Photocatalysis Smells … Coffee and Caramel

This behind-the-article post highlights rational approach of designing a photocatalytic reaction between toluene and elemental sulfur that gives artificial flavourant - dibenzyldisulfide.

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Dec 15, 2019
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The project that eventually transformed into the edge article in Chemical Science started after I realized that synthesis of organosulfur compounds is tricky. Typically, it requires sulfur-transfer agents such as Lawesson reagent. Sulfur content in Lawesson reagent is 32%. In other words, only up to 32% of the reagent is actually used, while the remaining organic part is converted to by-products.

In additions, sulfur-transfer agents decompose slowly over the time, therefore one can notice ‘strange smell’ from the cupboard where these reagents are stored.

On the other hand, octa sulfur is built on 100% from sulfur atoms. It is crystalline yellow solid without any smell. And it is cheap. Definitely elemental sulfur is much more convenient reagent than sulfur-transfer agents. Therefore, I decided to explore S8 in C-H functionalization of toluene.

However, there is a big problem. Elemental sulfur does not react with toluene under standard conditions. In order to activate the intrinsically reactive benzylic position in toluene for interaction with S8 I invoked to carbon nitride photocatalysis. In particular I took potassium poly(heptazine imide) photocatalyst because of two main reasons:

  • It is free of transition metals therefore catalyst poisoning by sulfur or organosulfur compounds can be excluded.
  • Valence band potential in K-PHI (2.22 V vs NHE) is suitable for toluene oxidation (Eox = 2.02 V).

Having chosen the catalyst, I optimized reaction conditions using toluene as a model compound, extended the scope of substrates to other alkylarenes and studied the mechanism.

As for dibenzyldisulfide, pure compound indeed has ‘smoky’ flavour. It was approved for use as synthetic flavouring substance of coffee and caramel, when used in ppm amount.[1],[2]

The article in focus of this blog post was published at the beginning of 2018 and summarized one of the very first reactions developed in my group at the Max Planck Institute of Colloids and Interfaces. If you think about ‘classic organic photocatalysis’ that normally is associated with molecular photoredox complexes and complex organic substrates, combination of heterogeneous carbon nitride photocatalyst, sulfur and toluene was unusual. Therefore, at the end of this post I would like to thank Prof. Kazunari Domen, the editor of Chemical Science, for seeing relevance of the work to the scope of the journal and his support during the submission and revision of the manuscript.

If you would like to be kept up-to-date of the latest research in carbon nitride organic photocatalysis, follow us on Twitter Aleksandr Savateev, InnovateMPIKG.


References

[1]  Section 172.515 'Synthetic flavoring substances and adjuvants’ in 'Code of Federal Regulations Title 21'. Revised as of April 1, 2019. Can be found via the link

 [2] Page 83 in 'Chemicals Used in Food Processing', 1965. Can be found via the link.

 


Go to the profile of Aleksandr Savateev

Aleksandr Savateev

Group leader, Max Planck Institute of Colloids and Interfaces

Photocatalysis, organic chemistry, material chemistry

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