basis for predicting reactivity trends among dibenzo‑annulated α,β‑unsaturated systems. The results support systematic extension of nucleophilic partner choice and mechanistic hypotheses across related annulated electrophiles using Mayr reactivity parameters and targeted computational characterization.