Stilbene adducts

The chemistry of 306 differs from that of its seven-membered analogue in that its reaction with rram-stilbene gives reductive coupling product 308 in addition to the stilbene adduct of zirconocene, the sole product of the latter (vide infra). This was attributed to differences in ring strain.116... 

These peroxides also form 1 1 adducts with styrene and form hydroben2oin diarenesulfonates with stilbenes. Di(ben2enesulfonyl) peroxide decomposes in water to phenol and sulfuric acid (33). 

Other isocyanates undergo [2 + 2] cycloaddition, but only with very electron rich alkenes. Thus phenyl isocyanate gives /3-lactams with ketene acetals and tetramethoxyethylene. With enamines, unstable /3-lactams are formed if the enamine has a /3-H atom, ring opened amides are produced 2 1 adducts are also found. Photochemical addition of cis- and traH5-stilbene to phenyl isocyanate has also been reported (72CC362). 

In contrast to other furoxans, the cycloreversion of 3,4-dinitrofuroxan to nitro-formonitrile oxide was observed even at room temperature. The nitrile oxide could be trapped in situ with electron-deficient nitriles (Scheme 149) (95MC231). Attempts to obtain cyclo adducts with styrene, phenylacetylene, rran.s-stilbene, and cyclohexene failed. 

Similar results were obtained in methanol (ref. 30), where for the bis-(trifluoromethyl) derivative the ratio between the return of the trans bromonium ion to the trans olefin and its collapse to products 2 and 6 is 4.5 and is again very strongly reduced by the presence of LiC104 (Table 9). In this solvent, however, return was not observed for unsubstituted stilbene, either. It can be observed that both cis - and trans -stilbene gave methoxybromo adducts in an anti stereospecific way, suggesting a nucleophilic assistance by the solvent. 

Thermolysis of 16e,f in either solution or gas phase (150-350 °C) gave deuteriated ethylenes (i.e. 40e from 16e and 41f from 16f) with about 95% retention of stereochemis-try ". Similarly, pyrolysis of the stereoisomeric 2,3-diphenylthiirane oxides 16g,h proceeded smoothly to yield stilbenes and sulfur monoxide in more than 70% yield . The extrusion of SO from the trans-isomer proceeds almost stereospecifically, while that from the cis-isomer occurs with complete loss of stereochemistry. This indicates the intervention of a stepwise mechanism, and not a symmetry-allowed nonlinear chelatropic reaction . Based on the fact that all attempts to trap the intermediate with 1,3-dipolarophiles were in vain, whereas a 1 1 adduct was obtained in good yield (about 60%) with the carbon radical scavenger di-p-anisyl thioketone, a mechanistic scheme as depicted in equation 10 has been proposed . Although the radical intermediates are capable of internal rotation about the carbon-carbon bond, for the 2,3-diphenyl case (i.e. 16g,h), the rotation would be... 

Cis- and fro/w-stilbenes have been reported to add to 2,3-dihydropyran upon photolysis to yield adducts (44) and (45) as well as two stilbene dimers(88> ... 

Methanesulfonates 844, obtained by addition of diphenyl phosphite to aldehydes At1 Cl IO and mesylation of the hydroxyl group of the adducts, react with benzotriazole to give diphenyl a-(benzotriazol-l-yl)benzylphosphonates 845. Lithiation and treatment with aldehydes Ar2CHO converts phosphonates 845 into stilbenes 846, which can eliminate benzotriazole to give diarylacetylenes 847 (Scheme 135) <2002ARK(xiii)17>. 

The bromination of /razw-stilbenes in methanol, trifluoroethanol and in acetic acid leads almost exclusively to the erythro adducts via a 100% anti addition, regardless the substituents and the solvent. In contrast, the stereochemistry of the reaction of c/s-stilbenes exhibits a considerable dependence on the substituents and on the solvents the reaction of p-methoxystilbenes is always stereo-convergent, that of p,p -bis(trifluoromethyl)stilbenes is stereo-specific in all investigated solvents, whereas unsubstituted stilbenes can produce variable stereochemical outcomes ranging from stereo-specific to stereo-convergent in going from methanol to trifluoroethanol as solvent. 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Irradiation of o-methyl-fraws-stilbene with diethylamine in acetonitrile solution results in the formation of two regioisomeric adducts in a 1 1 ratio, together with comparable... 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

A novel thermal rearrangement with loss of sulfur dioxide leading to the stilbene or styrene derivatives 169 and 170 in highly stereospecific manner was carried out by heating (in dioxane, DMSO, dioxane-water or THF) the sulfene-tropone adducts (y-sultones) 167 or 168 (equations 51 and 52)68. 

The treatment of the bromoacetal (255) with catalytic amounts ( 10%) of a Co(I) species, generated by the electroreduction of cobaloxime (232) in an Me0H-LiCl04 system at —1.8 V, produces the cis-fused adduct (256) in 60 70% yield (Scheme 96) [392]. Cathodic reduction is used for the synthesis of a [Co(CO)3PBu3] complex in a methanol-methyl formate medium, which catalyzes the alkoxycarbonylation of dichloromethane to dimethyl mal-onate in up to 75% yield [393]. The Co(II) complexes are found to be effective for the homogeneous reduction of gem-dichlorocyclopropanes in the presence of anthracene [394]. The formation of the C—C double bond of (258) may be ascribed to the a-elimination of the Co-H species. Thus, benzalchloride (257) can be converted to a mixture consisting primarily of ds- and trans-stilbenes (258) by the action of electrogenerated Co(I)(salen) (Scheme 97) [395-398]. 

