The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

DDQ/H+ system readily oxidizes a variety of electron donors with oxidation potential as high as ∼1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H+ together with commonly utilized FeCl3 as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl3) or DDQ/H+ for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials ≤1....

Probing the Arenium-Ion (ProtonTransfer) versus the Cation-Radical (Electron Transfer) Mechanism of Scholl Reaction Using DDQ as Oxidant

Solvent-to-polymer chirality transfer was examined using conjugated polymer with intramolecular stack structure (IaSS). When achiral poly(diphenylacetylene)s (PDPAs) dissolved in limonene, the solvent chirality was successfully transferred to the side phenyl stack structure, leading to intramolecular axial chirality. The phenyl–phenyl IaSS was under thermodynamic control to readily undergo asymmetric changes in chiral limonene, leading to optical activity in the isotropic structure between the main chain and resonant side phenyl rings. The axial chirality was significantly affected by the chain length and substitution position of the side alkyl groups. The longer alkyl chains and bulkier alkyl group prevented direct intermolecular interactions between the side phenyl rings and the chiral limonene molecules. PDPA with sterically congested, highly stable, and regulated IaSS was not favorable for efficient solvent-to-polymer chirality transfer.

Solvent-to-Polymer Chirality Transfer in Intramolecular Stack Structure

Spin‐Center Shift (SCS) – A Versatile Concept in Biological and Synthetic Chemistry

1 2 3 a (R = Et, R' = H) 94 : 6 b (R = iPr, R' = Me) 0 : 100 c (R = Ph, R' = H) 100 : 0 LG = CH 3 CO 2 - , PhCH 2 CO 2 - , C 6 H 5 CO 2 - , 4-NCC 6 H 4 CO 2 - , CH 3 CONHCH 2 CO 2 - , BocAla, GABA, Glu In aqueous media, α-keto amides LGCH 2 COCON(R)CH(R')CH 3 (la, R = Et, R' = H; 1b, R = i Pr, R' = Me; 1c, R = Ph, R' = H) with various carboxylate leaving groups (LG) at the C-3 position undergo photocleavage and release of carboxylic acids with formation of diastereomeric 5-hydroxy-oxazolidin-4-ones 2a,c in the cases of la,c or 5-methyleneoxazolidin-4-ones 3b in the case of 1b. For 1a,b, Φ(photocleavage) = 0.24-0.38, whereas Φ(photocleavage) = ca. 0.05 for 1c. The proposed mechanism involves transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates 4a-c that eliminate carboxylate anions. The resultant imminium ions, H2C=C(OH)CON+(R)=C(R')CH3 5a-c, cyclize intramolecularly to 3b or undergo intermolecular addition of water followed by tautomerization and cyclization to give 2a,c. These inter- or intramolecular trapping reactions of 5 release protons that decrease the pH and cause bleaching of the 620 nm band of the pH indicator, bromocresol green. Determination of the bleaching kinetics by laser flash photolysis methods in the case of la gives time constants of 18-137 μs, depending on the leaving group ability of the carboxylate anion, whereas amides 1b show only a small leaving group effect. For la, the large leaving group effect is consistent with rate-limiting carboxylate elimination from 4a, whereas the proton release step would be largely rate determining for 1b. Photolyses of la (LG = CH 3 CO 2 - , PhCH 2 CO 2 - ) in neat CH 3 CN results in carboxylate elimination to form imminium ion 5a, followed by internal return to give aminals.

Photochemical Cleavage and Release of Carboxylic Acids from α-Keto Amides

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200463103

Erica J. Kopatz

Papers

Photochemical Cleavage and Release of Carboxylic Acids from α-Keto Amides

Redox‐Induced Transformation from an Extended to a π‐Stacked Conformer in Acyclic Bis(catecholacetal)s of Acetylacetone

Time-resolved pH jump study of photochemical cleavage and release of carboxylic acids from α-keto amides