Catalytic Oxidative Cyclization of 2′-Arylbenzaldehyde Oxime Ethers under Photoinduced Electron Transfer Conditions

Abstract

A series of 2′-arylbenzaldehyde oxime ethers were synthesized and shown to generate the corresponding phenanthridines upon irradiation in the presence of 9,10-dicyanoanthracene in acetonitrile. Mechanistic studies suggest that the oxidative cyclization reaction sequence is initiated by an electron transfer step followed by nucleophilic attack of the aryl ring onto the nitrogen of the oxime ether. A concave downward Hammett plot is presumably the result of a change in charge distribution in the radical cation species with strongly electron-donating substituents that yields a less electrophilic nitrogen atom and a decreased amount of cyclized product. The reaction is selective (no nitrile byproduct is formed unlike other photochemical reactions involving aldoxime ethers) as well as regiospecific when using 2′-aryl groups with meta-substituents, making this reaction a useful alternative for preparing substituted phenanthridines.

Effect of symmetric and asymmetric substitution on the optoelectronic properties of 9,10-dicyanoanthracene

“…This is indeed the case for 9,10-dicyanoanthracene (DCA). Besides many reports in which this compound has been used in fundamental research of electron transfer, [1][2][3][4][5][6][7][8][9] it has been applied, for example, as a photosensitizer 10 for the oxidation of alkynes to diketones, 11 for oxidative cyclization reactions, 12 alkynylation of carboxylic acids, 13 benzylic amination by cross dehydrogenative coupling, 14 as well as for aromatic substitutions. 15 Furthermore, DCA is a key (photo)catalyst in the synthesis of the antimalarial drug artemisinin.…”

Probing the hydrolytic reactivity of 2-difluoromethyl pyrroles

“…To confirm the concave up parabola, we synthesized and analysed 22f (R = CN), following the same protocols as previously, and were delighted to observe an excellent fit with the data generated from 22a-22e. The non-linear relationship 39,40 revealed on the Hammet plot points to the influence of the electronic features of the substituents on the aryl ring of pyrroles 22. The ρ value for electron-donating groups was found to be -0.4, suggesting a build-up of positive charge (or neutralisation of negative charge, negative slope, Fig 2).…”

Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals

“…Oxygenation took place by attack of radical 102 onto the NO 2 group of 103 leading to intermediate 104 . Homolysis of the N–O bond of 104 gave nitroso-phenoxide 106 and an O -centred radical that rapidly abstracted hydrogen to furnish the product imino-alcohol 105 (Scheme 15) [107]. The scope of the process was found to be wide affording iminoalcohols in good to high yields from alkenes with a range of substituents and including bicyclic products.…”