Photoredox reaction intermolecular

Photoredox reactions occur either via imermolecular or via intramolecular processes. The most general scheme of an intermolecular photoredox reaction (Rehm and Weller, 1970) is shown in reaction 1 A compound C undergoes a transition from the ground state to an electronically excited state by absorption of light, C - C. If such an excited compound encounters a reaction partner Q (either an electron donor or an electron acceptor), electron transfer may take place, leading to C+ and Q ... 

Ruthenium (II) polypyridyl complexes promote the photoredox reactions such as Diels-Alder reaction (see formulae 29 31) and azomethine ylide formation followed by [3 + 2] cycloaddition (32-34) by use of visible light. Yoon found 2,2 -bipyrazine ligand is better than 2,2 -bipyridine. Bach reported enantioselective intramolecular [2 + 2] and intermolecular [3 + 2] photocycloaddition by use of chiral hydrogen bond templates (37) and (41). ... 

The great majority of photoredox reactions rely on reductive electron-transfer process to an alkyl halide (usually a bromide) generating an electron-deficient alkyl radical. The so-formed electrophilic radicals can be trapped by electron-rich olefins by intra- or intermolecular reaction [25]. 

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... 

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. 

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