1 - -1,2-diphenylethylene isomerism

Stilbenes have a 1,2-diphenylethylene as their basic structure (C6-C2-C6). Resve-ratrol, the most widely known compound, contains three hydroxyl groups in the basic structure and is called 3,4, 5-trihydroxystilbene. In plants, piceid, the glucoside of resveratrol, is the major derivative of resveratrol. Stilbenes are present in plants as cis or trans isomers. Trans forms can be isomerized to cis forms by UV radiation (Lamuela-Raventos and others 1995). 

Soveral 1,1-diarylethylcne oxides have been subjected to Ihr action of magnesium bromide or boron trifluoride (Eq. 453). Again, migratory aptitude in the epoxide-isomerization process appears to l -a function of the overall substitution pattern. Thus, 1,1-diphenyi ethylene oxide and 2-methyl-1,1 -diphenylethylene oxide -J suffer hydride shifts exclusively. On the other hand, 2-ethyl-i.i-diphenylethylene oxide reportedly undergoes hydride shift and cthvl migration... 

Aoyama et al. in relation to their studies on photochemical synthesis of (3-lactams [91] reported the synthesis of 4-spirocyclopropylazetidin-2-one [92] via photocycloaddition of 4-thioxoazetidin-2-one to alkenes followed by subsequent desulfurization. A solution of 1-isopropyl-3-phenyl-4-thioxoazetidin-2-one 70 and 1,1-diphenylethylene in benzene on irradiation with a high pressure mercury lamp afforded a [2 + 2] adduct 72 (R = Ph), in 67% yield which, on desulfurization with Raney-nickel [93] in anhydrous ethanol gave two isomeric... 

Isomerization at a nitrogen-nitrogen double bond (azo derivatives XLVIII) occurs by inversion at one of the nitrogen atoms. The observed isomerization barrier of azobenzene (XLVIII, a = c = phenyl) is about 23 kcal/mol, as compared with 18 kcal/mol for the analogous imine (XLVII, a = c = phenyl, b = H) and with 43 kcal/mol for diphenylethylene (see above) [56]. 

The 1,1,4,4-tetraphenylbutane disodium salt was found to eject electrons scavenged by added 1,1-diphenylethylene [59, 60]. In the case of trans-stilbene dianion, the photoejection of an electron was accompanied by isomerization in ds-stilbene radical anion while biphenyl was used as electron scavenger [61]. ds-Stilbene itself was used as scavenger of electron photoejected from aromatic radical anions like sodium perylenide [46], These scavenging experiments are... 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

Tetraphenylethylene oxide—no reaction 1,1-Diphenylethylene oxide—no reaction Burgstahler prepared the reagent by stirring lithium shot in ethylamine until the blue color was completely discharged and used it for isomerization of the non-conjugated diene (1) to (2). Mineral acids failed to effect isomerization. Lithium di-... 

The name for stilbene (1,2-diphenylethylene) was derived from the Greek word stilbos, which means shining. There are two isomeric forms of 1,2-diphenylethylene ( )-stilbene (trans-stilbene), which is not sterically hindered, and (Z)-stilbene (cis-stilbene), which is sterically hindered and therefore less stable. 

A variation of the reaction involved the use of the alkene itself as nucleophile. In this case, a radical cation dimer was formed by attack of the alkene radical cation by the neutral alkene, forming a distonic radical cation (Scheme 14.9, left part). With a-methylstyrene (17) as the alkene, a cychzation took place and the neutral radical resulting from the ensuing deprotonation coupled with the radical anion of the acceptor (in this case TCB), leading to the NOCAS adduct 18 as a diastereo-isomeric mixture in overall 90% yield [55]. The irradiation of aromatic nitriles in the presence of aUcenes may lead to different products, particularly when carried out in an apolar medium. As an example, 1,4-dicyanobenzene gave isoquinohnes by a [4-1-2]-cycloaddition with a cyano group through irradiation in the presence of diphenylethylenes in benzene via a polar exciplex [56]. 

Both direct irradiation and sensitized photolysis can lead to isomerization, often with differing results. An extensively studied system is stilbene (1,2-diphenylethylene) and its substituted derivatives (Eq. 16.19). Studies involving dozens of different stilbene derivatives and dozens of different sensitizers have produced a detailed view of the excited state structures of these systems. Indeed, the strongest experimental support for a scheme such as shown in Figure 16.15 comes from studies of stilbenes. 

Upon irradiation at ca. -70 C, 6 was isolated in 72% yield, almost identical with the 76% yield obtained in a parallel run at 15°C. Hence, Bruce concluded that an in-cage-controlled radical combination might play only a minor role in the mechanistic scenario (see Scheme 14). To further verify this assumption, Bruce irradiated a solution of benzoquinone and acetaldehyde in the presence of several scavengers, of which 1,1-diphenylethylene (46) proved to be the most suitable one. Along with a small amount of compound 6, an isomeric mixture of the thermally unstable trapping product 47 was isolated in 39% yield (Scheme 15). That free radical intermediates were involved was further indicated by the thermal... 

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