Conjugated triene, electrocyclic

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. 

Photochemical electrocyclic ring-closure in a 4-electron system works well for many acvclic dienes (2.17) and related cvclic systems (2.18). The situation with conjugated trienes is more complex, and they can act as 6-e ectron systems (2.19) leading to cyclohexal, J-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond (which must be cis if a 6-electron electrocyclic nng-closure is to take place). 

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). 

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-... 

The type of motion the orbital of the terminal carbon undergoes in an electrocyclic reaction can be detected only if substituents are bonded to these atoms. Substituents move as orbitals move. The thermal cyclization of (2A,4Z,6T)-octatriene provides an example of this effect. We consider 713 because it is the HOMO. It contains the highest energy electrons, so it is the frontier molecular orbital. As outlined above, disrotatory motion is required for a O bond to form at the ends of a conjugated triene. Disrotatory motion of the terminal 2p orbitals causes simultaneous disrotatory motion of the C-1 and C-8 methyl groups and yields r-5,6-dimethyl-l,3-cyclohexadiene (Figure 25.6a). 

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,... 

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... 

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

Electrocyclic ring closures occur in the gas phase and in solution, are relatively insensitive to solvent polarity and do not require catalysis. Typically, they are also stereospecific bond formation between terminal carbons of triene systems occurs in a disrotatory manner, and bond formation between termini of conjugated diene and tetraene systems occurs in a con-rotatory manner Kinetic data on electrocyclic ring closures are summarized in Table . 

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