Nitrostilbenes, substituted

The benzylidenemalononitriles and the a-nitrostilbenes substituted at the a-phenyl ring present electrophilic activation when the hydrogen atom at the para-position of the aromatic ring in compounds 129 and 135 are replaced by the strong EW group —N02 to give compounds 124 and 130, respectively. Note that the presence of a -Me group at the same position in compound 136 results in electrophilic deactivation of 135. 

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). 

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) ... 

The reactions of /J-methoxy-a-nitrostilbene (82) with anionic sulphurated nucleophiles159 showed UV/VIS spectroscopic evidence for the presence of the intermediate 83 in the formation of the substitution product (84) illustrated in Scheme 36. 

The p for stilbene derivatives are much higher than their benzene analogs (JL). To determine the effect of substitution on the nonlinearity, 4-methoxy-41-nitrostilbene... 

Substituted a-nitrostilbenes were hydrogenated to 1,2-diphenylethylamines in 52-87% yields, conveniently in methanol over in situ-prepared Pd-C catalyst (eq. 9.28).53 In these cases the hydrogenation was carried out in presence of the hydrogen chloride formed from palladium chloride, the amount of which, however, was much smaller (-0.0022 mol) for the nitrostilbenes hydrogenated (0.037-0.042 mol). In one instance, a-nitro-4 -methoxystilbene gave rise to a small amount of the corresponding oxime when the hydrogen uptake was less than theoretical. 

Workers at the Batelle Institute have done a considerable amount of work on photochromism resulting from cis-trans isomerization (36). Most of their studies involved substituted stil-benes (e.g., 4-methoxy-4 -nitrostilbene) in a variety of matrices. Previously studied thioindigo derivatives were also examined and it was shown that the latter materials could be used to write holograms. 

As a preparative method the direct decarboxylation of olefinic acids is almost limited to the formation of styrenes and stilbenes from substituted cinnamic acids. Thermal decomposition of cinnamic acid gives styrene (41%). The yield is nearly quantitative if the reaction is carried out in quinoline at 220° in the presence of a copper catalyst. The yields of substituted styrenes where the aryl radical contains halo, methoxyl, aldehyde, cyano, and nitro groups are in the range of 30-76%. cis-Stilbene and cis-p-nitrostilbene are prepared in this way from the corresponding a-phenylcinnamic acids (65%). One aliphatic compound worthy of mention is 2-ethoxypropene, prepared by heating -ethoxycro-tonic acid at 165° (91% yield). The mechanism of acid-catalyzed decarboxylations of this type has been studied. Isomerization of the double bond from the a,/5- to the /5, y-position before decarboxylation very likely occurs in many instances. ... 

Nitro alcohols ate usually isolated (method 102) but are sometimes dehydrated directly to olefinic nitro compounds as in the preparation of co-nitro-2-vinylthiophene from nitromethane, thiophenecarboxaldehyde, and sodium hydroxide (78% yield). Many substituted /3-nitrostyrenes have been obtained by condensation of nitromethane or nitroethane with substituted benzaldehydes. A methyl group on the benzene ring is sufficiently activated by nitro groups in the orlio or para position to cause condensation with aldehydes. A series of nitrostilbenes has been made in this way from substituted benzaldehydes. ... 

The addition of substituted benzylamines to a-cyano-4-nitrostilbene (125) has been found to involve the formation of the zwitterionic species (126) in an equilibrium, and its subsequent decomposition to (127) catalysed by a second molecule of the amine. 

A quantitative relationship between Hammett substituent constant (a) for substituted ethylene and the global electrophilicity index has been found.122 Therefore, it is not surprising to find a good correlation between In k for the addition of HOCH2CII2S to substituted a-nitrostilbenes reported by Bernasconi et a/.116 and the global electrophilicity index co, as shown in Figure 4. 

Figure 4 Plot of ln( ) vs the electrophilicity index a> for the reaction of the addition of HOCH2CH2S to substituted a-nitrostilbenes. Rate coefficients k from reference 116...

The regioselectivity is well represented by a local reactivity picture in asymmetrically substituted ethylenes.121 The local electrophilicity values, tok, for the two carbon atom belonging to the C=C double bond, named as Cl and C2 (C2=C1(CN)2 and C2=C1N02), of the series of benzylidenemalononitriles and the a-nitrostilbenes 124-137 are given in Table 14, while local electrophilicity values of the electron-poor... 

According to Pitacco, Valentin and coworkers secondary ketoenamines react generally with nitroolehns to give alkyl- and aryl-substituted tetrahydroindole-7-ones under thermodynamic control in good yield without any catalyst, if the a-ketoenamine is an N-alkyl derivative. The course of the reaction depends on the substituents and the conditions which result in different cleavage or rearrangement reactions. In the case of a-nitrostilbene a Michael adduct is obtained in low yield. If, however, for example, 1-nitrocyclopentene is used in the reaction with iV-t-butylenaminone under kinetic control, an unstable [4 -i- 2]cycloadduct can be isolated. The reaction clearly demonstrates the concurrence of Michael addition and subsequent cyclization to 7-indolones... 

In liquid solutions in the absence of a sensitizer, T-T absorption has hitherto been observed predominantly with nitro-substituted frans-arylethy-lenes (Tables 16c-e) such as nitrostilbenes [31,200], nitroazastilbenes [172,416], and nitrostyrenes [417], Apparently, the nitro group enhances strongly thioindigo dyes [418,419]. No triplet could be measured for stilbene under direct excitation conditions at room temperature [96,200] because the triplet yield is too small (see Section 1V.B.6). 

Since the lowest triplet state of nitrostilbenes at room temperature can be probed by laser flash photolysis, quenching rate constants could be measured directly [188,200], Values for kq are close to the diffusion-controlled limit (Table 17). Comparable results have been obtained for naphthyl-[410] and 2-anthrylethylenes [433] and for ADBs [33, 143, 145-147, 232, 411], For substituted 1-NPEs, conclusions about the position of the triplet equilibrium were drawn on the basis of rate constants for quenching by ferrocene and oxygen [441]. 

The rate constant for the individual transitions may depend on a variety of features, such as substitution, medium and temperature. One important parameter is the height of the activation barrier between t and p. If the rate constant for the step 11 — 1 p is of the size comparable to that for the step t -> 3t, both processes may contribute to tram-> cis photoisomerization. For most nitrostilbenes examined, is substantial and for 4-nitro-, 4,4 -dinitrostilbene, and 4,4 -NMS a singlet pathway has been ruled out. However, for 4,4 -NDS in nonpolar solvents [160] and for quaternary salts of 4-nitrostyrylquinolinium [229], a contribution of the singlet mechanism has been concluded. 

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