Efficient Visible Light Photocatalysis of [2+2] Enone Cycloadditions

Abstract

We report that Ru(bipy)3Cl2 can serve as a visible light photocatalyst for [2+2] enone cycloadditions. A variety of aryl enones participate readily in the reaction, and the diastereoselectivity in the formation of the cyclobutane products is excellent. We propose a mechanism in which a photogenerated Ru(bipy)3+ complex promotes one-electron reduction of the enone substrate, which undergoes subsequent radical anion cycloaddition. The efficiency of this process is extremely high, which allows rapid, high-yielding [2+2] cyclizations to be conducted using incident sunlight as the only source of irradiation.

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…These experiments revealed an increase in k obs associated with an increase in [ Building upon these early observations that Brønsted acids can play a significant role in catalyzing electron transfers, Yoon and coworkers reported an acid-catalyzed reductive cyclization of enones in the context of visible light photoredox catalysis ( Figure 46) [253]. The Yoon lab had previously demonstrated the ability of Lewis acid salts to activate similar systems [254][255], however different products were formed based on the method of enone activation. Specifically, it was proposed that Lewis acid activation affords a radical anion intermediate, whereas Brønsted acid activation results in a neutral radical.…”

Eosin Y Catalyzed Visible-Light-Promoted Aerobic Oxidative Cyclization of 2-Aminobenzothiazole

“…In this sequence visible light photoredox catalysis has recently received much attention in organic synthesis owing to readily availability, sustainability, non-toxicity and ease of handling of visible light. [10][11][12][13][14][15] In their revolutionary work in this area, MacMillan, [16] Yoon [17] and Stephenson [18][19] have used Ruthenium and Iridium complexes as the photoredox catalyst, which has inspired the development of several powerful methods for various chemical transformations useful in organic synthesis.…”

Semiconductors in Organic Photosynthesis

“…Irradiation of aryl enones, in the presence of a mixture of Ru(bpy) 3 Cl 2 , i-Pr 2 NEt and LiBF 4 in acetonitrile, allowed to develop a highly diasteroselective intramolecular [2 + 2] cycloaddition reaction, which, in all the examples reported, led to the formation of the corresponding cis-cyclobutanes (Eq. 17) (Ischay & al., 2008). In the reaction mechanism, iPr 2 NEt has the role to reduces the excited species Ru(bpy) 3 2+ * to Ru(bpy) 3 + , which seems to be the real initiator of the process, while LiBF 4 , being a Lewis acid, favours the solubility of Ru complex in acetonitrile.…”