L, 3-Hexadiene

Before examining the reaction of deactivated alkenes, the phosphonium salt synthesis was applied to 1,3-dienes.21 When ( )-6-phenyl-l,3-hexadiene was treated with equimolar amounts of PPh3 and CF3S03H in the presence of RhH(PPh3)4 (2.5 mol%) in THF at 0 °C for 3 h, ( )-(6-phenyl-3-hexenyl)triphenylphosphonium salt was obtained in 89% yield after anion exchange with LiPF6 and recrystallization (Scheme 16). The addition of phosphine and hydrogen occurred at the 1- and 2-carbon atoms of the 1,3-diene, respectively. The reaction of (7)-1,3-dienes was then performed for comparison. 

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

If both ethylene subunits of a l,3-hexadien-5-yne derivative are members of benzene entities, the cycloaromatization, caused by flash vacuum thermolysis, may give rise to bowl-shaped molecules such as corannulene or semibuckminsterfullerene. However, in those cases, the initial step is not a Hopf cydization but an isomerization of the ethynyl to a vinylidenecarbene group [125]. 

Catalytic forms of copper, mercury and silver acetylides, supported on alumina, carbon or silica and used for polymerisation of alkanes, are relatively stable [3], In contact with acetylene, silver and mercury salts will also give explosive acetylides, the mercury derivatives being complex [4], Many of the metal acetylides react violently with oxidants. Impact sensitivities of the dry copper derivatives of acetylene, buten-3-yne and l,3-hexadien-5-yne were determined as 2.4, 2.4 and 4.0 kg m, respectively. The copper derivative of a polyacetylene mixture generated by low-temperature polymerisation of acetylene detonated under 1.2 kg m impact. Sensitivities were much lower for the moist compounds [5], Explosive copper and silver derivatives give non-explosive complexes with trimethyl-, tributyl- or triphenyl-phosphine [6], Formation of silver acetylide on silver-containing solders needs higher acetylene and ammonia concentrations than for formation of copper acetylide. Acetylides are always formed on brass and copper or on silver-containing solders in an atmosphere of acetylene derived from calcium carbide (and which contains traces of phosphine). Silver acetylide is a more efficient explosion initiator than copper acetylide [7],... 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Heptadien-l-yne is less volatile and presumably more stable than l,3-hexadien-5-yne. Distilladve separation at atmospheric pressure from El O should therefore be possible without involving the risk of polymerization or decomposition. This allows the preparation of the tosylate and its conversion into the dienyne in the same pot, using EtjO as solvent. Other enynes with a b.p. >100 C/760 mmHg can be prepared in a similar way. 

VI 5-Methyl-l,3-hexadien A1(C2H5)3 -f- VClj in n-Heptan 88 trans-isotaktisch 4,85 (73)... 

Hexatriene has been prepared by many workers. The more successful methods are the catalytic pyrolyses (alumina, 260-325°) of l,3-hexadien-5-ol6 7 8 and 2,4-hexadien-1 -ol.9 Other methods which give hexatriene of questionable purity or involve less convenient laboratory methods are dehydration of 1,5-hexadien-3-ol by sodium bisulfate at 170°,10 or by phthalic anhydride at 160-200°,2 and by catalytic hydrogenation of di-vinylacetylene.11 Additional methods are listed in footnote 5. The present procedure is a practical laboratory method of preparing pure hexatriene in satisfactory yields. 

The VEEL spectra of the species formed from cyclohexane on Pt(lll) show that at least two intermediate species occur along the decomposition pathway to benzene. These spectra are discussed in Sections VI.A and VI.C, in the context of spectra of species formed from adsorbed cyclohexene (239) and cyclo-l,3-hexadiene (240) on the same surface. On Pt(100) hex, in contrast to Pt(lll), most of the cyclohexane molecules desorb before conversion to benzene, but the latter was formed after adsorption at 300 K. An intermediate in the conversion of cyclohexane into benzene on Pt(100) (1 X 1), stable between ca. 200 and 300 K, was recognized spectroscopically, but not structurally identified, by RAIRS (230) and by VEELS (224). It seems that there is a smooth transition from the spectrum of adsorbed cyclohexane on Pd(100) to that of benzene at temperatures exceeding 250 K without the detection of intermediate spectra (220). 

Methylenecyclopropane is known to have an extra strain of 11 kcalmol-1 (theory)59 and 13 kcalmol-1 (experiment) over cyclopropane60. Alkylidenesiliranes (E/Z-79 and 80) were prepared by photolysis of 2,2-dimesitylhexamethyltrisilane in the presence of tert-butyl allenes61-64. Whereas these alkylidenesiliranes were stable in air at temperatures below 80 °C for at least one day, under photochemical conditions they rapidly decomposed. The photolysis of (isolated) 81 gave (E/Z)-3-silyl-l,3-hexadienes... 

Treatment of 1-phenyl-2,4-hexadien-l-ol with the acid chloride of fumaric acid monomethyl ester in ether/triethylamine gave a quantitative yield of the crude ester as an oil. Attempted purification by Kugelrohr distillation at 180-190°C under high vacuum gave the bicyclic lactone 1 in 45% yield. It was subsequently found that the fumarate esters of the isomeric alcohols 1-phenyl-l,4-hexadien-3-ol and l-phenyl-l,3-hexadien-5-ol also gave compound 1 in 43 and 35% yield respectively when distilled in a Kugelrohr apparatus. 

The formulas of 37-39 show the substitution of three carbonyls by a conjugated diene. Due to the fact that dienes are four-electron donors, the complexes are electron-deficient species. The electron deficiency is reduced by formation of C—H—Cr three-center, two-electron bonds, previously found for a number of other complexes (43). When 2-methyl-5-isopropyl-l,3-cyclohexadiene is used as a potential ligand, the expected complexes 37r and 39r are not obtained, but instead complexes with the isomeric l-methyl-4-isopropyl-l,3-cyclohexadiene group (37r and 39r ) are formed [Eq. (19)]. Similarly, lk and 11 form not only the corresponding complexes 39k and 391, but also the isomeric complexes with (Z)-l,3-hexadiene (39k ) and with (Z)-2-methyl-1,3-pentadiene (391 ). 

A study on the homo- and copolymerization of a variety of dienes such as 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, E-l,3-pentadiene, E-l,3-hexadiene, E-l,3-heptadiene, E-l,3-octadiene, E,E-2,4-hexadiene, E-2-methyl-l,3-pentadiene, 1,3-cyclohexadiene mainly focused on mechanistic aspects [139]. It was shown that 1,4-disubstituted butadienes yield frans-1,4-polymers, whereas 2,3-disubstituted butadienes mainly resulted in cis- 1,4-polymers. Polymers obtained by the polymerization of 1,3-disubstituted butadienes showed a mixed trans-1,4/cis-1,4 structure (60/40). The microstructures of the investigated polymers are summarized in Table 26. 

Malaga et al. [114] also proposed a diradical intermediate based on picosecond laser spectroscopic data. Excitation of l,3-hexadienes/9-cyanoanthracene showed an unidentified absorption band in addition to the respective ion radicals, which might be attributed to 1,6-diradical intermediates. 

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