Stilbenes reactions

Before we cite another experiment that supports the idea that isomerization occurs from the phantom triplet state, it will be helpful to complete our set of stilbene reactions and construct an experimental potential energy diagram. 

A paper by Meijer et al. described the synthesis and electronic properties of a series of PPE-PPV hybrid oligomers. The approach is shown in Scheme 11 [70]. Starting from a brominated stilbene, reaction with a series of diethynyl-benzenes furnished pentameric PE-PV hybrids in yields around 80%. The materials are used as emitters in LEDs and other electrooptical devices. It was noted that fibrous aggregates formed upon melting and recooling of thin films of these pentameric PE-PV hybrids. 

A rather indirect mode of reaction accounts for 60% of the product trans-stilbene (reaction sequence 1, 2, 4, 5, and 6 in Table VII). The importance of this path may be traced to both the low disproportionation/recombination ratio for resonance stabilized radicals, and the high rate constant for 3-bond scission of the intermediate radical recombination product, 1,2,3,4-tetraphenylbutane. Analogous decomposition routes involving formation of an adduct, followed by irreversible destruction of the adduct, may be important reaction paths in many other coal related reaction systems. 

A lesser degree of loss of configuration was observed in the reactions of cis- 36> and iraMS-stilbene 35> with tetrachloro-o-benzoquinone to give dioxenes. The m-stilbene reaction (in acetone or acetonitrile) afforded about 80% of cis-adduct while 88% of trans-a.d6.net 35> was obtained from the reaction of trans-stilbene (both reactions at 15°, A >400 nm). Curiously, irradiation of the quinone and cfs-stilbene (or diphenyl-acetylene) in benzene solution produced the mono-phenyl ether (33)... 

St5u-ene can be converted into stilbene, reaction (13), using W(CO)3(PhMe) in... 

Catalyst precursor 5% of [Pd((i ,i )-Me-DuPhos)(fran -stilbene)], reactions performed at room... 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

RRKM fit to microcanonical rate constants of isolated tran.s-stilbene and the solid curve a fit that uses a reaction barrier height reduced by solute-solvent interaction [46],... 

As a multidimensional PES for the reaction from quantum chemical calculations is not available at present, one does not know the reason for the surprismg barrier effect in excited tran.s-stilbene. One could suspect diat tran.s-stilbene possesses already a significant amount of zwitterionic character in the confomiation at the barrier top, implying a fairly Tate barrier along the reaction path towards the twisted perpendicular structure. On the other hand, it could also be possible that die effective barrier changes with viscosity as a result of a multidimensional barrier crossing process along a curved reaction path. 

Abrash S, Repinec S and Hochstrasser R M 1990 The viscosity dependence and reaction coordinate for isomerization of c/s-stilbene J. Chem. Phys. 93 1041-53... 

The phenylation of styrene with phenyl Grignard reagents as a hard carbon nucleophile proceeds in 75% yield in the presence of PdCl2, LiCl, and K2CO3 at room temperature to give stilbene (207). Selection of the solvent is crucial and the best results are obtained in MeCN. The reaction can be made catalytic by the use of CuCl2[197]. Methyllithium reacts with styrene in the presence of Pd(acac)2 or Pd(OAc)2 to give /3-methylstyrene (208) in 90% yield[198]. 

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]. 

Chlorides are inert. However, the reaction ofp-chlorobenzophenone (9) with a styrene derivative proceeds satisfactorily at 150 C by u.sing dippb [l,4-bis(-diisopropylphosphino)butane] as a ligand to give the stilbene derivative 10. However, dippp [l,3-bis(diisopropylphosphino)propane] is an ineffective ligand[13]. On the other hand, the coupling of chlorobenzene with styrene proceeds in the presence of Zn under base-free conditions to afford the cis-stilbene 11 as a main product with evolution of H . As the ligand, dippp is... 

By the reaction of bromotoluene with ethylene under pressure, p-methylsty-rene and stilbene (45) are obtained[44,45]. A polymer 47 is obtained by the reaction of >-bromostyrene (46) with ethylene. The reaction has been applied to polymer synthesis[46]. One example is the reaction of 1,4-divinylbenzcne (48) with 9,10-dibromoanthracene to give the oligo(arylenevinylene)s 49[47]. 

Vinyl acetate reacts with the alkenyl triflate 65 at the /3-carbon to give the 1-acetoxy-1,3-diene 66[68]. However, the reaction of vinyl acetate with 5-iodo-pyrimidine affords 5-vinylpyrimidine with elimination of the acetoxy group[69]. Also stilbene (67) was obtained by the reaction of an excess of vinyl acetate with iodobenzene when interlamellar montmorillonite ethylsilyl-diphenylphosphine (L) palladium chloride was used as an active catalyst[70]. Commonly used PdCl2(Ph3P)2 does not give stilbene. 

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]. 

Stilbene Derivatives. Most commercial brighteneis aie bistria2inyl derivatives (1) of 4,4 -diarmnostilbene-2,2 -disulfonic acid (Table 1). The usual compounds are symmetric preparation begias with reaction of 2 moles of cyanuric chloride derivatives with 1 mole of... 

Bis(benZoxaZol-2-yl) Derivatives. Bis(benzoxazol-2-yl) derivatives (8) (Table 3) aie prepared in most cases by treatment of dicaiboxyhc acid derivatives of the central nucleus, eg, stilbene-4,4Cdicarboxyhc acid, naphthalene-l,4-dicarboxyhc acid, thiophene-2,5-dicarboxyhc acid, etc, with 2 moles of an appropriately substituted o-aminophenol, followed by a ring-closure reaction. These compounds are suitable for the brightening of plastics and synthetic fibers. 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

A different approach, taken by both Monsanto (58) and Gulf Research and Development Company (59), involved the oxidative coupling of two molecules of toluene to yield stilbene. The stilbene is then subjected to a metathesis reaction with ethylene to yield two molecules of styrene. 

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. 

The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

The reaction of cinnoline 2-oxide with phenylmagnesium bromide gives phenanthrene, trans- and cfs-stilbene, 2,3-diphenyl-l,2-dihydrocinnoline and 2-styrylazobenzene in yields of 1-15%. Analogous results are also obtained from 4-methylcinnoline 2-oxide. 

N- Aminoaziridines have been converted to alkenes by reaction with a variety of oxidizing agents (70JA1784). Usually, the deamination reaction is stereospecific. The oxidation of l-amino-2,3-diphenylaziridines with manganese dioxide, however, was not stereospecific. The trans compound gives entirely frans-stilbene, whereas the cfs-aziridine forms a mixture of 85% trans- and 15% c -aikene. cw-Stilbene is not isomerized to trans under the reaction conditions, and the results are explained in terms of an azamine intermediate which can isomerize through a tautomeric equilibrium. 

Fig. 13.9. Schematic reaction profile for the EZ isomerization of stilbene. The reaction coordinate 6 is the torsion angle about the double bond. [From FI. Meier, Angew. Chem. Int. Ed. Engl. 31 1399 (1992)]. Reproduced by permission of Wiley-VCH.

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

The reaction of benzylmagnesium chlorides wnth thiophenealde-hydes and thienyl ketones has been used for the preparation of styrylthiophenes and 1,2,2-triarylethylenes, which are of biological interest. In stilbene and 1,2,2-triphenylethylene the reactivity toward electrophilic reagents is transferred with deactivation to the double bond. However, styrylthiophene is formylated and acylated... 

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