Stilbene radical reactivities

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

It is hardly surprising that different chemical reactivity might be expected from the exciplex and the radical ion pair formed by complete electron exchange. Lewis observation (50) that in the excited state interaction of trans-stilbene with either electron-rich or electron-poor alkenes cycloaddition is more efficient from the relatively less polar exciplex than from radical ion pairs is typical for many such cycloadditions. 

In parallel to this example, Lewis and coworkers have also observed inverse reactivity for electron transfer and cycloaddition efficiency in the interaction of trans-stilbene with unsaturated esters (52). This result is understandable if the exciplex, rather than the radical ion pair, arranges the olefins in the correct geometry for cycloaddition. Analogous results have also been reported in the (2+2) cross photoreaction of cyano-substituted stilbenes with dienes, where all possible regioisomers were formed, eq. 16 (53a) ... 

The first example of the use of a polymer-bound cinchona alkaloid in the AD was described in 1990 by Sharpless [48,49], The polymer was readily obtained by radical co-polymerization of 9-(p-chlorobenzoyl)quinidine acrylate with acrylonitrile. First applications in dihydroxylations of frans-stilbene using NMO as co-oxidant yielded products with enantioselectivities in the range of 85 -93 % ee. It is interesting that a repetitive use of the polymer was possible without great loss of reactivity, indicating that the metal was retained in the polymeric array. 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

It can be seen that the cation radical of stilbene, but not stilbene itself, is subjected to acetoxylation. Stilbene in trans form yields the trans form of the cation radical, which undergoes further reaction directly. Stilbene in cis form gives the cation radical with the cis structure. Such a cis cation radical at first acquires the trans configuration and only after that adds the acetate ion. It is the isomerization that causes the observed retardation of the total reaction. It is the absence of adsorption at the electrode surface that allows the nonace-toxylated part of cis stilbene to isomerize and to turn into the more rich stereoisomeric set of final products. To support this point of view, one can mention the cation radical epoxi-dation and cylopropanation of stilbenes. In the aminiumyl ion-catalyzed reactions, cis stil-benes react about 2.5 times slower than trans stilbenes, whereas in electrophilic oxidations the cis isomers are more reactive (Kim et al. 1993 Bauld Yeuh 1994 Mirafzal et al. 1998 Adamo et al. 2000). 

The reactivity of a polar monomer can be considerably enhanced in copolymerization with a species of the opposite polarity. Maleic anhydride does not homopolymerize under normal free-radical reaction conditions but it forms 1 1 copolymers with styrene under the same conditions and even reacts with stilbene (7-6), which itself will not homopoloymerize. 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

Clearly, the relative reactivity of a monomer does depend upon the nature of the radical that is attacking it. Maleic anhydride is much more reactive than stilbene toward radicals ending in a stilbene unit, and stilbene is much more reactive than maleic anhydride toward the other kind of radical. (Indeed, these two compounds, individually, undergo self-polymerization only with extreme dilhculty.) A more modest—and more typical- tendency toward alternation is shown by styrene and methyl methacrylate. Here, toward either radical (- M ) the opposite monomer (M2) is about twice as reactive as the same monomer (Mj). 

Product distributions from various olefins clearly show that at least two mechanisms are involved. Cyclohexene provided a higher yield of oxide than that of cyclohexene-1-one. The formation of cyclohexene-1-one suggests that the ROO radical serves as a reactive intermediate in this system, but a radical process, operating alone, would produce mostly the ketone and alcohol products with very little epoxide. Most significantly, czs-stilbene gave czs-stilbene oxide as the dominant product with only a minor yield of the frazzs-stilbene oxide, whereas a radical process would give mostly trzzzzs-product [81]. 

Unsaturated Lignin Model Compounds Double bonds in lignin model compounds are attacked by peracetate ions. Dehydro-di-woeugenol (XXI, Figure 12.9) reacted with epoxidation of the aliphatic double bond and formation of the diol. The double bonds in stilbenes [59] and coniferaldehyde [90] are also cleaved. FemUc acid (IVa) and its ethyl ester reacted slowly at 50°C the methyl ether, 3,4-dimethoxy cinnamic acid, was much less reactive and was almost quantitatively recovered [55]. The reactions of ferulic acid and its ethyl ester (both in the trans form) were accompanied by trans-cis isomerization, perhaps an indication of reversible phenoxy radical formation. HomovanilUc acid (XXXa) was also formed the proposed mechanism involved epoxidation of the a-P double bonds followed by decarboxylation. 

Aerial oxidation of this affords anthraquinone in 37% yield. Both 1-naphthyl and 2,4-dimethylbenzene sulphonates follow the same path but yield only traces of the corresponding quinone. In the last case, 2,4-dimethylbenzenesulphonate, the quinone is accompanied by 0.8% of 2,4-dimethylphenol. The second path involves loss of sulphur trioxide to yield aryl radicals which afford the products, the arene and/or the biaryl, shown in Scheme 17212,213. Other studies have shown that anthraquinone-1-sulphonic acid (256)214,215 and anthraquinone-2-sulphonate216 are also photochemically labile. A study of the photochemical reactivity of azulene sulphonic acids has also been reported217. Photochromism has been studied with respect to the stilbene derivative 257218. 

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