Imines photoisomerization

A variety of heteroannulated 1,2-diazepines 12 has been obtained by photoisomerization of the bicyclic /V-inunes 11, where R1, R2 represents part of a five-or six-mem bered heterocycle.143 The precursors of the imines, the AT ami no mesitylenesulfonates 10 (Ar = Mes), are prepared from the parent heterocycles by reaction with O-mesitylenesulfonylhydroxylamine.144... 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

Tetra-r-butylpyridazine (34) is converted into its Dewar isomer (35) when irradiated in pentane with UV light of wavelength > 300 nm. Irradiation of this product at shorter wavelengths, or thermolysis, gives rise to further reaction (91TL57). Irradiation of 4-amino-2,6-dimethylpyrimidine gives the acyclic amino imine via the Dewar pyrimidine as shown in Scheme 2a. The photoisomerization of perfluoropyridazines to pyrazines is considered also to involve Dewar diazine intermediates. 

Rearrangements.- E-Photoisomerization occurs readily in imines and in azo compounds. The syn-isomer (1), for example, is the major product of irradiation of nitrofurazone (2) in solution and is formed together with the corresponding azine on exposure to laboratory illumination. The photoisomerization of azobenzene derivatives in solution, in membranes, in host-guest complexes of cyclodextrins, and in polymers continues to attract attention. The reversibility of E-photoisomerization of azobenzene in cyclo-hexane solution has been established, and the E/ -ratios generated by irradiation of various azobenzene derivatives adsorbed on... 

The photoisomerization of several indole derivatives has been studied while isomerization around the C=N double bonds of N-methoxy-l-(l-pyrene)meth-imine has been described. In certain cases, the reversibility of the isomerization step depends on the spin multiplicity of the reacting state." Several reports have been concerned with the photophysical properties of cyanine dyes promoted into upper excited states. Indeed, for a series of cyanine dyes having different polyene backbones it was possible to establish that isomerization takes place... 

Rearrangements.- Examples of, E-photoisomerization in imines have again been reported. The photoisomerizations of methanimine and of fluorinated methanimines have been examined and the study of the photoisomerization of protonated and unprotonated imines of 9-cis-, 11-cis-, 15-cis-, and all trans-retinals has attracted attention in view of the continuing interest in the mechanism of the visual process. Evidence from flash photolysis studies indicates that the photoisomerization of the E-hydrazone (1) to... 

A photochemical diaziridine synthesis, analogous to the photoisomerization of nitrones, was worked out at suitable azomethine imines. By the action of sunlight, 104 is isomerized to the unstable diaziridine 105 in nearly 80% yield. 

Like alkenes (Section 6.1.1), chromophores containing the N=N (azo compounds) or C=N (imines, oximes, etc.) bonds can undergo E Z (or trans cis) photoisomerization (Scheme 6.156) and the resulting isomer concentration ratio in the photostationary state (PSS) reflects the absorption properties of the isomers and isomerization quantum yields (see Scheme 6.1 in Section 6.1.1). Since conventional (dark) synthesis generally provides access to more stable E-isomers, photochemistry is an exceptional tool for preparing sterically hindered Z-isomers.1061,1062 The photoisomerization reaction can be induced by a direct irradiation or by sensitization and it often competes with other phototransformations, such as photofragmentation or photorearrangement (Section 6.4.2). 

Rearrangements.- E-Photoisomerization has been observed in imines and related systems containing carbon-nitrogen double bonds, although in the case of imines, only one stereoisomer is normally stable at room temperature. The 9,10-dicyanoanthracene-... 

Azomethane imines undergo an analogous photoinduced cyclization to give diaziridines. The dihydroisoquinoline derivative (99), for example, is converted on irradiation in cyclohexane or benzene into the diaziridine (100) the transformation is thermally reversible. In contrast, the pyrazolidinone azomethine imines (101) undergo photoreversible conversion into the diaziridines (102), providing in this way a useful reversible photochromic system. The analogous photoisomerization of a pyrene-substituted pyrazolidinone azomethinimine has... 

Photolysis of the pyrazole-4-carboxyaldehyde 169 has been found to afford imidazole 172. This reaction is analogous to the isoxazole-oxazole photoisomerization discussed above. It is most likely that a 1,5-electrocyclic ring closure of iminonitrile imine 171 is responsible for imidazole formation. ... 

Nobel Laureate Jean-Marie Lehn has conceptually reduced the light-powered motor to a very simple system the chiral imine (Figure 4) 25). As observed for chiral alkenes, absorption of a photon produces out-of-plane rotation of the tt bond in imines. Molecular asymmetry should cause the 90 rotation to occur with some preference for clockwise or coimterclockwise direction. Photoisomerization of the anti imine to the syn geometry constitutes directional, 180° rotation. At room temperature, the syn isomer would rapidly undergo in-plane inversion to the original anti form. This brilliant simplification of the molecular motor based on photochemical/thermal cycling has not yet been experimentally reduced to practice. 

The oxime ethers (such as outlined in Scheme 6) are attractive candidates for mechanistic studies concerning the E,Z-photoisomerization of imines (vide infra) because the presence of the alkoxyl group drastically reduces the rates of thermal interconversion (K x 10 at 60 C) and allows mechanistic studies to be carried out at ambient temperatures.Padwa and Albrecht studied photochemical ,Z-isomeriza-tion regarding the carbon-nitrogen bond of O-methyl oxime ethers in some detaU. They found that upon direct irradiation of the O-methyl oximes of acetophenone, a photostationary state Z E ratio of... 

Many simple imines are susceptible to facile hydrolysis to give their parent ketones in the presence of even a trace of moisture. The small amount of ketones generated affects the results of the photoreaction of imines. Thus, a major activity in investigating the photochemical behavior of imines has been on imine derivatives, such as O-alkyl oximes and hydrazones, which are less susceptible to hydrolysis. Extensive studies were thus carried out on the E,Z-photoisomerization of the configurationally stable oxime ethers, which have aheady been described vide supra). 

The ground-state synthesis of imines, oximes, and azo compounds invariably produces the more stable -isomers and photochemical isomerization is practically the only means for preparing their less-stable Z-isomers, as in the case of olefins such as stilbene. For example, the pharmacologically active Z-isomer of isonicotinaldehyde was prepared by photoisomerization (Scheme 50)." A less stable E-isomer of methyl p-nitrophenylhydroximate O-methyl ether can be isolated in 38% yield by UV irradiation of the Z-isomer in benzene, followed by preparative TLC (Scheme 51)." ... 

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