Hexatriene closing

This method is trivially extendable to longer systems. Hexatriene closes to cyclo-hexadiene in just the manner predicted by the frontier orbitals. The only significant change in going from butadiene to hexatriene is that we go from four to six n electrons. This means that the HOMO for hexatriene has one more node than that for butadiene (or, the HOMO for a 2n tt-electron system is like the LUMO for a 2/2—2 tt-electron system insofar as end-to-end phase relations are concerned). The net effect is that the predictions for hexatriene are just the reverse of those for butadiene. That is, hexatriene closes thermally by the disrotatory mode and photochemically by the conrotatory mode. The general rule, called a Woodward-Hojfmann rule, is this the thermal electrocyclic... 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

The amount of high precision experimental structural data on conjugated polyenes is limited. Some structure results are presented in Table 5. In gas electron diffraction studies it is difficult to determine closely spaced bond distances accurately, because these parameters are highly correlated with the corresponding vibrational amplitudes. Today it is possible to calculate the vibrational amplitudes accurately, if the vibrational frequencies are known. This was, however, not the case when the GED studies presented in Table 5 were carried out. The observed differences between the terminal and central C=C bonds in the GED studies of traw.s-l,3,5-hexatriene and c/s-l,3,5-hexatricne are probably too large29. A very accurate X-ray study of traw.s-l,3,5-hexatriene has, however, been carried out also in connection with the preparation of this chapter4. Figure 4 shows the molecular structures of trans-1,3-butadiene and trans-l,3,5-hexatriene as found in the crystal lattice. 

Deprotonation of Cl gives an enolate ion, which in this compound is actually a 1,3,5-hexatriene. As such it can undergo an electrocyclic ring closing. Protonation gives the product. 

We see the same pattern, but with opposite stereochemistry in the disrotatory reactions of the hexatrienes 4.47 and 4.49. The direction the reaction takes is determined by thermodynamics, ring-closing in this case, but the stereochemistry is not, since the formation of the cis disubstituted cyclohexadiene 4.48 is counter-thermodynamic. If you try the frontier... 

Although the resonance structures of benzene show it as a cyclo-hexatriene, because of its fully delocalized n system and the closed shell nature of this n system, benzene does not undergo addition reactions like ordinary unsaturated compounds. The destruction of the n electron system during addition reactions would make the products less stable than the starting benzene molecule. However, benzene does undergo substitution reactions in which the fully delocalized closed n electron system remains intact. For example, benzene may be reacted with a halogen in the presence of a Lewis acid (a compound capable of accepting an electron pair) to form a molecule of halobenzene. 

The interactions of TDZ (6) with model membranes composed of different phospholipids were also studied by the same group [78]. Calorimetric studies demonstrated that TDZ (6) altered the thermotropic properties of negatively charged DMPC membranes to a larger extent than of zwitterionic phospholipids (PC and PE). The character of the drug-induced changes of the transition parameters of all studied lipids indicated that TDZ (6), similarly to other phenothiazine derivatives, was likely to be localized close to the po-lar/apolar interface of the bilayers. Experiments in which fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) was employed revealed that TDZ (6) reduced the mobility of lipid molecules in a concentration-dependent manner and thus decreased membrane fluidity. The influence of TDZ (6) on isolated... 

Similar results are also obtained for the next higher homologue of the reaction, the ring closing reaction of the 1,3,5-hexatriene radical cation 6 + to give the 1,3-cyclohexadiene radical cation 7 +. Even though 6 + is spectroscopically well... 

The existence of different rotamers of 6 + and the relatively high barriers separating them is also the reason why the ring closing reaction is not observed experimentally. The most stable isomer for both neutral 1,3,5-hexatriene and its... 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

The easiest way to rationalize the stereospecificity in electrocyclic reactions is by examining the symmetry of the HOMO of the open (non-cyclic) molecule, regardless of whether it is the reactant or the product. For example, the HOMO of hexatriene is 3, in which orbital lobes (terminal) that overlap to make the new a-bond have the same phase (sign of the wave function). Thus, in this case, the new cr-bond between these two terminal orbital lobes can be formed only by the disrotation suprafacial overlap) (Fig. 8.45). If the terminal orbital lobes of HOMO of hexatriene were to close in a conrotatory antarafacial overlap) fashion, an antibonding interaction would result. 

The monomers distribution in HTI chains has been investigated by H-NMR and C-NMR on both hydrogenated and raw copdlymer. Both the methods ow the presence of triads attributable to in, HE (EH) and EIE (where I = isobutene and E = 1,3,5-hexatriene), and their intensity is very close to the calculated random distribution of monomeric units ... 

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