The Concept of Photozymes: Short Peptides with Photoredox Catalytic Activity for Nucleophilic Additions to α‐Phenyl Styrenes

Abstract

In memory of Klaus HafnerConventional photoredox catalytic additions of alcohols to olefins require additives, like thiophenol, to promote back electron transfer. The concept of "photozymes" assumes that forward and backward electron transfer steps in a photoredox catalytic cycle are controllable by substrate binding to photocatalytically active peptides. Accordingly, we synthesized a short tripeptide modified with 1,7-dicyano-perylene-3,4 : 9,10-tetracarboxylic acid bisimide as photoredox catalyst. This peptide undergoes an unconventional photoredox catalytic cycle with the radical anion and dianion of the perylene bisimide-peptide as intermediates. The photoredox catalytic reactions with αphenyl styrenes as substrates require remarkably low catalyst loadings (0.5 mol%) and give the methoxylation products in high yields. The concept of "photozymes" for photoredox catalysis has significant potential for other photocatalytic reactions, in particular with respect to enantioselective photocatalysis.

Photocatalytic Synthesis of Acetals and Ketals from Aldehydes and Silylenolethers without the Use of Acids

“…[35] PBI shows a reduction potential of E red (PBI/ PBI *À ) = À 0.15 V (vs. SCE). [35] With E 00 = 2.3 eV (from the crossing point of the normalized absorbance and the normalized emission, λ = 535 nm) the excited state reduction potential of PBI can be estimated, E red (PBI*/PBI *À ) = 2.15 V, which shows that this chromophore is a potent photo-oxidant. The cyclovoltammetric measurements of the silylenolether as substrates S revealed oxidation potentials of E ox (S * + /S) = 2.0-2.1 V (Figures S107 and S108).…”

“…We synthetically modified the perylene bisimide chromophore by two cyano substituents at the core to make it even more electron‐deficient. This bis‐cyanolated PBI (Figure 2) was successfully applied in the nucleophilic addition of alcohols to styrene derivatives, [34] and PBI ‐modified peptides as “photozymes” [35] . PBI shows a reduction potential of E red ( PBI/PBI ⋅ − )=−0.15 V (vs. SCE) [35] .…”

“…This bis‐cyanolated PBI (Figure 2) was successfully applied in the nucleophilic addition of alcohols to styrene derivatives, [34] and PBI ‐modified peptides as “photozymes” [35] . PBI shows a reduction potential of E red ( PBI/PBI ⋅ − )=−0.15 V (vs. SCE) [35] . With E 00 =2.3 eV (from the crossing point of the normalized absorbance and the normalized emission, λ =535 nm) the excited state reduction potential of PBI can be estimated, E red ( PBI*/PBI ⋅ − )=2.15 V, which shows that this chromophore is a potent photo‐oxidant.…”

“…It is important to mention here, that despite this H-atom transfer PhSH or any other additive typically used for Hatom transfer in photoredox catalysis is not needed. This is very likely due to the fact that the radical anion of the photocatalyst PBI *À is extremely long lasting in the range of minutes to hours (under exclusion of oxygen) [34,35] which is a significant advantage of this organo-photocatalyst. The formation of the ester as side product can be controlled by several experimental parameters, as already discussed above.…”

Photoredox Catalytic Access to N,O‐Acetals from Enamides by Means of Electron‐Poor Perylene Bisimides

“…Inefficient second electron transfer in photocatalytic cycles due to the early dissociation of the photocatalyst and the (pre)product radical ions is a known problem leading to low yields and to undesired byproducts, which can be solved by peptides with substrate binding sites. [16] In most cases, this problem is solved by additives, in particular thiophenol as an H-atom transfer reagent (an H-atom transfer comprises electron and proton transfer). [17] The second perylene bisimide, N,N-di-(2,6-diisopropyl)-1,6,7,12-tetrabromo-2,5,8,11tetracyano-perylen-3,4,9,10-tetracarboxylic acid imide (PBI2), follows the idea that the strong electron-deficiency of the perylene core should increase the electrostatic interaction with the intermediate substrate/product radicals and radical ions and thus make additives dispensable.…”