Selective Peroxygenase-Catalyzed Oxidation of Toluene Derivatives to Benzaldehydes

Yutong Wang Postdoctoral Researcher

October 13, 2024

Introduction

The selective oxidation of toluene derivatives into benzaldehydes presents an ongoing challenge in organic chemistry. Traditionally, the oxidation of toluene derivatives has resulted in a mixture of products, including alcohols, aldehydes, and acids, making it difficult to achieve the desired product with high specificity. Industrial processes often operate at low conversion rates to maintain some level of selectivity, but this results in lower yields and necessitates extensive purification efforts.

Recent advances in biocatalysis, particularly with peroxygenase enzymes, offer a potential solution. Unspecific peroxygenases (UPOs) are capable of facilitating selective oxyfunctionalization reactions, but challenges remain, such as achieving regio- and chemoselectivity in these processes. In this blog, we explore a study that addresses these issues using both substrate engineering and reaction optimization techniques.

Biocatalytic Oxidation with Peroxygenases

Peroxygenases, like the well-known AaeUPO from Agrocybe aegerita, have gained attention for their ability to catalyze benzylic oxidations. However, in their native form, AaeUPO often produces a complex mixture of oxidation products when reacting with toluene, yielding both benzylic and ring-hydroxylation products.

To improve this selectivity, the researchers employed a two-fold strategy:

1. Substrate Engineering: By modifying the substitution pattern on the aromatic ring of toluene derivatives, the study hypothesized that selective binding to the enzyme’s active site could be achieved, promoting oxidation at specific positions.
2. Reaction Engineering: The study also moved from aqueous to non-aqueous reaction conditions, aiming to enhance both the concentration of the product and its chemoselectivity.

Key Findings

• Regioselectivity Enhancement: In silico docking studies confirmed that the position of substrate binding to the AaeUPO active site strongly influences the regioselectivity of the reaction. For example, p-chloro-toluene showed a highly selective transformation, yielding only p-chloro-benzaldehyde, while unmodified toluene led to a mixture of products.
• Reaction Optimization: The use of a non-aqueous solvent system improved the chemoselectivity of the reaction, with the aldehyde product being favored over alcohols and acids. Interestingly, the intermediate alcohols did not accumulate significantly, and no benzoic acid derivatives were detected under these conditions.
• Substrate Scope: The study tested a range of toluene derivatives with various substituents on the aromatic ring. All tested compounds, except p-ethynyl-toluene, were converted into their corresponding benzaldehydes with selectivity greater than 92%, and in most cases, over 96%.
• High Product Yields: Product concentrations of up to 185 mM were achieved, corresponding to over 18 g/L in some cases. This high yield, combined with the reusability of the immobilized enzyme, points to the potential for industrial applications of this method.

Conclusion

This study demonstrates the power of combining substrate engineering with reaction engineering to tackle the longstanding challenges of regio- and chemoselectivity in biocatalytic oxidation reactions. By moving away from aqueous media and optimizing the interaction between enzyme and substrate, the researchers achieved highly selective benzylic oxidations with promising product yields.

Future work could focus on further optimizing enzyme immobilization and scaling up the process to make it suitable for large-scale industrial applications. Additionally, refining the product isolation methods could further enhance the efficiency of this biocatalytic approach.