Hexa-1,3,5-triene

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Fukui at Kyoto Imperial University in Japan introduced a simplified version. According to Fukui, we need to consider only two molecular orbitals, called the frontier orbitals. These frontier orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In ground-state 1,3,5-hexa-triene, for example, 1//3 is the HOMO and excited-stale 1,3,5-hexatriene, however, 5 is the LUMO. 

The other possible linear vinylallene vinylog, 1,2,4,6-heptatetraene (22), has been prepared by the DMS route from the mono dibromocarbene adduct to 1,3,5-hexa-triene, 210 [78], practically simultaneously with the methyl derivative 211 [79] and several higher alkylated derivatives of 22, 214 (Scheme 5.31) [80],... 

Fig. 5 a and b. Schematic representation of a highest occupied MO, b lowest unoccupied MO in hexa-triene... 

A solution of sodium hydroxide (106 g., 2.7 moles) in 535 ml. of water is placed in a 2-1. flask equipped with a sealed mechanical stirrer and an outlet arranged for downward distillation into an ice-cooled receiver. The aqueous solution of the quaternary bromide is added dropwise to the boiling solution of sodium hydroxide during a period of 2.5-4 hours (Note 9). The hexa-triene and dimethylbenzylamine which form are distilled with the water. Distillation is continued for 10-15 minutes after the final addition of quaternary bromide solution. The clear upper layer of the distillate is separated, cooled to 5-10°, washed with three 170-ml. portions each of cold 2N hydrochloric acid and water, and dried over anhydrous sodium sulfate. The oil is then distilled,... 

Figure 14.6. (ia) Orbital correlation diagram for the photochemical electrocyclic reaction of butadienes. (b) Orbital correlation diagram for for the photochemical electrocyclic reaction of hexa-trienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

Acetone, methyl ethyl ketone (2-butanone) and methyl isobutyl ketone (4-methyl-pentan-2-one) (6.8 mmol/kg bw for 3 days) increased the hepatotoxicity of carbon tetrachloride to Sprague-Dawley rats (Raymond Plaa, 1995a) this enhancement of toxicity was coincident with increased microsomal aniline hydroxylase activity (Raymond Plaa, 1995b). In addition to the effect on cytochrome P450, acetone, but not the other ketones, increased basal canalicular membrane fluidity, as measured by fluorescence polarization of 1,6-diphenyl-1,3,5 -hexatriene or 1 - [4-(trimethylammoniumphenyl)-6-phenyl] -1,3,5 -hexa-triene (Raymond Plaa, 1996). 

The preparation of unsymmetrically terminally disubstituted 1,3,5-hexa-trienes by twofold Heck coupling of 162-m with two different alkenes cannot be achieved, since the second coupling step turned out to be significantly faster than the first one [96]. Sufficient differentiation could not be brought about with a triflate and a bromide leaving group as in 168. How-... 

Quite often, when calculating the delocalization energies of cyclic polyene systems, the latter are compared with isoconjugated acyclic polyenes, e.g. benzene with hexa-triene etc. 

It is not possible within the restrictions of this review to treat all carbocyclic frameworks (e.g. spirocy-clics) and a conscious decision was also made specifically to exclude the heterocyclic version of this electrocyclization. The number and variety of heteroatoms and the possible permutations along the hexa-triene system that can a priori be considered would justify a separate treatment. The reader is referred to the excellent monograph by Marvell for references to the treatment of electrocyclizations leading to heterocycles. However, in order to provide at least a few leading references, several selected examples of hetero electrocyclizations are included in this chapter together with comments concerning retro electrocyclizations and higher-order electrocyclic processes. 

Excited States. The first absorption region in the ultraviolet spectrum of cycloheptatriene extends from 3300 to 2250 A. The absorption is intense and the transition is presumably a ir — ir excitation from the ground singlet to an upper singlet state. The radiative lifetime has been calculated to be about 10 sec. No fluorescence has actually been observed. There are no data available on the triplet states, but by comparison with the singlet — triplet absorption spectrum of 1,3,5-hexa-triene, which has been recorded by Evans, it is reasonable to assume that l,3,o-cycloheptatriene has its lowest triplet state at 47 kcal./mole or less. A second triplet state has also been reported for 1,3,5-hexatrienc by Evans. ... 

Figure 2.6. Theoretical and experimental excitation energies of butadiene, hexa-triene, and benzene. The calculations include all singly excited configurations (S). all singly and doubly excited configurations (D) or complete Cl (C), respectively (by permission from Schulten et al.. 1976a).

The quantum chain process has also been observed in benzophenone-sensitized isomerization of 2,6-dimethyl-2,4,6-octatriene (25) [120]. The effect of azulene on the photostationary state isomer composition suggests that both tt and tc are the stable conformers in the triplet state and are equilibrated. However, equilibration of all excited intermediates is not complete within the lifetime of the excited triplet state ( 50 ns). The triplet lifetime of 1,3,5-hexa-triene (26) is reported as 100 ns (Table 5) [121]. 

Diphenyl-cyclo- hexadien-(l,3)d unsensibilisiert 1,1- Diphenyl-hexa -trien-(l,3,3) it 4... 

Andrich, M.P., and Vanderkooi, J.M., Temperature dependence of 1,6-diphenyl-1,3,5-hexa-triene fluorescence in phospholipid artificial membranes, Biochemistry, 15, 1257, 1976. Blitterswijk, W.J.V., Hoeven, R.P.V., and Dermeer, B.W.V., Lipid structural order parameters (reciprocal of fluidity) in biomembranes derived from steady-state fluorescence polarization measurements, Biochem. Biophys. Acta, 644, 323, 1981. 

Are electrons still delocalized even when there is no ring To consider this, we ll look at hexa-triene—three double bonds and six carbons, like benzene, but without the ring. There are two isomers of hexatriene, with different chemical and physical properties, because the central double bond can adopt a cis or a trans geometry. The structures of both cis- and trans-hex-atriene have been determined by electron diffraction and two important features emerge ... 

A similar consideration leads to the following symmetry properties for the four Tc-molecular orbitals of butadiene and six Tt-molecular orbitals of hexa-triene and are summarized in Figure 1.11. 

B. Wustenberg, N. R. Branda, A photoswitchable donor-n-Unker-acceptor system based on a modified hexa-triene backbone, Adv. Mater., 17, 2134-2138 (2005). 

Figure 5.2. Rudimentary correspondence diagram for cyclization of czs-l,3,5-hexa-triene...

---