One-Way and Two-Way Geometrical Isomerizations of 1, 1-Diarylketone Oxime O-Methyl Ethers Induced by Photosensitized Electron Transfer

Under an oxygen atmosphere, the E and Z isomer ratio of N-methoxy-4-methoxyphenyl-4'-methylphenylmethanimine (one of the title compounds; initially, ⌈E⌉/⌈Z⌉ = 1/1) reached to 4/96 upon irradiation (> 360 nm) of 9, 10-dicyanoanthracene (DCA) as a photosensitizer in acetonitrile. Key intermediate is presumed to be a putative distonic radical cation which is a unique one having a spatially separate radical and ionic centers on the molecular framework. The whole scenario of the reaction was supported by molecular orbital calculations. An excited singlet state of DCA generated by light absorption was quenched by oxime ethers to give the DCA anion radical and radical cations of E- and Z-oxime ethers. Then the subsequent single electron transfer from the DCA anion radical to molecular oxygen generates superoxide anion radical, which reacted with distonic radical cations of E- and Z-oxime ethers bya ⌈supra + antara⌉ manner. Geometry selection occurs at a stage of just making a new bond between nitrogen and oxygen terminals in the ensuing peroxyaminy1-1,4-diradical. The final oxime ether is afforded by releasing molecular oxygen without successful single bond formation. Instead, ring closure to azadioxetane gives rise to formation of byproducts, benzophenones and methyl nitrite, by decomposion of it in a metathesis fashion.

One-Way Geometrical Isomerization of 1,1-Diarylethenes Induced by Photosensitized Electron Transfer

One-way geometrical isomerization around a C=C bond of 1,1-diary1-2-t-butylethene is achieved upon photoirradiation of cyanoaromatics as a light-absorbing sensitizer. In the thermochemical view, there is no difference between both E and Z isomers of the ethene. Key intermediate is a putative distonic radical cation which is a unique one having a spatially separate radical and ionic centers on the molecular framework. Generation of such an intermediate is due to the presence of a π-donating substituent on an aromatic ring and a bulky t-butyl group. Molecular oxygen interacts as superoxide with the C=C bond of the radical cation in a [supra + antara] manner to give a decomposition product, i.e. a benzophenone derivative and pivalaldehyde, via a dioxetane and competitively, one geometrical isomer of the ethene is afforded by splitting oxygen before making two bonds with the ethene radical cation and superoxide.