1.3.5- Hexatriene arylation

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

In the absence of nucleophiles, the elimination of /3-hydrogen from tr-allyl-palladium intermediates to give the (E, )-l-aryl-l,3-dienes 267 and 268 takes place[173,180]. l-Aryl-l,3,5-hexatriene 269 reacts with an aryl bromide to give the 1,6-diaryl-1,3,5-hexatriene 270. The best results are obtained with aryl halides having EWGs in the para-position[181]. 

Hexatriene may be arylated similarly to 1,3-butadiene. Iodobenzene and aryl halides with electron-donating substituents require more catalyst (as in the butadiene reactions) than aryl halides with electron-withdrawing substituents to give 1,6-diarylated hexatrienes in a reasonable time. 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

It has been successful to isolate intermediates and to identify them by NMR spectroscopy as 3-aryl-6-dialkylamino-l-hydroxy-1,4,5-triaza-1,3,5-hexatriene. Treatment with potassium permanganate of these open-chain structures gave ring closure into 3-dialkylamino-l,2,4-triazine-4-oxides. Also, in these reactions it was observed that the regiospecificity of the addition is temperature dependent. Whereas at —40°C the addition occurs at C-3, at -70 °C the addition takes place at C-5 (01H127). Whether on oxidation at —70°C the 5-dialkylamino-6-phenyl-l-2-4-triazine-4-oxide is formed is unknown. 

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

Scheme 11 Cyclopropylallenes as precursors for 2-aryl-1,3,5-hexatrienes and their subsequent [4 + 2] cycloadditions [53,54]...

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

Planar, conjugated molecules fluoresce, whereas saturated molecules and those with only one double bond do not usually a molecule must possess at least one aromatic ring if one is to observe fluorescence or phosphorescence. Even conjugated olefins such as 1,3,5-hexatriene do not fluoresce however, aryl-substituted olefins do. For example, rrun -stilbene, which is planar, fluoresces c -stilbene does not, presumably because it is not planar. 

Knoke, M. and de Meijere, A. (2003) A versatile access to l-cyclopropyl-2-aryl-1,3,5-hexatrienes - domino Heck-Diels-Alder reactions of l,3-dicyclopropyl-l,2-propadiene. Synlett, 195-8. 

SCHEME 3.14 Synthesis of 1,3-butadienes and 1,3,5-hexatrienes by palladium-eatalyzed cross-eoupling of aryl iodides, diarylacetylenes,... 

H. Horiguchi, K. Hirano, T. Satoh, M. Miura, Adv. Synth. Catal. 2009, 351, 1431-1436. Palladium-catalyzed three-component 1 2 1 coupling of aryl iodides, alkynes, and aUcenes to produce 1,3,5-hexatriene derivatives. 

Shibata et al. demonstrated an intermolecular three-component coupling of aryl or vinylhalides, diarylacetylenes, andmonosubstitutedalkenes, leading to the corresponding 1,3-butadiene or 1,3,5-hexatriene derivatives in the presence of palladium species [40] (Scheme 6.23). 

Cationic diarylethenes were synthesized by appending an imidazolium directly connected to the central ethane unit as an aryl group in order to participate in the photochromic hexatriene-cyclohexadiene reaction. Scheme 4.9. [47] At least one of these compounds (1-open form) is described in the experimental part as being an oil, and probably is an intrinsically photochromic ionic liquid, see below. [48] The solvatochromic behaviour of 1 was studied in two ionic liquids, [EMIM][NTf2] and [EMIM] [EtS04], as well as in some common solvents. The closed ring form shows different absorption spectra depending on the anion of the IL. The red form is stabilized in [EMIM] [EtSOJ while the yellow form in... 

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