Hexatrienes, ring closure

Let s start with the hexatriene ring closure, first looking at the orbitals, and then following the same procedure that we taught you for cycloadditions and H sigmatropic rearrangements to see what the q j-. 

HOMO of cw-1,3,5-hexatriene anticipates the preferred direction of ring closure. 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

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. 

Thiemann and coworkers [68] sought novel types of steroids with different biological activity, and in doing so prepared areno-annulated compounds such as 6/1-133 (Scheme 6/1.35). This is achieved with a Heck reaction of 6/1-132 with an acrylate, followed by an electrocydic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

J. Rimmelin and G. Jenner, Tetrahedron, 30, 3081 (1974). A recent measurement of the pressure and temperature dependence of die electrocyclic ring-closure of Z-l,3,5-hexatriene to 1,3-cyclohexadiene in the range of 200 to 2500 bar and 100 to 125 °C does not show a significant temperature dependence of die activation volume (M. K. Diedrich and F. -G. Klarner, unpublished results). 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

The acyclic fluorinated 1,3,5-hexatriene system 454 is also resistant to 6 r-electron electrocyclic ring closure at temperatures up to 200 °C while the analogous hydrocarbons cyclize easily at 160 °C (equation 171)232. 

It has also been proposed that during ring closure of this I-chroma-1,3,5-hexatriene intramolecular complexation of the terminal C-C double bond may be important [330] (Figure 2.25). 

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... 

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. 

There are at least two C6-dehydrocyclization mechanisms one of these proceeds through arylalkene intermediates and corresponds to the hexatriene-type C6-dehydrocyclization of paraffins. The other pathway is direct ring closure. It is probably related to C5-dehydrocyclization. 2-Butylnaphthalene may differentiate between the two mechanisms phenanthrene is probably formed by the first reaction, anthracene by the second. 

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