Theoretical Study of the Reactivity of Phenyl Radicals Toward Enol Acetates

Leonardo Carneiro,Leticia Real,Camilla Buarque,Pierre Esteves

doi:10.21577/0103-5053.20200192

Abstract

Arylation reactions are an important class of reactions and allow the synthesis of natural and synthetic products. Despite the efficient, but high cost and toxic methodologies involving transition metals, radical arylations have gained importance after the advent of photoredox catalysis. Arylation of enol acetates is an important tool for obtaining aryl ketones but the scope of the reaction is limited to the pattern of substitution at phenyl radical and α-carbon of the enol. Theoretical calculations ((U)BHandHLYP/6-311G**) show that the polar effect is the key factor in this reaction. A good correlation of calculated rate constants with field effect explained why phenyl radicals with electron-withdrawing groups react faster toward enol acetate. The presence of alkyl groups at α carbon at the enol showed some influence of enthalpic effect but strong influence of steric effect, evidenced by great correlations with Taft and Charton parameters. Finally, substitution at β carbon showed no significant effect at reaction rates.

Highlights

The insertion of aryl groups in organic scaffolds is an important class of reaction in Organic Chemistry
The observation of the substituent effects on the reaction kinetics can be interpreted as how the two carbon atoms of the double bonds in the enol acetate are electronically demanded on the rate-controlling transition state (Scheme 1)
The negative slopes of LFER correlations indicate the electron density depletion or accumulation of positive charge at β-carbon on the transition state (TS) concerning this atom on the reactants, We explored the phenyl radical reactivity in additions to enol acetates

Summary

Introduction

The visible-light excitation of the photocatalyst allows the formation of aryl radicals via single electron transfer (SET) reactions.[5] The proposed mechanism[14] (Figure 2) shows that after the formation of the phenyl radical, it is added to the enol acetate generating a radical adduct. This species can donate an electron to the photocatalyst or to

Methods

Results

Conclusion