Front Cover: Photocatalytic Synthesis of Acetals and Ketals from Aldehydes and Silylenolethers without the Use of Acids (Chem. Eur. J. 13/2023)

Abstract

The photoredox catalytic cycle with an electron‐deficient perylene bisimide (PBI) runs as long as green light is provided. The substrate silylenolethers and aldehydes enter the cycle, are oxidized to their radical cations and converted to acetals and ketals, which leave the cycle as products. Acid catalysts are not needed. Ketal and acetal groups protect the carbonyl compounds like helmets. More information can be found in the Research Article by H.‐A. Wagenknecht and D. Steuernagel (DOI: 10.1002/chem.202203767).

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

“…The chromophore was further improved for its photocatalytic use by the 2,6‐di iso propylphenyl substituents at the imide nitrogens to enhance solubility in MeCN [13] and by the two cyano substituents at the core to make it even more electron‐poor. N , N ‐di‐(2,6‐di iso propyl)‐1,7‐dicyano‐perylen‐3,4,9,10‐tetracarboxylic acid imide ( PBI1 , Figure 2) has been successfully applied in the nucleophilic addition of alcohols to styrenes, turned out to give higher yields than the mesityl acridinium, [14] and the acid‐free synthesis of acetals from silylenol ethers and aldehydes [15] . PBI1 shows a reduction potential of E red ( PBI1 / PBI1 ⋅ − )=−0.15 V (vs. SCE); with E 00 =2.30 eV the reduction potential of PBI1 in the excited state can be estimated to be E red ( PBI1* / PBI1 ⋅ − )=2.15 V, showing that this chromophore is a strong photooxidant.…”

“…N,N-di-(2,6-diisopropyl)-1,7-dicyano-perylen-3,4,9,10-tetracarboxylic acid imide (PBI1, Figure 2) has been successfully applied in the nucleophilic addition of alcohols to styrenes, turned out to give higher yields than the mesityl acridinium, [14] and the acid-free synthesis of acetals from silylenol ethers and aldehydes. [15] PBI1 shows a reduction potential of E red (PBI1/ PBI1 *À ) = À 0.15 V (vs. SCE); with E 00 = 2.30 eV the reduction potential of PBI1 in the excited state can be estimated to be E red (PBI1*/PBI1 *À ) = 2.15 V, showing that this chromophore is a strong photooxidant. 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.…”

Tandem Photooxidation/Acetalization of Alcohols to Acetals/Ketals with Sulfur‐doped Carbon Nitride

“…Generally the high costs of such conversions and the inherent problem of separating expensive catalysts from products and waste generation prohibit their large‐scale acceptance. Therefore, only recently avoiding the use of acids, green solid photocatalytic systems have increasingly been employed to catalyse such transformations in tandem fashion [12,17,22–25] . Also, photocatalytic conversion of aldehyde to acetal with ethanol using TiO 2 and other photocatalysts has shown significant promise [23,25–27] .…”

“…Therefore, only recently avoiding the use of acids, green solid photocatalytic systems have increasingly been employed to catalyse such transformations in tandem fashion. [12,17,[22][23][24][25] Also, photocatalytic conversion of aldehyde to acetal with ethanol using TiO 2 and other photocatalysts has shown significant promise. [23,[25][26][27] Nikitas et al showed that thioxanthenone photocatalysts efficiently catalyzed the acetalization of aldehydes using a household lamp.…”