Stilbenes addition

Figure 14.17. Stilbene addition in Corey s osmium complex...

Nucleophilic attack by a carbanion on a rhodium-acetylene complex has been postulated in the reaction of tolane with MeMgBr in the presence of (Ph3P)3 RhBr (Michman and Balog, 1971). This interpretation is supported by the known formation of rhodium-alkyne complexes under these conditions (Muller and Segnitz, 1973) and by the predominant formation of trans-a-methyl-stilbene. Additional study of the scope of this reaction would seem worthwhile since recycling of the rhodium-containing by-product (Ph3P)3 RhOH, should be possible. 

Stilbene decolorises bromine only on heating. Proceed as above, but keep the stilbene solution hot during the addition of the CCI4 solution of bromine. Stilbene dibromide has m.p. 237 . 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Arenediazonium salts are also used for the couplina[563], (Z)-Stilbene was obtained unexpectedly by the reaction of the ti-stannylstyrene 694 by addition-elimination. This is a good preparative method for cu-stilbene[564]. The rather inactive aryl chloride 695 can be used for coupling with organostannanes by the coordination of Cr(CO)3 on aromatic rings[3.565]. 

Bis(azol-2-5l)stilbenes (2(i]ll such as (4) have been prepared. 4,4 -Dihydrazinostilbene-2,2 -disulfonic acid, obtained from the diamino compound, on treatment with 2 moles of oximinoacetophenone and subsequent ring closure, leads to the formation of (4) [23743-28 ]. Such compounds are used chiefly as washing powder additives for the brightening of cotton fabrics, and exhibit excellent light- and hypochlorite-stabiUty. 

Phytoalexins are low molecular weight compounds produced in plants as a defense mechanism against microorganisms. They do, however, exhibit toxicity to humans and other animals in addition to microbes (30). Coumarins, glycoalkaloids, isocoumarins, isoflavonoids, linear furanocoumarins, stilbenes, and terpenes aU. fall into the category of phytoalexins (31). Because phytoalexins are natural components of plants, and because their concentration may increase as a response to production and management stimuli, it is useful to recogni2e the possible effects of phytoalexins in the human diet. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

Electrochemical reduction of 2,3-diphenylthiirene 1-oxide yields acetylene (80%) and benzil (10%). Electrolysis of 2,3-diphenylthiirene 1,1-dioxide in DMF gives trans-stilbene (30%) but in the presence of acetic acid, 1,2-diphenylvinylmethyl sulfone (27%) is obtained in addition to the stilbene (40%) (81CC120). 

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). 

Further improvement in light stability may be achieved by addition of small quantities of ultraviolet absorbers. Typical examples include phenyl salicylate, 2,4-dihydroxybenzophenone, resorcinol monobenzoate, methyl salicylate and stilbene. 

Laudanosine contains four methoxyl groups. By exhaustive methyla-tion it yields trimethylamine and laudanosene (tetramethoxy-o-vinyl-stilbene), CH2=CH—C6H2(OCH3)2—CH=CH—C6H3(OCH3),. On oxidation with manganese dioxide and sulphuric acid it furnishes, in addition to the interesting by-product 2 3 6 7-tetramethoxy-9 10-dihydroanthracene, veratraldehyde and 4 5-dimethoxy-2 )3-methyl-... 

A versatile approach to the 5-ary ldibenzo[7>,/]oxcpin system is the electrophilic ring-closure reaction of 2-phenoxy-substituted diphenylacetylenes or 1-bromostilbenes. Cyclization of the alkynes 4 occurs after addition of a suitable electrophile such as H +, PhS+, Br+, or I+ to give products 5.100-101 Addition across the triple bond gives stilbenes as byproducts. 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

In fact, the stabilization of the diion formed from tetraphenyl ethylene is so great that the monoion cannot be isolated. On the other hand, the stabilization of the diion of stilbene is less pronounced, and hence both the monoionic and the diionic species are observed. In contrast to these cases, the addition of a second electron to the styrene" ion or to the 1,1 diphenyl ethylene- ion would yield a diion possessing one unstabilized end charge, i.e.,... 

Several compounds that inhibit vesicular glutamate transport have been identified These include the dyes Evans Blue and Rose Bengal. In addition, the stilbene derivative 4,4 -diisothiocyanatostilbene-2,2 -disulfonic acid (DEDS), a compound commonly used as a specific inhibitor of anion channels, inhibits vesicular glutamate transport. Most known inhibitors have limited use as they are membrane impermeant, with the exception of Rose Bengal. 

The lead tetraacetate reaction between jV-aminophthalimide and (E) stilbene was first described by Rees,5 and the hydrazinolysis of the addition product by Carpino.6 The procedures described here incorporate their methods, with improvements. The dimesylate-hydrazine reaction was first described by Paulsen7 in the carbohydrate series. 

In addition to the crosslinks already mentioned, there exist a number of possibilities for secondary reactions, so that the final resin may consist of a number of crosslinks whose origin is not immediately clear. For example, this is evidence that some resins contain units that are effectively derivatives of stilbene (vinylbenzene), implying that there are units in the cured resin corresponding to that illustrated (4.1). 

Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

The suspension may be warmed to dissolve the alkene more rapidly. (E)-Stilbene dissolves completely at ca. 65°. If the suspension is warmed, it must be cooled below 30° before proceeding further to prevent a vigorous reaction when the N-bromo-succinimide is added. The submitters recommend that the warm suspension be cooled under an atmosphere of nitrogen. If a volatile alkene is used, the mixture should be cooled prior to and during the addition of N-bromosuccinimide to prevent losses by evaporation. 

In addition to linear PPV oligomers, cyclic oligomers have been intensively investigated. Wennerstrom et al. described the synthesis of a cyclic cis,trans, cis,cis,trans,cis (Z,E,Z,Z,E,Z)-OPY hexamer (112) by a fourfold Wittig-type ole-fination, starting from two molecules of frfln5-stilbene-4,4 -dicarboxaldehyde and two molecules of l,4-xylylene-bis(triphenylphosphonium) dibromide [133]. 

The stereochemistry of addition reactions of Br2 to olefins is difficult to control. For example, the reaction of ( )-stilbene (1) with Br2 in CH2CI2 gives a 84 16 mixture of meso- (2) and rac-l,2-dibromo-l,2-diphenylethane (3) in 98%... 

Hydroarylation, (addition of H-Ar, Ar = aryl), of alkynes, catalysed by Pd(OOCCH3)2 or Pd(OOCCFj)j in acetic acid, is an atom-economic reaction, giving rise to substituted c/i-stilbenes (Fujiwara reaction). Catalytic conversions and improved chemoselectivity to the mono-coupled product under mild conditions can be achieved by modification of the metal coordination sphere with NHC ligands. Hydroarylation of mesitylene by ethylpropiolate (Scheme 2.19) catalysed by complex 107 (Fig. 2.18) proceeds in good conversions (80-99%, 1 mol%) under mild conditions at room temperature. 

A substantial amount of syn addition is observed for Z-l-phenylpropene (27-80% syn addition), E-1 -phenylpropcnc (17-29% syn addition), and ra-stilbene (up to 90% syn addition in polar solvents). 

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