Dimethylamino photoisomerization

After early unsuccessful attempts to direct the photoreduction of ketones with chiral secondary alcohols [8-10]. Weiss et al. examined the sensitized cis-trans photoisomerization of 1,2-diphenylcyclopropane in chiral solvents but obtained the product without detectable optical rotation [11]. Seebach and coworkers were the first to achieve asymmetric induction for a photochemical reaction by a chiral solvent [12-15]. They examined the photopinacolization of aldehydes and ketones in the chiral solvent (S,S)-( + )-l,4-bis(dimethylamino)-2,3-dimethoxybutane (DDB, 4). Irradiation of acetophenone in the presence of 7.5 equiv. of DDB yielded a mixture of chiral d,/-pinacols 3/ent-3 and achiral meso-pinacol 2. At 25°C pinacol 3 was obtained with 8% ee, with the (R, / )-( + )-enantiomer prevailing. At lower temperatures the asymmetric induction was more effective, up to 23% ee at — 78°C in a 1 5 mixture of DDB and pentane (Scheme... 

FIGURE 1.12 Photoisomerization of 4-dimethylamino azObenzene.The dotted tine is the extrapolated Z-spectrum. (From reference 128, by permission.)... 

At room temperature, the isomerization of e.g. 4-dimethylamino-4 -nitroazobenzene cannot be observed reasonably, because the thermal back isomerization of the Z-form is too fast. To determine the spectrum of the Z-form, Gabor and Fischer applied low-temperature and extrapolation techniques. The photoisomerization quantum yields are (l)g, z 0.20 and (t>z- E 0.75 at 163 K. They decrease to (j)E->z 0-06 to 0.09 at 143 K, whereas (1)z >e does not change much. Newer data regarding quantum yields are not available. 

Stilbene and its derivatives with substituents generating only low spin-orbit coupling (e.g., 4-methoxy, 4-dimethylamino, 4-chloro, and 4-fluoro). Direct competition of fluorescence with trans - cis photoisomerization is indicated by kinetic analysis. 

For unsubstituted stilbene, azastilbenes, and naphthylethylenes the CT interaction involves only singlet states as initial species. Introduction of a nitro group makes triplet states accessible for electron transfer [513], Because of the larger value of tt as compared to rs, smaller donor concentrations are required for electron transfer in the triplet state. The same Stern-Volmer constants for quenching of 4>, c and 1/tt (or l() indicate that trans - cis photoisomerization and electron transfer compete. This was also found when a positive charge was introduced by quaternization of 4-R-azastilbenes (A+, R = nitro or cyano) [170,489], but not for compounds with R = dimethylamino [171]. Under certain conditions (e.g., in a solvent of medium polarity such as dichloromethane and for X- = I"), a radical pair (A. . X ) is produced by excitation of the ion pair (Figure 20b) [172,489]. The same (neutral) radical can be formed by electron transfer from the amine, (e.g., DABCO) to the cation of a quarternary salt of 4-R-azastilbene in the case R = nitro the electron is transferred to the triplet state, in competition with trans- cis photoisomerization (Figure 21). 

The trans-cis photoisomerization rate of the 4-dimethylamino-4 -aminostilbene strongly depends on temperature and, hence, on viscosity up to a temperature about... 

The sensitized cascade reported in Ref [16] consisting of triplet ds-trans photoisomerization of the excited stilbene includes the triplet sensitizer (erythrosin B), the photochrome stilbene derivative probe (4-dimethylamino-4 -aminostilbene), and nitroxide radicals (5-doxyl stearic acid) quenching the excited triplet state of the sensitizer (Figure 10.11). 

Fig. 31a). The native FAD cofactor was extracted from GOx and the semisynthetic FAD cofactor was reconstituted into the apo-GOx (apo-GOx) (Fig. 31b). This reconstituted enzyme includes a photoisomerizable unit directly attached to the redox center of the enzyme, and hence, the enzyme is predisposed for optimized photoswitchable bioelectrocatalytic properties. The photoisomerizable enzyme was assembled on an Au-electrode as described in Fig. 31(c). The bioelectrocatalytic oxidation of glucose was stimulated in the presence of ferrocene carboxylic acid as a diffusional electron-transfer mediator. The (28a)-state of the reconstituted GOx was inactive for the bioelectrocatalytic transformation, whereas photoisomerization of the enzyme to the (28b)-state activated the system (Fig. 32). By the cyclic photoisomerization of the enzyme mono-layer between (28a) and (28b) states, the bioelectrocatalyzed oxidation of glucose was cycled between the off and on states, respectively (Fig. 32, inset). It was also found that the direction of the photo-bioelectrocatalytic switch of the (28a/28b)-FAD-reconstituted GOx is controlled by the electrical properties of the diffusional electron-transfer mediator [385]. With ferrocene dicarboxylic acid as a diffusional electron-transfer mediator, the enzyme in the (28a)-state was found to correspond to the switched off biocatalyst, while the (28b)-state exhibits switched on behavior. In the presence of the protonated 1-[1-(dimethylamino)ethyl]ferrocene, the direction of the photobioelectrocatalytic switch is reversed. This control of the photoswitch direction of the photoisomerizable GOx was attributed to electrostatic interactions between the diffusional electron-transfer mediator and the photoisomerizable unit linked to the FAD. The (28b)-state attracts the oxidized negatively charged... 

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