Continuous flow synthesis of sulfur heterocycles via cycloaddition reactions of photochemically generated transient thioaldehydes

• Applicant: Professor Dr. Christoph Schneider
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation ; Analytical Chemistry
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 251124697

Thioaldehydes are extremly labile compounds since they tend to undergo rapid oligomerization reactions and have to be prepared in situ in the presence of the respective reaction partner. As a consequence they have been much underinvestigated and underutilized in organic synthesis. A particularly straightforward and very mild method of preparation is the Norrish II-cleavage of phenacycl sulfides. However, it suffers from a number of drawbacks such as long irradiation times, heating of the reaction mixture, decomposition of products and losses in yield.To overcome these shortcomings we intend to develop a robust and broadly applicable continuous flow process for thioaldehyde production and fully exploit the synthetic potential of thioaldehydes. Preliminary studies have revealed that irradiation in the thin tubes of a flow reactor produce high yields of thioaldehydes after short residence times that can be successfully intercepted by an electronich diene in [4+2]-cycloaddition reactions. It is our objective to further optimize this continuous flow process and extend it to a broad range of aryl-, heteroaryl-, and alkyl-substituted thioaldehydes to furnish thiopyranes. It is further intended to also study ene reactions and [2+2]-cycloaddition reactions of thioaldehydes in continuous flow. Furthermore, thioaldehyde formation will be followed online by spectroscopic means and DFT-calculations are to be conducted to elucidate the mechanism of the thia Diels-Alder reaction. Finally, two asymmetric strategies will be pursued toward an enantioselective thiopyran synthesis.

• DFG Programme: Research Units
• Subproject of: FOR 2177: Integrated Chemical Micro Laboratories