Addition of the Carbonyl Oxygen to the ipso- or ortho-Carbon Atoms of the β-Phenyl Ring Followed by Intersystem Crossing and Rapid Relaxation to the Ground-State Ketones: A Mechanism for β-Phenyl Quenching of the First Triplet Excited States of Derivatives of β-Phenylpropiophenone

Taming Radical Pairs in Nanocrystalline Ketones: Photochemical Synthesis of Compounds with Vicinal Stereogenic All-Carbon Quaternary Centers

Here we describe the use of crystalline ketones to control the fate of the radical pair intermediates generated in the Norrish type I photodecarbonylation reaction to render it a powerful tool in the challenging synthesis of sterically congested carbon-carbon bonds. This methodology makes the synthetically more accessible hexasubstituted ketones ideal synthons for the construction of adjacent, all-carbon substituted, stereogenic quaternary stereocenters. We describe here the structural and thermochemical parameters required of the starting ketone in order to react in the solid state. Finally, the scope and scalability of the reaction and its application in the total synthesis of two natural products is described.

β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications

β-Phenyl quenching of 9-phenylphenalenones leads to the reversible formation of naphthoxanthenes and eventually to stable naphthoxanthenyl radicals or naphthoxanthenium cations.

Quenching of triplet benzophenone by benzene and diphenyl ether: a DFT study

The reaction of triplet benzophenone with benzene and diphenyl ether has been studied by density functional theory. Quenching of the triplet ketone is predicted to occur by addition of the carbonyl oxygen to the arene chromophores. The reaction is accompanied by a significant degree of charge transfer. In case of the reaction of triplet benzophenone with diphenyl ether (DPE), addition is predicted to occur preferentially at the ortho position of the DPE molecule. Addition to the ipso-position of DPE, which provides a pathway for formation of the phenoxy radical, is predicted to occur as a minor reaction pathway.

Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Photolysis of p- and m-azidomethylacetophenone (1a, 1b) in argon-saturated solutions yields predominantly imine 2a, 2b, whereas irradiation of 1a, 1b in oxygen-saturated solutions results in heterocycles 3a, 3b, aldehydes 4a, 4b and nitriles 5a, 5b. Density functional theory calculations place the energy of the first and second excited state of the triplet ketones (T1K and T2K) in 1a, 1b in close proximity to each other. The triplet transition state for cleaving the C–N bond in 1a, 1b to form azido and benzyl radicals 1aB, 1bB is located only 3 kcal mol–1 (1 kcal = 4.184 kJ) above T1K, indicating that azido cleavage is feasible. The calculations place the energy of the triplet azido group (TA) in 1a, 1b ∼25 kcal mol–1 below T1K; thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for TA to expel N2 and form triplet nitrenes is less than 1 kcal mol–1 above TA in 1a, 1b. Laser flash photolysis of 1a, 1b reveals the formation of the triplet excited ketones of 1a, 1b, which decay to form benzyl radicals 1aB, 1bB and triplet alkylnitrenes. The triplet ketones and the benzyl radicals are quenched with molecular oxygen at rates close to diffusion, whereas the triplet nitrenes react more slowly with oxygen (∼5 × 105 M–1 s–1). We conclude that the triplet alkylnitrenes intercept the benzyl radicals to form 2 in argon-saturated solution, whereas the benzyl radicals are trapped to form 4 in oxygen-saturated solution; thus, the triplet nitrenes react with oxygen to form 3.