A Highly Efficient Chromium Functionalized, Imidazolium Di-Cationic Ionic Liquid Catalyst System for the Oxidation of Aromatic Primary Alcohols
Authors: Barot Yash , Anand Vivek , Mishra Satyendra , Mishra Roli 
Published: January 2025

Background:

The oxidation of aromatic primary alcohols is pivotal in organic synthesis,
converting accessible starting materials into valuable intermediates. Traditional methods often rely
on chromium-based reagents, which are hazardous and environmentally problematic. Ionic liquids,
particularly those based on imidazolium cations, offer an attractive alternative due to their unique
solvent properties and chemical stability. However, their application in oxidation reactions has been
limited by challenges such as selectivity and efficiency. Recent advancements have focused on
integrating chromium complexes into imidazolium ionic liquids to harness their catalytic potential.
Understanding the catalytic efficiency and mechanistic insights of chromium-functionalized imidazolium
di-cationic ionic liquids in alcohol oxidation is crucial for developing sustainable and efficient
synthetic methodologies aiming to mitigate environmental impact and improve synthetic efficiency
in organic chemistry.

Objectives:

The aim of this study was to synthesize, characterize, and explore the catalytic efficiency
and mechanism of chromium-functionalized imidazolium di-cationic ionic liquids in the oxidation
of aromatic primary alcohols.

Methods:

The oxidation of benzyl alcohol was optimized by varying solvent and temperature parameters.
Initially, benzyl alcohol was subjected to oxidation in different solvents: water, DMF,
ACN, chloroform, 1,2-dichloroethane, and DMSO at room temperature. Solvent effects were evaluated,
with DMF, ACN, and DMSO yielding approximately 80% conversion to the desired aldehyde.
Interestingly, DCE did not yield the desired aldehyde. CHCl3 emerged as the optimal solvent,
achieving a high yield of 94% in minimal reaction time. Temperature optimization revealed that at
room temperature, the reaction required 40 minutes to reach 94% yield. Increasing the temperature
to 60 °C reduced the reaction time to 10 minutes while maintaining a high yield of 98%. Thus, 60
°C was identified as the optimal temperature for maximizing both yield and reaction speed. The
methodological adjustments of solvent and temperature parameters provided crucial insights for
optimizing the oxidation of benzyl alcohol using chromium-functionalized imidazolium di-cationic
ionic liquid.

Results:

Reactions at room temperature required longer times and yielded lower product amounts
compared to reactions conducted at higher temperatures. Importantly, no over-oxidation to carboxylic
acids was observed. Electron-donating groups on aromatic alcohol substrates led to higher
yields of aldehydes in shorter times. Conversely, substrates with electron-withdrawing groups
showed reduced yields (84% to 92%) over extended periods. Primary aliphatic alcohols exhibited
lower yields even with prolonged reaction times, while secondary alcohols yielded fewer oxidation
products. Recycling [DIL]2+[Cr2O7]2- for four cycles showed decreased yields over successive
uses, highlighting its potential for continuous catalytic use in alcohol oxidation.

Conclusion:

In this study, imidazolium-based Di-cationic ionic liquid [DIL]2+[Cr2O7]2- was synthesized,
and its ionic liquid properties were demonstrated using TGA and DSC. Our developed
catalyst efficiently converts primary aromatic alcohols to aldehydes using [DIL]2+[Cr2O7]2- or
[DIL]2+[Cr2O7]2-/H5IO6, offering solvent-free rapid oxidation, catalyst recyclability for up to
four cycles, and facile catalyst recovery. In comparison to other available oxidants, the developed
protocol has a superior yield, ease of workup, ease of handling, and low hygroscopicity.