Of hexatrienes

The reaction was very vigorous and external heating was not necessary. A mixture of hexatriene and hexanol condensed in the receiver (note 1). The conversion was terminated by external heating, so that 5-10 ml of hexanol distilled at 55-50 C/ /15 mmHg. The contents of the receiver were "redistilled", using the apparatus... 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

Notice molecular symmetry at work. The Huckel rr-electron model is in many ways a blunt instrument, because we would get exactly the same answers for either of the following possible conformers of hexatriene (Figure 7.3). 

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

Vibrational frequencies of hexatriene and octatetraene have been reported by several authors21,24-26,36. The increase in the size of these molecules with respect to butadiene limits the use of highly accurate levels of calculation, so that a good choice of scaling factors is necessary to obtain useful results. Kofraneck and coworkers21 have shown that employing scale factors determined from vibrational data for trans structures alone does not give a balanced description of cis and trans structures. 

FIGURE 23. Gas-phase photodissociation (PD) spectra of the radical cation of hexatriene (a) at low resolution, (b) expanded scan of the first absorption band at high resolution219,220... 

The thermal disrotatory cyclization of hexatrienes leads to cyclohexadienes (equations 11 and 12)11"13. 

The ground state bonding orbitals of cyclohexadiene is correlated with the ground state bonding orbitals of hexatriene and so it will be a thermally allowed reaction. 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

The photochemistry of vitamin D and its various isomers has been reviewed comprehensively many times129,171,172,207,226, but as it lies at the very heart of hexatriene/cyclo-hexadiene photochemistry, it seems fitting to provide a very brief overview of the salient features here. 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

Tracer Studies on the Possibility of Formation of Hexatriene and Various Six-Membered Rings During Dehydrocyclization of 1-Hexene (20, 53)... 

The process is much more rapid over platinum. Dautzenberg and Plat-teeuw (23) assumed the formation and thermal cyclization of hexatriene [similarly to the earlier suggestion with respect to oxides (22)]. However, it is not likely that such an extremely unstable intermediate would leave the catalyst surface just in order to cyclize and then rapidly readsorb to complete aromatization. Still, thermal cyclization cannot be a priori excluded at high temperatures where the equilibrium concentration of triene is higher and its adsorptivity lower, but its appearance may be rather exceptional. We suggest, instead, a surface cyclization step of dj-l,3,5-hexatriene. 

This time, although not critical, was chosen to prevent accumulation of unreacted quaternary base or of hexatriene in the reaction vessel. 

Figure 14.3. (a) Orbital correlation diagram for electrocyclic reaction of butadienes (b) Orbital correlation diagram for electrocyclic reaction of hexatrienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

Answer to 4(d). We can consider the orbitals of cycloheptatrienylidene to arise from the interaction of the orbitals of hexatriene and the valence orbitals of a di-coordinated carbon atom (a 2p orbital and an sp" hybrid orbital). The orbitals of hexatriene may be obtained from an SHMO calculation. The interaction diagram is shown in Figure B7.2. The p orbital of the carbene site is raised as a result of the dominant interaction with 713 of hexatriene. The orbital 714, which is closest in energy to the carbene s p orbital, does not interact because of symmetry, and 5 interacts less strongly because the coefficients at the terminal positions of the hexatriene are... 

Figure B7.2. Electronic structure of cycloheptatrienylidene from the interaction of the orbitals of hexatriene and methylene. Symmetry labels refer to the p and n orbitals and a vertical mirror.

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

Aromatization, however, may also be envisaged as taking place via stepwise dehydrogenation of an unbranched hydrocarbon molecule followed by ring closure of the polyunsaturated intermediates. In fact, the formation of dienes was proved during the aromatization of C6 and C7 alkanes to the corresponding aromatics over monofunctional metal oxides and metal black, and bifunctional catalysts.307 308 Radiotracer studies even allowed the detection in very low concentration of hexatriene during the aromatization of n-hexane over Pt black.309 It was also proposed that aromatics are formed from the cis isomers, whereas trans isomers may be coke precursors.213 Direct experimental evidence has recently been... 

Simulation of the difference signal using known theoretical and experimental structures of the 1,3,5-hexatriene product molecules matches the observed signal to within 10%, without the use of any adjustable parameters. Our fit indicates that the molecule opens up to the completely open tZt form of 1,3,5-hexatriene, within about 20 ps. Within the noise of our instrument, there is no indication for any intermediate existence of the cZt or cZc forms of hexatriene. 

It should also be noted that Leach and Migirdician37 have produced adducts of hexatriene in mixtures of benzene with certain other molecules in glassy matrices at low temperatures. Thus, along with benzene isomers which retain the empirical formula of the monomer, there may be formed photochemically other products which result from a rupture of the ring. Just possibly some of the cuprenelike polymer formed at very short wavelengths20 may be made in this way. We will return to this point in later sections. 

Platinum catalyzes at least two types of C6- dehydrocyclization, one of which involves olefinic intermediates (13, 28, 29). In the case of paraffins, this latter reaction involves the ring-closure of hexatrienes (30, 31). In the C6-dehydrocyclization of n-butylbenzene and n-pentylbenzene, phenyl-butadiene and phenylpentadiene could correspond to these triene intermediates (13, 14). The second C6-dehydrocyclization mechanism is similar to C5-dehydrocyclization, and may not involve olefinic intermediates. 

Figure 11.11 Correlation diagram for the forbidden conrotatory closure of hexatriene.

It is instructive now to turn to the correlation diagrams in Figures 11.11 and 11.12 for conrotatory and disrotatory closure of hexatriene, a six tt electron system. The disrotatory mode is now allowed, the conrotatory forbidden. If correlation diagrams for larger systems are constructed, it will be found that with each addition of two carbons and an electron pair the predicted selectivity will reverse. 

Figure 11.12 Correlation diagram for the allowed disrotatory closure of hexatriene.

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