Para-substituted stilbenes

A variety of para-substituted stilbenes have been subjected to direct photolysis (Sch. 34). Upon irradiation at 313 nm, 4-halostilbenes give... 

The latter limitation did not prove a disadvantage until recently. The paradigm of second-order nonlinear optics used to be an asymmetrically para-substituted (azo)-benzene or stilbene, and hence a dipolar molecule,... 

Aldol-type condensation of an aromatic aldehyde with activated methylarene or phenylacetic acid is a useful reaction for preparing stilbene derivatives. Starting from para-substituted toluenes or para-substituted aromatic aldehydes, one can obtain 4,4 -disubstituted stilbenes. This reaction is relatively simple but has low yield. As an example, condensation of 2,4-dinitrotoluene and 4-nitrophenylacetic acid with aromatic aldehyde was studied [26]. The reaction involves carbanion addition to the carbonyl group. The carbanion is formed by the extraction of proton from the active methylene group of 2,4-dinitrotoluene by the base (usually, piperidine). The carbanion then adds to carbon atoms of the carbonyl group of the aldehyde. The reaction will therefore be facilitated by the ease of both the formation of the... 

All the other -substituted styrenes 75, similar to cis and trans-stilbene, were also markedly less reactive than styrene with MA. Rates of copolymerization at 60°C in benzene, with AIBN initiator, were found to decrease approximately with the increase in electron-attracting characters of the substituents, e.g., OCH3 > CH3 > CO2CH3 > Cl > COCHa- For the copolymerization of para-substituted -methylstyrene 76 in benzene at 60°C... 

As shown in Scheme 3.9, para-substituted trans-stilbenes were found to be effective substrates for ketone 9, and the enantioselectivity of the epoxidation varied with the size of the substituents on the olefins. The ee values of the epoxide product increased (84 to 95 % ee for 9i) as the substituents became larger (from H to Me to Et to i-Pr to t-Bu). On the other hand, Httle effect on enantioselectivity was observed by the meta-substituents on the phenyl group of the stilbene. 

In 1998, Yang and coworkers reported a ketone with a quaternary carbon at the a-position at one side and a substituent at the fi-position of the other side of the carbonyl group (75) (Scheme 3.54) [88]. Studies on a series of meto- and para-substituted trons-stilbenes with 75b showed that the ee of the epoxide varied with the substituent... 

Brick et al. have studied this bromination in more detail and showed that the extent of the bromination can be controlled by changing the ratio of the reagents. The first substitution was found to be in the para position but subsequent intramolecular rearrangements allowed the formation of 2-5-dibrominated species. Brick et al. also reported the functionalization of such species using Pd-catalyzed reactions such as Heck and Suzuki couplings to give fully substituted p-stilbenes, p-biphenyls, diarylamines, and methylcinnamates. Hydrogenation of... 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

O. Lhost and J. L. Bredas, ]. Phys. Chem., 96, 5279 (1992). Theoretical Study of Torsion Potentials in traws-Stilbene and Substituted truns-Stilbenes Modeling Torsions in Poly(para-phenylene vinylene) and Derivatives. 

Up to now, photochemical studies are still confined to the reactions of simple azobenzene compounds, such as those derived by coupling a phenol or aromatic amine with a diazotised aniline. Azobenzene is isoelectronic with stilbene, its derivatives undergo facile trans cis photo isomerisation reactions in a manner analogous to that of stilbene. Azo dyes which possess a hydroxyl or amino substitutent ortho or para to the azo group... 

Two DHPs were obtained from tetramethoxystilbene [279]. Substitution by a nitro group in the meta or para position reduces distinctly [82], Saltiel et al. have questioned whether the values may be erroneous [105], On the basis of quenching measurements with azulene they proposed additional routes for bromostilbenes from c via excited states of DHP which may relax back to 3c or c. A consequence of a higher value of for the mechanism of cis -> tram isomerization is that the ratio of c decaying to the trans isomer may have to be reexamined. For trans-ct-bromostilbene and the / -phenyl substituted derivatives several photoreactions (e.g., debromination) compete with photocyclization [475]. Interestingly, no evidence for photocyclization could be found for several fluorinated stilbenes [481]. Rotamers can be distinguished in the cyclization of c/s-2,2 -DNE [482],... 

The formation of the benzyl radical by photolysis of toluene in benzene at 4.2 and 77 K has been investigated as a function of light intensity and shown to be unimolecular and biphotonic, and to occur via the lowest excited triplet state. The gas-phase photodissociation of [2,2]paracyclophane (71) into two molecules of /7-quinodimethane and analogous dissociations of two methyl-substituted derivatives have been shown to be efficient two-photon processes.A hot molecule formed by internal conversion from the initially formed singlet electronic excited state is proposed as an intermediate, a mechanism which is completely different from that of the two-photon dissociation of (71) in low-temperature matrices, which involves a triplet state. The photolyses of [2,2]para-cylcophane and [2,2]paracyclophane-l-ene have also been studied in THF and hexane matrices at 77 K, using detection by both luminescence and absorption spectroscopy. The products from both these compounds in THF matrices included alkyl derivatives of rra/15-stilbene, and in hexane matrices alkylphenan-threnes. Cycloreversion of so-called cyclodimers of naphthalene derivatives with benzene or furan, e.g. (72), occurs efficiently upon UV irradiation. ... 

Hexane. 3500—1300 cm. Thin film 2-Methylpentane. 3500—1300 cm . Thin film Dec-l-ene. 3500—1300 cm. Thin film /rfl 5-Stilbene. 1300—400 cm". KBr disc Styrene. 3500—1300 cm". Thin film Styrene 1300—400 cm". Thin film Phenyl Acetylene. 3500—1300 cm". Thin film Phenyl Acetylene. 1300—400 cm". Thin film Aromatic substitution patterns. 2000—1600 cm" para-Cresol. 3500—1300 cm". I.M. CCI4 solution tert-Butyl methyl ketone. 4000—650 cm". Thin film -Heptaldehyde. 4000—650 cm". Thin film Di-/sopropyl ether. 4000—650 cm". Thin film Acetic Anhydride. 4000—650 cm". Thin film Propionic acid. 4000—650 cm". Thin film Propionic acid. 3500—2000 cm". Solution 0.005M CCI4 Methyl salicylate. 4000—650 cm . Thin film n-Butylamine. 4000—650 cm". Thin film Benzamide. 3500—1300 cm". KBr disc Methionine. 4000—650 cm". KBr disc Benzonitrile. 3500—1300 cm". Thin film Benzonitrile. 1300—400 cm". Thin film A-Methyl acetamide. 3500—650 cm". Thin film Methyl acrylate. 4000—650 cm". Thin film Benzoyl chloride. 4000—650 cm". Thin film Triphenyl phosphate. 3500—1300 cm". Melt Triphenyl phosphate. 1300—400 cm". Melt Di-wopropyl sulphone. 4000—650 cm". Melt Nitrobenzene. 4000—650 cm". Thin film Dimethyl sulphoxide. 4000—650 cm". Thin film Polymeric silicone. 4000—650 cm". Thin film Calcium sulphate Dihydrate. 4000—650 cm". KBr disc... 

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