Like the trichloromethyl peroxide radical, peroxothio compounds can perform even nucleophilic oxygenation of substrates that are inert to Oj" in aprotic solvents. For example, stilbene is not changed in dry benzene containing 18-crown 6-ether and KOj. In the presence of diphenylsul-fide, however, the interaction takes place and results in the formation of stilbene epoxide. According to Oae et al. (1981), stilbene initially gives PhCH(00 )CH Ph anion-radical adduct. Abstraction of O from the adduct leads to stilbene epoxide with 40% yield (Oae et al. 1981). 

In some cases, the initial adduct can combine with an additional nucleophile to give a diadduct. This process is particularly likely when the driving force for aro-matization of the monoadduct is weak or absent. For example, Novak has shown that heteroarylnitrenium ion 85 is trapped at first by water and then by an additional nucleophile to give the diadduct 87 (Fig. 13.45). Likewise, Bose et al. reported the dihydroxylation of the nitrenium ion 88 derived from stilbene... 

Benzil undergoes a cycloaddition reaction with stilbene and 1,1-diphenyl-ethylene to form adducts containing the 2,3-dihydro-[l,4]-dioxin ring system and with visnagin to form an oxetane. These reactions and the cycloaddition reactions of the analogous o-quinones are reviewed by Schonberg.87... 

It is of interest to note that the addition of nitrosyl chloride to a molecule such as cis,trans, trans-1,5,9-cyclododecatriene takes place only at one of the trans bonds [72], It would appear from this that, in a competitive reaction between cis and trans double bonds, the reaction at the trans bond is favored. However, further work is required to substantiate this generalization, particularly in view of the fact that in an experiment involving both cis- and trans-stilbene the nature of the nitrosyl chloride adduct was not fully determined [55]. 

Thermolysis of indenone oxides (151) is an equally useful route to the betaines (150). Dimethyl acetylenedicarboxylate and compound 151 (R = R = Ph) at 175°C give adduct 153, and cyclohexanone at 150 C gives adduct 154. - Similarly, 1,3-dipolar adducts (e.g., 155) have been obtained using a wide variety of olefins—including cis- and [rans-, 2-dichloroethylene, dimethyl maleate, dimethyl fumarate, maleic anhydride, cis- and tran -stilbene, fran -dibenzoylethylene, tra .y-l,2-dicyanoethylene, A -phenylmaleimide, vinylene carbonate, acenaphthylene, and norbor-nadiene. With cis olefins the endo adduct (155) is usually the predominant isomer. Diphenylcyclopropenone gives compound 156 by spontaneous elimination of carbon monoxide from the initial adduct (157). Adduct 156... 

Barton and co-workers have studied30 the reactions of trifluoromethyl hypofluorite with various alkenes and found that (Z)- and (T -stilbene give vicinal fluoro trifluoromclhoxy products and vicinal difluorides in chlorotrifluoromethane or diethyl ether, while vicinal fluoro methoxy adducts predominate when the reaction is carried out in methanol at —78 C. svn Addition predominates in all cases on the basis of the fluorination regioselectivity of many alkenes, the formation of a /1-fluoro carbcnium ion is proposed.30... 

The reaction of phenyl-substituted alkenes (2-phenylprop-l-ene, ( )-l-phenylprop-l-ene, 1,1-diphenylethene, 1,1-diphenylprop-l-ene) with F-Teda BF4 (6) in the presence of various alcohols results in the formation of vicinal fluoro alkoxy adducts with Markovnikov-type regioselec-tivity.89,94 The stereochemistry of the fluorination-methoxylation addition reaction is slightly syn predominant in the case of (Z)-stilbene, indene, and dibenzosuberenone, while equal amounts of both diastereoisomers are formed in the case of ( )-l-phenylprop-l-ene and acenaphthylene. 

Fluorination.1 The reagent reacts with Irani- and cu-stilbene, preferentially by s> -addilion. Only the sr/i-adducts are isolable in the reaction of 1 with ethyl trans-anti e -cinnamate (equation I). OAc... 

The formation of 59 and related adducts in the reactions of t-1 and several of its ring-substituted derivatives with 1,4-cy-clohexadienes (110) provides the only example of acyclic adduct formation between a stilbene and electron-rich or electron-poor alkenes (see, however, Section VI). Kaupp has proposed that... 

TABLE 13. trans-Stilbene-Amine Adduct Yields and Ratios3... 

The results described in this article establish that the stilbenes are among the most versatile of organic reactants in bimolecular photochemical reactions. Only triplet cyclo-alkenones can rival the ability of It to dimerize, form [2+2] adducts with both electron-rich and electron-poor alkenes, and form acyclic adducts with amines, heterocycles, and noncon-jugated dienes. All of the known bimolecular photochemical reactions of excited stilbenes involve It as the reactive excited state. The failure of - -c, and 3C to undergo bi-... 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

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