Photoredox organocatalysis

Photoredox Organocatalysis. Photoredox catalysis is emerging as an extremely powerful tool in organic synthesis [103]. Photoredox catalysis relies on the ability of some compounds, such as ruthenium(II) tris(bipyridyl) complex (Ru(bipy)3 ), to absorb visible light affording a relatively kinetically stable excited species (usually in the triplet state) with enhanced redox properties (Scheme 2.22). 

Nagib DA, Scott ME, MacMillan DWC (2009) Enantioselective a-trifluoromethylation of aldehydes via photoredox organocatalysis. J Am Chem Soc 131 10875-10877... 

Scheme 15.107 Enantioselective trifluoromethylation of aldehydes mediated by coupled photoredox/organocatalysis.

Nicewicz and MacMillan merged later photoredox catalysis and asymmetric organocatalysis to an efficient approach to the otherwise difficult asymmetric a-alkylation of aldehydes 118 by activated alkyl bromides 117 (Fig. 30) [183]. The concept of face differentiation at the a-position of aldehydes via chiral enamines 121 provides the basis for the method. This allows the formation of functionalized... 

Fig. 19. Different ways to introduce oxyl radical reactivity nature employs metal bound tyrosyl radicals (19) or high-valent metal oxo fragments in many active sites (65,153). Nitroxyl radicals such as 2,2,6,6,-tetramethylpiperidin-l-oxyl (TEMPO, 20) are reactive species used in organocatalysis (154). The excited states of carbonyl functional groups (21) and metal oxo-fragments (22) display a radical pair character, which may become very attractive for biomimetic photoredox processes upon spectral sensitization (3,5).

The merging of photoredox catalysis with asymmetric organocatalysis was reported by MacMillan and Nicewicz in 2008 [104]. Initial studies in the MacMillan group were devoted to the imidazolidinone-catalyzed asymmetric a-functionaliza-tion of aldehydes with a-bromo ketones and esters [104a] and were subsequently expanded to enantioselective perfluoroalkylations [105] and benzylations [106]. The... 

Hong BC, Lin CW, Liao WK, Lee GH (2013) Sequential asymmetric catalysis in Michael-Michael-Michael aldol reactions merging organocatalysis with photoredox catalysis in a one-pot enantioselective synthesis of highly functionalized decalines bearing a quaternary carbon stereocenter. Org Lett 15 6258-6261... 

Nicewicz and MacMillan reported an elusive asymmetric intermolecular a-alkylation of aldehydes by elegantly merging Ru photoredox catalysis with chiral amine organocatalysis [30]. The reaction mechanism proposed for the synergistic catalysis is illustrated in Scheme 2. 

Nicewicz DA, MacMillan DWC (2008) Merging photoredox catalysis with organocatalysis the direct asymmetric alkylation of aldehydes. Science 322 77-80... 

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