Nitrostilbenes

As another example of nitrene formation, the reaction of o-nitrostilbene (96) with CO in the presence of SnCU affords 2-phenylindole (97). The reaction is explained by nitrene formation by deoxygenation of the nitro group with CO, followed by the addition of the nitrene to alkene. Similarly, the 2//-indazole derivative 99 was prepared by reductive cyclization of the A-(2-nitrobenzyli-dene)amine 98[89]. 

Miscellaneous Dyes. Other classes of dyes that stiU have some importance are the stilbene dyes and the forma2an dyes. Stilbene ( es are in most cases mixtures of dyes of indeterminate constitution that are formed from the condensation of sulfonated nitroaromatic compounds ia aqueous caustic alkah either alone or with other aromatic compounds, typically arylarnines (5). The sulfonated nitrostilbene [128-42-7] (79) is the most important nitroaromatic, and the aminoa2oben2enes are the most important arylarnines. Cl Direct Orange 34 [2222-37-6] (Cl 40215-40220), the condensation product(s) of (79) and the aminoa2oben2ene [104-23-4] (80), is a typical stilbene dye. 

In 1911 Heim observed that the reaction of two moles of phenylnitromethane and one mole of benzaldehyde in the presence of an aliphatic amine gave cis- and frans-nitrostilbenes and three other products. One of the products was triphenylisoxazole (11CB2016). In 1924 Kohler reinvestigated the reaction (24JA2105). He observed that the reaction of phenylnitromethane (367) and cfs-a-nitrostilbene (368) in the presence of sodium methoxide gave... 

It was also found that bromonitrophenylmethane (375) reacted with sodium aryl-methanenitronate (374) in DMSO to give the isoxazoline iV-oxide (369) in 60% yield. Both reactions probably involved the formation of a vicinal dinitroethane derivative (376), which lost nitrous acid to give cfs-a-nitrostilbene (368). As mentioned, the reaction of (368) with (374) gave the isoxazoline iV-oxide (369). 

This class was first reported in 1924 and was formed 62HC(17)l) by cyclization of a-bromo-/3-aryl-y-nitroketones. The direct synthesis by oxygenation of 2-isoxazolines has not been reported. To date only 3-substituted derivatives have been prepared. Aryl-nitromethanes react with nitrostilbene to form isoxazoline A-oxide by a nitrile ion displacement (Scheme 138) <62HC(17)1, 68TL3375). 

D. O., Warrington, B. H., Wong, S. Y. F., The preparation of a series cf nitrostilbene ester compounds using micro-reactor technology. Analyst 126 (2001) 7-10. 

Arylindoles from o-nitrostilbenes and triethyl phosphite, 48,115 Arynes, addition of side-chain carban-ions, 48, 55 Azelaic acid, 45, 31... 

It should be possible in principle to determine the orientation of chromophores in a single monolayer on an OWG by the absorption of transverse electric (TE, s-polarized) and transverse magnetic (TM, p-polarized) modes laser. Swalen et al. [109] reported that much stronger absorption was observed for a thin evaporated film of 4-dimethylamino-4 -nitrostilbene with the TM mode and for seven monolayers of cyanine dyes with the TE mode. These results corresponded... 

HPLC and GC methods were used for analysis of water-soluble nitro-substituted aromatic sulfonic acids523. For example, 4-amino-4 -nitrostilbene-2,2 -disulfonic acid (248)... 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

Identical spectra and lifetimes were observed for the first three compounds listed above. It is assumed, that the observed transients are triplet states of the nitrostilbenes on the basis of the following results ... 

It is concluded that there is no common intermediate in the direct cis - trans-and trans - cis-isomerizations of the nitrostilbenes. 

Since similar transients are not observed from 4-cyano-4 methoxystilbene, 4,4 -dimethoxystilbene, 4-aminostilbene and 4,4 -dimethylaminostilbene, the possibility of observing such transients in the nitrostilbene series is certainly due to the tendency of the nitro group to enhance Si - -Ti transitions D. 

It should be mentioned that only inefficient photocyclizations to the respective dihydrophenanthrenes have been observed with 4-nitrostilbene (< =0.0007) and 3-nitrostilbene <0.0001) compared to more efficient ( = 0.28) cycli-zation in 3-aminostilbene... 

If the styryl substituent retained its donor nature in the anion-radical state, an increase, not a decrease in the value of the nitrogen HFC constant (a(N)) would have been observed. Experiments show that fl(N) values for anion-radicals of nitrostilbenes decrease (not increase) in comparison with the fl(N) value for the anion-radical of nitrobenzene (Todres 1992). Both naked anion-radicals and anion-radicals involved in forming complexes with the potassinm cations obey such regularity. In the cases of potassinm complexes with THF as a solvent, a(N) = 0.980 mT for PhNOj anion-radical and fl(N) = 0.890 mT for PhCH=CHCgH4N02-4 anion-radical. In the presence of 18-crown-6-ether... 

Under the same conditions, 4-styryl nitrobenzene (4-nitrostilbene) undergoes cis-to-trans isomerization only, with no changes in the nitro group (Todres et al. 1984, 1985 Scheme 1.9). 

This means that 4-nitrostilbene is a more effective electron acceptor than nitrobenzene. This theoretical conclusion is verified by experiments. The charge-transfer complexes formed by nitrobenzene or 4-nitrostilbene with Af,Af-dimethylaniline have stability constants of 0.085 L mol or 0.296 L mol respectively. Moreover, the formation of the charge-transfer complex between cis-4-nitrostilbene and A/,Af-dimethylaniline indeed results in cis-to-trans conversion (Dyusengaliev et al. 1995). This conversion proceeds slowly in the charge-transfer complex, but runs rapidly after one-electron transfer leading to the nitrostilbene anion-radical (Todres 1992). The cis trans conversion of ion-radicals will be considered in detail later, (see sections 3.2.5.1, 6.4, and 8.2.1). 

In contrast, the nitro and ethylenic fragments in trani-4-nitrostilbene form the united conjugation system. Such a conjugation is a necessary condition for the whole-contour delocalization of an unpaired electron in arylethylene anion-radicals. Whether this condition is the only one or there is some interval of allowable strength for the acceptor is a question left to future experiments. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

Interestingly, treatment of a-nitrostilbene in water-ethyl acetate mixture by the cation-radical of A,A -dioctyl-4,4 -bipyridinium (the octylviologen cation radical) leads to the formation of products derived from the nitro group reduction. No dimerization is observed (Tomioka et al. 1986). Water is responsible for the fixation of transferred electron within the nitro group. Further reactions result in the formation of the corresponding oxime and ketone as shown in Scheme 2.10. 

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