Visible‐Light‐Mediated Photocatalytic Synthesis of 2‐Substituted Oxazole‐5‐carbaldehydes Promoted by Benzeneseleninic Acid

Abstract

Benzeneseleninic acid was used under visible light irradiation (white LEDs) to promote the oxidative cyclization of N-propargyl amides to the respective 2-substituted oxazole-5-carbaldehydes. A total of twelve 2-aryl oxazole-5-carbaldehydes, six of them so far unprecedented, were prepared in 30-90 % yield in just 1 hour of reaction at room temperature. No metal catalysts, nor strong oxidants or heating were necessary, making this a mild method to access a valuable class of compounds. The only coproducts are water and diphenyl diselenide, that can be recovered and converted to new benzeneseleninic acid for a new reaction.

Arylseleninic acid derivative decomposition towards Se(ii)-based electrophiles: an elegant approach to construct 3-selanyl-imidazopyridines

“…Thus, based on the results observed above, as well as on our previous results, 15,16 we have proposed a plausible reaction mechanism describing the full conversion of the substrates 1 and 2 toward the product 3 , which can be systematically divided into three main steps, among which the first two play an essential role, delivering the Se-based electrophilic reactive species in the reaction medium. Thus, the initial event is a slight thermal decomposition of BSA 2 , that could be promoted by DMF, 18 releasing (PhSe) 2 4 , O 2 , CO, and Me 2 NH.…”

“…Thus, based on the results observed above, as well as on our previous results, 15,16 we have proposed a plausible reaction a In a round-bottomed flask were added the substrates 1a (0.25 mol) and 2a, and 2 mL of solvent. Then, the mixture was heated and stirred at the tabled temperature and time, under open air conditions.…”

“…The aryl seleninic acids were easily prepared from the respective diphenyl diselenides and hydrogen peroxide, as previously described. 15–17 An interesting substrate tolerance was observed, and the respective products 3a–h were prepared in good to excellent yields, among which some cases can be highlighted. For instance, naphthalene-1-seleninic acid 2b reacted smoothly with 1a to afford 3b in 84% yield.…”

“…Thus, the initial event is a slight thermal decomposition of BSA 2 , that could be promoted by DMF, 18 releasing (PhSe) 2 4 , O 2 , CO, and Me 2 NH. Then, under thermal conditions, PhSeO 2 H 2a and (PhSe) 2 4 trigger a comproportionation process, 15,16,19 being converted to the Se-based species II and III , which are able to transfer the organoselenium group to a nucleophilic reaction partner. Eventually, the species II and III can be protonated, affording the strong electrophilic species IV , which in the presence of imidazopyridine 1a , triggers a S E Ar mechanism, by the formation of the σ-complex intermediate V , that is finally deprotonated to reach the aromatic product 3a (Scheme 3, plausible reaction mechanism).…”

“…The formation of a vinyl selenide in situ , promoted by BSA, was identified as the key step of the reaction (Scheme 1). 16 Thus, as a continuation of our efforts in the development of environmentally friendly synthetic protocols to prepare valuable structures, we disclose here an efficient approach to prepare 3-selanylimidazopyridines using BSA as an electrophilic selenium source, under thermal conditions. This new approach avoids the use of additional oxidant, catalyst, or auxiliary reagents.…”