1.2- Distyrylbenzene

Heck in one of his first papers already demonstrated the feasibility of applying the palladium-catalyzed crosscoupling of aryl and alkenyl halides with alkenes repetitively on appropriate oligofunctional substrates. For example, twofold coupling of 1,4-diiodobenzene with styrene furnished 1,4-distyrylbenzene in 67% yield (Scheme 1). Since then, a large number of ortho-, meta-, and / r< -dihaloarenes and -heteroarenes have been subjected to twofold Heck reactions with various alkenes (Schemes 2-4). 

Fourfold Heck coupling (twofold on each dibromobenzene moiety) and Jeffery conditions has been applied on the two easily accessible tetrabromo[2.2]paracyclophane isomers 21 and 23, respectively, to prepare new double-layered 1,2- and 1,4-distyrylbenzene chromophoric systems 22-R and 24-R respectively, to enable studies of intramolecular charge-transfer phenomena in such systems (Scheme... 

Scheme 7 New double-layered 1,2- and 1,4-distyrylbenzene chromophores by fourfold Heck couplings. A Pd(OAc)2, NBu4Br, K2CO3,...

Cyano-substituted 1,4-distyrylbenzenes (Table 7.1) are carbocyclic brighteners with a very high whiteness and good efficiency for plastics and synthetic fibers, especially polyesters [28], Asymmetrical compounds, mixtures of two or three dif-... 

Both decks of the [2.2]paracyclophane derivatives 160 and 161 could be transformed into 1,4-distyrylbenzene chromophores via the palladium-catalyzed coupling (Heck reaction) with styrene and substituted styrenes adopting the protocol of Jeffery [106]. The fourfold coupling products 162 and 163 were thus obtained with yields up to 70% [107a]. As four new carbon-carbon bonds are formed in this transformation, each single coupling step must proceed with excellent yield. All these reactions lead stereoselectively to products with (E)-configurated double bonds. 

These phanes consisting of 1,4-distyrylbenzene units could also be reduced with alkali metals (Li, Na, K) in THF solution to give multiply charged species. The tetraanion of 163 (R = C6H5) is produced in four steps, its charge distribu-... 

Solutions of both the tetrastyryl[2.2]paracyclophanes 162 and 163, respectively, show intense blue-green fluorescence when exposed to sunlight. In comparison to that of 2,5-dimethyl-1,4-distyrylbenzene the UV/vis spectrum of 163 (R=Ph) shows a bathochromic shift of 41 nm for the longest wavelength absorption Amax= 394 nm) and a significant increase in the extinction coefficients. Excitation of 163 (R=Ph) at 394, 355 or 339 nm leads to fluorescence with a broad, unstructured emission band at 465 nm, which is shifted by 61 nm towards longer wavelengths in comparison to that of 2,5-dimethyl-1,4-distyrylbenzene with a well structured band at 404 nm. The relative quantum yield

[Pg.127]

The (Z)-stilbene (6) does not form a rotaxane with dibenzo-24-crown-8 but, when the stilbene is isomerized (sensitized by benzil) to the E form, a pseudo-rotaxane is formed.Lewis and Crompton have reported that the photoacidity of the stilbene derivatives (7) and (8) is dependent on the position of the hydroxy group on the stilbene, and as a consequence the meta derivative undergoes isomerism.The influence of the nitrogen atoms on the photochemistry of the 1,4-distyrylbenzene analogue (9) and related compounds has been assessed. The photophysics and conformational analysis of the stilbene dimer (10) have been studied. Modelling of the optical properties of the stilbenophane (11) has been reported. 

Srinivasan et al. have presented dynamics studies of guest molecules (partially deuterated 1,4-distyrylbenzene (2PV)) in the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of 2PV is investigated over a wide temperature range by NMR as well as fluorescence spectroscopy. The obtained results are further corroborated by MD simulations. 

For the calculation of /F/j and I/F we have first calculated the valence force field by the MNDO method on the molecule of trans-trans 1-4 distyrylbenzene (DST) [176]. Full geometry minimisations were carried out by keeping the molecule coplanar. Further improvement of the force field was obtained by least squares fitting of a... 

The multiple coupling of oligohaloarenes with alkenes was described early on by Heck and Nolley in one of their first papers on this new type of palladium-catalyzed cross-coupling reaction. Twofold coupling of 1,4-diiodobenzene with styrene furnished the 1,4-distyrylbenzene in 67% yield, shortly afterward, the double Heck... 

In principle, the double photocycHzation of 1,4-distyrylbenzenes offers a convenient route to dibenz[a,h]anthracenes and [5]helicenes dependent on the sites of reaction. In the first report of the photochemistry of 1,4-phenylene bisjphenylmaleic anhydride) 27, the product was identified as the dibenz[a,h]anthracene 28," but more recently it has been shown that a mixture of isomers is formed (overall yield of 80%), in which 28 comprises less than 5%, with the (5]helicene 29 as the major product. This approach towards the synthesis of polynuclear arenas, including helicenes, is complemented by the photocyclizations of styryl-naphthalenes, -phenanthrenes, etc. and of 1,2-dipolyarylethenes. Examples of these processes pubhshed during the review period are now considered. 

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