Cicloadição [2 + 2] estereosseletiva entre azalactonas de Erlenmeyer-Plöchl utilizando a catálise fotorredox mediada por luz visível

Abstract

The increasing environmental concern in recent years has demanded the search for alternative methodologies in chemical processes. In this context, sunlight was already known as a source of clean and renewable chemical energy. However, most organic molecules only absorb wavelengths in the UV region, not being able to absorb the energy of visible light, which is abundant in the solar spectrum. Inspired by natural processes such as photocisthesis, photoredox catalysis consists of using metallic complexes and organic dyes as an antenna in the reaction medium, capturing the energy of visible light and transforming it into chemical energy. The advent of photoredox catalysis allowed access to new patterns of reactivity under mild conditions, allowing the increase in molecular complexity and access to important building blocks such as cyclobutane rings, in a simple and effective manner. Previous works in the literature reported that direct irradiation of ErlenmeyerPlöchl azlactones using visible light resulted in the formation of 1,3-diaminotruxillic cyclobutane derivatives, as a mixture of isomers. However, selective methodologies for obtaining 1,2-diaminotruxinic derivatives, resulting from the head-to-head coupling of Erlenmeyer-Plöchl azlactones, have not been described yet. In this work, three highly regio- and diastereoselective methodologies are presented to obtain 1,2-diaminotruxinic derivatives through photoredox catalysis mediated by visible light. Using the photocatalyst Ru(bpy)3(PF6)2, it was possible to access 8 symmetrical cycloadducts, with the opening of both azlactone rings, with yields ranging from 22 to 56% and diastereoisomeric ratio >19:1. In contrast, the use of the organic dye Eosin Y as a photocatalyst resulted in the formation of 14 bicyclic cycloadducts, with the maintenance of the azlactone rings. Then, the versatility of this ring through one-pot opening reactions was explored. Using different aliphatic amines, three opening products were obtained, with yields ranging from 36 to 51% yield. Control experiments and theoretical calculations suggest that the mechanism proceeds via electron transfer. The photocatalyst [Ir(dtbbpy)(ppy)2]PF6 showed good cisergy with Ni(OTf)2 acting as Lewis acid, leading to the formation of 8 diacid derivatives, with yields ranging from 46 to 67% and diastereoisomeric ratio >19 :1. Based on the literature reports and control experiments, it was proposed that this mechanism takes place via energy transfer and that the presence of Lewis acid and the power used are decisive for the stereochemistry of the product.