Hexatriene electrocyclic reaction

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic 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. 

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. 

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

One of the simplest electrocyclic reactions occurs when hexatriene is heated to 500 °C. 

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 electrocyclic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

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. 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

Scheme 80. A 1,3,5-hexatriene to 1,3-cyclohexadiene-type anion radical electrocyclic reaction.

Simple examples of electrocyclic reactions are the formation of cyclobutene from butadiene and cyclohexadiene from hexatriene ... 

The presence of a 1,3-cyclohexadiene or a 1,3,5-hexatriene in the starting material or the product may indicate a six-electron electrocyclic reaction. 

Electrocyclic Reactions. Electrocyclic reactions are those pericyclic reactions in which a ring is formed (or opened). Thus, cyclobutene (157), on heating, gives butadiene (158), and hexatriene (159) gives cyclohexadiene (160). 

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