A new method in radical chemistry: Generation of radicals by photo-induced electron transfer and fragmentation of the radical cation

The photochemical reaction between an excited state of carbonyl compounds and an alkene to give the oxetane deriving from a cycloaddition reaction (the Paterno–Buchi reaction) is presented. The mechanism of the reaction is discussed on the basis of kinetic, spectroscopic, and theoretical evidence, showing that the reaction can involve both singlet and triplet excited states of the carbonyl compounds, the high probable presence of an exciplex, and the possible presence of electron transfer processes, and when triplet excited carbonyl compounds are involved, the presence of a biradical intermediate is observed. The reactivity of electron-poor and electron-rich alkenes is discussed. Regio- and stereochemical behavior of the reaction are presented showing that, in some cases, very high regio- and stereoselectivities are obtained. The possible theories explaining this behavior are presented. In this field, the stereoselectivity observed, when chiral auxiliaries are used, when the reaction is performed in organized media, and when the presence of a hydroxyl directing effect can be supposed, is presented. The effect of light, temperature, solvent on the reaction is discussed. The possible application of this reaction in organic synthesis is discussed, presenting both previous synthetic schemes where the synthesis of an oxetane is a key step, the possible use of the oxetane in synthesis, and comparing the photochemical approach with the other approaches used to synthesize the oxetane ring.

The Paternò-Büchi reaction - a comprehensive review.

A strategy for oxidative Csp3–H/N–H cross-coupling is presented. This reaction successfully utilizes 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tert-butyl nitrite (TBN) as co-catalysts to construct the biomedical applicable biheteroarenes under aerobic conditions. Notably, this amination reaction is successful with a wide range of alkylheteroarenes and could be used as a functionalization tactic for pharmaceutical research and other areas. Furthermore, preliminary mechanistic studies indicate that the C–N bond formation proceeds through the nucleophilic attack of azole to the carbon cation.

DDQ-Catalyzed Direct C(sp3)–H Amination of Alkylheteroarenes: Synthesis of Biheteroarenes under Aerobic and Metal-Free Conditions

In the 80 years since the original report1 of a photochemical addition to form an oxetane, the Paterno—Buchi reaction2 has become increasingly familiar to chemists engaged in the synthesis of complex molecules. Existing reviews3–6 have summarized mechanistic considerations and defined the scope and limitations of the photocycloaddition reactions. In confluence with the widespread advances in synthetic methods and asymmetric synthesis, the Paterno–Buchi reaction has prospered and led to significant developments in the area of organic synthesis.

The Paterno–Büchi Reaction

Fast hydrogen atom transfers from various methylbenzenes (ArH) to photoactivated quinones Q* show primary kinetic isotope effects k(H)/k(D) of 2.4−5.6. The quantitative effects of added inert salt on the kinetics and on the yields of the intermediate cation radical ArH+• demonstrate that hydrogen transfer proceeds via a two-step sequence involving an initial electron transfer to form the ion-radical pair [ArH+•, Q-•] which subsequently undergoes proton transfer according to the electron-transfer mechanism in Scheme 1.

Kinetic Isotope Effects for Electron-Transfer Pathways in the Oxidative C−H Activation of Hydrocarbons

The photochemistry of chloranil (Q) in the presence of the arenes hexamethylbenzene, 1-methyl-, 1,3-dimethyl-, 1,4-dimethyl-, and 2,3-dimethylnaphthalene, and acenaphthene has been investigated. Irradiation in a nonpolar solvent provided ether adducts resulting from formal addition of the quinone to the arene side chain, the hydroquinone, (QH/sub 2/), and in one case, an identifiable oxidation product, acenaphthylene. The quantum yield for these transformations was shown to have a complex dependence on the concentration of the arene and irradiation wavelength. On irradiation at 366 nm under conditions in which free chloranil absorbs, the preferred product was the reduced species, QH/sub 2/. On irradiation at 436 nm under conditions in which the ground-state charge-transfer (CT) complex of Q and the arene chiefly absorbs, the preferred product was the ether adduct. Excitation of complexes at 546 nm resulted in a sharply reduced quantum yield. Flash photolysis studies show that the quenching of Q triplets by the arenes leads to free radicals (QH/sup ./ and DH/sup ./) by a sequential electron-transfer, proton-transfer mechanism. Irradiation of QDH/sub 2/ complexes appears alternatively to lead to singlet radical ion pairs (Q/sup .-/, DH/sub 2//sup .+/) which evolve into singlet radical pairs (QH/sup ./, DH/sup ./) and final products inmore » competition with back reactions which return radical pairs to ground-state reactants.« less

Photoaddition and photoreduction of chloranil with arenes via singlet and triplet excited complexes. Effects of irradiation wavelength and radical-ion pair spin multiplicity

Divergent stereoselectivity in the photoaddition of alkanals and medium ring cycloalkenes

Photoaddition and Photoreduction of Chloranil with Arenes via Singlet and Triplet Excited Complexes: Effects of Irradiation Wavelength and Radical Ion Pair Spin Multiplicity.

Xuan T. Phan

Papers

Photoaddition and photoreduction of chloranil with arenes via singlet and triplet excited complexes. Effects of irradiation wavelength and radical-ion pair spin multiplicity

Divergent stereoselectivity in the photoaddition of alkanals and medium ring cycloalkenes

Photoaddition and Photoreduction of Chloranil with Arenes via Singlet and Triplet Excited Complexes: Effects of Irradiation Wavelength and Radical Ion Pair Spin Multiplicity.