Transition-Metal-Controlled α-C–H Acidity in Keto–Enol Tautomers: A DFT Study of Metal, Tautomer, and Substituent Effects
- 1Department of Chemistry and Material Science, School of chemical Engineering
- 2Aalto University
Abstract
Metal coordination provides an effective strategy for modulating the acidity of otherwise weak C–H bonds and controlling proton-transfer reactivity. Herein, we use density functional theory to investigate the effects of metal identity, tautomeric state, and substituent electronics on α-hydrogen acidity in metal-coordinated keto–enol systems. A systematic set of approximately 128 Au(III)-coordinated structures containing electron-donating and electron-withdrawing substituents was examined, with multiple coordination geometries and tautomeric forms considered to identify the thermodynamically preferred states. Au(III) coordination substantially enhances α-C–H acidity, with many systems exhibiting highly favorable deprotonation and strongly acidic computed pKa values. Electron-withdrawing substituents generally increase acidity, whereas electron-donating groups reduce it, although deviations observed for OMe and CF3 highlight the additional roles of charge delocalization and electrostatic stabilization. Kinetic analysis further reveals a strong dependence on tautomeric state: deprotonation of Au(III)-coordinated enol structures proceeds through barrierless pathways, whereas the corresponding keto structures require activation barriers of approximately 37–46 kcal mol−1. Comparative calculations with Co(III)-and Rh(III)-porphyrin complexes show substantially weaker acidification than Au(III), with both metals preferentially stabilizing the keto tautomer. These results demonstrate that metal identity, tautomeric structure, and substituent effects collectively govern α-C–H acidity and proton-transfer reactivity, highlighting the distinctive ability of Au(III) to promote α-hydrogen acidification in keto–enol systems.
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BibTeX
@misc{mahdian2026transitionmetalcontrolled,
title = {{Transition-Metal-Controlled α-C–H Acidity in Keto–Enol Tautomers: A DFT Study of Metal, Tautomer, and Substituent Effects}},
author = {Amir Mahdian and Kari Laasonen},
howpublished = {ChemRxiv},
year = {2026},
month = oct,
doi = {10.26434/chemrxiv.15010004/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010004/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Transition-Metal-Controlled α-C–H Acidity in Keto–Enol Tautomers: A DFT Study of Metal, Tautomer, and Substituent Effects
AU - Amir Mahdian
AU - Kari Laasonen
PY - 2026
DA - 2026/10/06
PB - ChemRxiv
DO - 10.26434/chemrxiv.15010004/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010004/v1
AB - Metal coordination provides an effective strategy for modulating the acidity of otherwise weak C–H bonds and controlling proton-transfer reactivity. Herein, we use density functional theory to investigate the effects of metal identity, tautomeric state, and substituent electronics on α-hydrogen acidity in metal-coordinated keto–enol systems. A systematic set of approximately 128 Au(III)-coordinated structures containing electron-donating and electron-withdrawing substituents was examined, with multiple coordination geometries and tautomeric forms considered to identify the thermodynamically preferred states. Au(III) coordination substantially enhances α-C–H acidity, with many systems exhibiting highly favorable deprotonation and strongly acidic computed pKa values. Electron-withdrawing substituents generally increase acidity, whereas electron-donating groups reduce it, although deviations observed for OMe and CF3 highlight the additional roles of charge delocalization and electrostatic stabilization. Kinetic analysis further reveals a strong dependence on tautomeric state: deprotonation of Au(III)-coordinated enol structures proceeds through barrierless pathways, whereas the corresponding keto structures require activation barriers of approximately 37–46 kcal mol−1. Comparative calculations with Co(III)-and Rh(III)-porphyrin complexes show substantially weaker acidification than Au(III), with both metals preferentially stabilizing the keto tautomer. These results demonstrate that metal identity, tautomeric structure, and substituent effects collectively govern α-C–H acidity and proton-transfer reactivity, highlighting the distinctive ability of Au(III) to promote α-hydrogen acidification in keto–enol systems.
ER -