Assessing the reactivity of the enol form of Acetophenones – 1-Phenylethenol Laura S. Sánchez B., Nanette Wachter-Jurcsak, Daniel P. Miller Hofstra University Department of Chemistry
Introduction
Computational Details
Most substituted acetophenones were experimentally found to form symmetrically substituted dibenzoylfuroxans when treated with nitric acid (Scheme 1). However, the reactivity of hydroxyacetophenones differed as a result of tautomerization (Figure 1). In this study, a HOMO and Hirshfeld charge analysis were conducted to assess the Lewis basicity of the C⍺ (C1) towards electrophilic addition.
Geometry Optimizations were performed with the revPBE functional, a TZP basis set, and a frozen core.
O H
O
X
O H
:B
HNO3
NO2
SLOW
X
O
H CH2NO2
X
X
-
O C N O
X O
O
O
O
X
-H2O
O C N OH H
C N O O N O N
X
[3+2] Cycloaddition
X
Scheme 1. Mechanism for furoxan formation from acetophenones.
Figure 1. Keto-enol tautomerization of 4-hydroxy-1phenylethenol at the 4 position.
X
Figure 3. Hirshfeld charge analysis results and 1-phenylethenol template. C8 corresponds to where the functional group substituent would attach.
Results The HOMOs show that there is immense electronic character in the alkene group, represented primarily as a pi-bonding feature (Figure 2). This HOMO character indicates that the alkene off of the enol acts as a Lewis base susceptible to electrophilic attack (nitration). This was validated by the Hirshfeld charge analysis (Figure 3). We saw that activating substituents (OH, CH3O) strengthen the Lewis base character, while deactivating groups decrease the negative charge at the C⍺. Furthermore, the charge analysis reinforces that the different groups direct electrophilic ring attack at particular positions.
Conclusion
Figure 2. HOMOs for the differently substituted acetophenone tautomerized products, 1-phenylethenols.
We can conclude that activating groups, those with available electrons, will contribute to the Lewis basicity of the C⍺ site of 1-phenylethenol. Conversely, deactivating groups, those that are electron deficient, will pull the electrons towards themselves and decrease the Lewis base character. In the future we hope to explore other computational methods for studying the reaction pathway towards ring nitration.