Organo-photoredox catalysis

Asymmetric a-Alkylation of Aldehydes and Organo-photoredox Catalysis... 

One year later, an extension of organo-photoredox catalysis into enantioselective a-trifluoromethylation and a-perfluoroalkylation of aldehydes was reported by MacMillan and co-workers [150, 151] that was accomplished by using a readily available iridium photocatalyst [Ir(ppy)2(dtb-bpy)]" and a commercial imidazolidinone catalyst (Scheme 8.38). A broad range of perfluoroalkyl iodides and bromides as well as a variety of aldehydes bearing various functional or bulky groups afforded corresponding a-fluoroalkylated products in high yields with excellent diastereo-and enantioselectivity (up to 99% ee). 

Although the results of the last two approaches below previously reported results, this new extension of organo-photoredox catalysis to classic organic dyes would find broadly utility across many applications. 

Zeitler et al. have developed a metal-free version of such a cooperative organo-photoredox catalysis. They used eosin as a powerful photocatalyst in the asymmetric a-alkylahon of enolizable aldehydes. Their results are comparable to those obtained in the transition metal series [40]. 

As stated in the previous section, an extensive effort has been devoted to the development of intermolecular catalytic enantioselective a-alkylation reactions of aldehydes. In 2008, Nicewicz and MacMillan [147] accomplished the direct asymmetric a-alkylation of aldehydes with a-bromo carbonyl compounds (Scheme 8.36) via merging two types of catalysis, photoredox [148] and organo-catalysis. A variety of aliphatic aldehydes and electron-deficient a-bromo carbonyl compounds can be used, affording a-alkylated aldehydes in excellent yields (up to 93%) with outstanding enantioselectivities (up to 99% ee) in a relatively short reaction time. Furthermore, this alkylation protocol represents a technically simple... 

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