Conjugated diene electrocyclic reactions

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The cyclization step of Equation 28-8 is a photochemical counterpart of the electrocyclic reactions discussed in Section 21-10D. Many similar photochemical reactions of conjugated dienes and trienes are known, and they are of great interest because, like their thermal relatives, they often are stereospecific but tend to exhibit stereochemistry opposite to what is observed for formally similar thermal reactions. For example,... 

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. 

The [4+2]-cycloaddition of a conjugated diene and a dienophile (an alkene or alkyne), an electrocyclic reaction that involves the 4 71-electrons of the diene and 2 7t-electrons of the dienophile. The driving force of the reaction is the formation of new o-bonds, which are energetically more stable than the 7t-bonds. 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

An important group of conjugated diene/triene systems are those in the vitamin D series. The key reactions in the commercial manufacture of vitamin D (and probably also in its formation in skin exposed to daylight) are a photochemical, conrotatory electrocyclic ringopening in the provitamin, and a thermal 1,7-shift of hydrogen in the previtamin so formed (2.23). High conversions to the vitamin are not normally possible because all three species absorb appreciably at the... 

We noted previously that photochemical electrocyclic reactions take a different stereochemical course than their thermal counterparts, and we can now explain this difference. Ultraviolet irradiation of a polyene causes an excitation of one electron from the ground-state HOMO to the ground-state LUMO. For example, irradiation of a conjugated diene excites an electron from il/2 to and irradiation of a conjugated triene excites an electron from i/f j to (/ 4 (Figure 30.6). 

Only the singlet state interconversions of conjugated dienes and cyclobutenes are taken into consideration here, because the triplet state is generally not involved in this process.596 There are two important factors that affect electrocyclization (1) reaction... 

Benzene and its substituted analogues also undergo electrocyclization reactions characteristic of other conjugated triene systems. When 1,2,4-tri-tert-butylbenzene (67) was irradiated in ether solution using a Hanovia Type L lamp and Vycor filter, the corresponding bicyclo[2.2.0]-hexa-2,5-diene (Dewar benzene) 68 was isolated 45>. Nonbonded interactions of the fer/-butyl substituents are partially relieved upon cycliza-... 

For conjugated tetraenes, n = 8, conrotation should be preferred. The expectation that cyclization of eight TT-electron systems will be conrotatory has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature. The unsubstituted system, has an of 17.0 kcal/mol and A// of —11.2 kcal/mol. At slightly higher temperatures, the cyclooctatriene system that is formed undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene. ... 

Fig. 10.27. Summary of relative and A/7 relationships in kcal/mol for electrocyclic reactions of conjugated dienes, trienes, and tetraenes.

Electrocyclization of conjugated dienes occurs in competition with cis-trans isomerization. The cyclization occurs from the s-cis conformation of the diene. Cyclobutene formation is favored in cyclic dienes and for other dienes where the s-cis diene conformation is dominant. For several dienes, the quantum yield in nonpolar solvents at 257 nm is about 0.1. As the cyclized alkenes do not absorb at this wavelength, the reaction can give substantial preparative yields, despite the competing cis-trans isomerization. 

Huntsman first observed that 1,5-hexadiyne gave 3,4-dimethylenecyclobutene upon heating at 250°C with an activation enthalpy of 34.4 kcal/mol and an activation entropy of — 9.4 e.u. " A reasonable reaction pathway would invoke a 3,3-shift to bisallene followed by electrocyclization of the destabilized conjugated diene moiety (Scheme 7.15). 

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