Photochemical syn-anti isomerization about the carbon-nitrogen double bond

Abstract

Upon irradiation with ultraviolet light, the O-methyl oxime ethers of acetophenone undergo facile syn-anti photoisomerization. The photostationary state ratio obtained by direct irradiation of the syn and anti oxime ethers was 2.20 ± 0.03. Photoisomerization about the C-N double bond could also be induced by triplet excitation. A photostationary state vs. triplet energy of the sensitizer plot shows (1) a high-energy region in which the stationary state ratio is ca. 1.5, (2) a gradual increase in the syn/anti ratio from 72 down to 59 kcal of triplet energy, and (3) a sharp decrease from 59 down to 54 kcal. The latter two observations were explained by nonvertical excitation of the acceptor. Analysis of the sensitized quantum yields for the syn-anti isomerization reaction shows that energy transfer to the oxime ethers does not proceed with identical rates. The inefficiency encountered in these sensitized experiments suggests that not all triplet sensitizers are effective in generating a twisted imine triplet. A double sensitization experiment using the oxime ethers and r/ww-stilbene indicates that triplet energy transfer can occur from the oxime ethers to trans-stilbene. Since the direct irradiation could not be quenched with high concentrations of piperylene, it would seem as though an electronically excited singlet is the reactive state in the direct isomerization. ne of the most active areas of organic photochemistry has been the study of systems which possess a carbonyl group.2-5 As a result of these studies the photochemical transformations of organic molecules containing this functional group have been categorized into a number of primary photochemical processes.6 This state of affairs contrasts sharply with the present status of the structurally related imine system, the photochemistry of which is mainly qualitative with relatively little available in the way of quantum yield data and kinetic studies. While photochemical reactions have been observed with a number of systems possessing a C-N double bond,7-31 a more (1) NDEA Title IV Fellow, 1967 (2) N.

Additional Insights on the Bastadins: Isolation of Analogues from the Sponge Ianthella cf. reticulata and Exploration of the Oxime Configurations

“…First, ( E , E )-bastadin 19 ( 1a ) was subjected to photoisomerization using an incident light of 310–330 nm with acetophenone as triplet sensitizer. 30 A mixture of recovered starting material plus new diastereoisomers was obtained consisting of: 1a [ E , E , 36%], 1b [ E , Z , 25%], ( 1c) [ Z , E , 20%], and ( 1d) [ Z , Z , 19%]. Their geometries were confirmed based on the diagnostic δ C shifts for C 1 /C 25 discussed above and shown in Table 2.…”

Photochemistry of Anils in NaY Zeolite

“…[17,18] Therefore, a reasonable explanation that can reconcile all the observations of the photochemistry of anils adsorbed on NaY with the pathways reported in other media is that upon light absorption the only photoprocess undergone by the zwitterionic form is an anti-syn isomerization through the CϪC and CϪN bonds with partial double bond character. This rationalization is supported by the following facts: i) our studies on the steady-state irradiation and product isolation by continuous extraction gave only anils back (no stable products formed); ii) such stereoisomers have been detected earlier in the photochromism of anils in solution by time-resolved spectroscopy, but they were assumed not to play any role in the photochromism; [17,18,20] iii) the parent N-phenylimine of benzaldehyde adsorbed on NaY only undergoes syn/anti isomerisation upon irradiation as was reported in solution for anils and other carbon-nitrogen double bond-containing molecules; [40,41] iv) stereoisomers of the related zwitterionic merocyanine form of spiropyrans have also been reported: depending on the nature of the solvent used at least four isomers have been identified (see Structure 1) by transient Raman spectroscopy; [42,43] and v) the occurrence of cis/trans isomerization is compatible with the observation of only minor changes in the absorption wavelength seen in the steady-state diffuse reflectance UV/Vis spectra recorded after the irradiation in NaY.…”

Aqueous photodegradation of sethoxydim herbicide: Qtof elucidation of its by-products, mechanism and degradation pathway

“…These mechanisms are usually complex and depend on the substituents and the reaction conditions. With respect to the Z-isomer (S9), the isomerization of oximes seems to occur through a triplet state (Mallavia et al, 2001;Padwa and Albrecht, 2002b). From that excited state, the isomerization process can follow two different mechanisms (Fig.…”