TiO2 in Organic Photosynthesis: Sunlight Induced Functionalization of Heterocyclic Bases

Abstract

In the last decades photocatalyzed organic processes have attracted the great interest, due to their low environmental impact. Various oxidation and reduction processes of organic substrates have been developed as alternative routes to the "classical" synthesis of more complex molecules. In this attempt TiO2 is actually largely studied and employed, especially as catalyst in the organic pollutants photodegradation to products with less environmental impact. From the synthetic point of view, the attention has been focused particularly on the TiO2-mediated synthesis of alcohols, carbonyl-and carboxylic-derivatives. Recently our group has developed new TiO2-photocatalyzed synthetic protocols for the functionalization of N-heteroaromatic bases as an alternative to the "classic" well known redox metal-catalyzed methods. These substrates are suitable for the nucleophilic free-radical addition and the photocatalytic activity of microcrystalline TiO2 has shown a key role in the success of our protocol.

TiO2 Photocatalyzed C–H Bond Transformation for C–C Coupling Reactions

“…In another example of activating two C-H bonds, N-heterocyclic arene C-H bond was also cleaved by sequential SET and radical attack event via TiO2 photocatalysis (see Figure 14). Sunlight induced the functionalization of some heterocyclic bases in the presence of polycrystalline TiO2 [43,117]. In this work, polycrystalline TiO2 mediated two C-H bond cleavage events to facilitate C-C formation between sp 2 C-H bond in heterocyclic bases such as quinoline, quinaldine, lepidine, and quinoxaline and sp 3 C-H bond in formamide, dimethylformamide, and dimethylacetamide.…”

“…These reports successfully fulfilled the organic transformations with high yield, excellent selectivity, mild conditions, and wide substrate scope. Corresponding to this new field, a number of excellent reviews have focused on TiO2 photocatalysis in organic synthesis [39,40,42,43,47,50,52,56,57,63,[86][87][88]. However, the incredible potential of C-H bond functionalization by TiO2 photocatalysis for C-C coupling reactions has not been fully tapped yet.…”

Semiconductors in Organic Photosynthesis

“…Recently, some reviews on this field have been published (Gambarotti, 2010;Pastori, 2009;Shiraishi, 2008;Anpo & Kamat, 2010). Herein we report some examples taken from the literature which can be used to take a view on the recent state of the art.…”

“…In fact only catalytic amount of catalysts or initiators, depending on the reaction types, are needed to promote the reactions, whereas the classic Friedel-Crafts protocol needs stoichiometric quantity of Lewis acids. TiO 2 photocatalysis offers an alternative to the free-radical functionalization of heteroaromatic bases in the presence of classic metalperoxide system (Gambarotti et al, 2010;Augugliaro et al, 2010). Caronna and co-workers reported the carboxyamidation of bases in the presence of H 2 O 2 or air under sunlight (Eq.…”

Recent Progress in Semiconductor Photocatalysis for Organic Fine Chemical Synthesis

“…In early 1980s, great effort was placed on organic synthesis by semicon-ductor photocatalysis [10]. Photocatalysis in synthetic route has attracted many researchers, because this method presents a greener approach to organic synthesis [11,12]. Recent studies have revealed that highly selective redox reactions could be achieved by visible light irradiation.…”