Dienes thermal electrocyclic reactions

For the thermal electrocyclic reaction of dienes, the HOMO for the diene is n2, since there are four electrons to accommodate in the n-orbitals (two paired electrons per orbital). Thus, for hexa-2,4-diene the conrotatory mode of reaction gives the trans isomer (Scheme 8.3). 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

The effect of the number of n electrons upon the stereochemistry of a reaction is illustrated by the cyclization of a diene system compared to a triene system, as shown above. Although the methyl groups in both compounds have the E configuration, the products have different stereochemistry. Although both reactions are thermal, only the trans isomer results from the diene and only the cis isomer results from the triene. Thermal electrocyclic reactions of systems with 4 n electrons have the opposite stereochemistry to structurally related systems with 4 + 2 ti electrons. Furthermore, the stereochemistry of the thermal and photochemical pericyclic reactions is opposite. Photochemically initiated cychzation of the triene gives the trans isomer, whereas the ds isomer forms in the thermal cyclization. 

The frontier molecular orbitals control the motions of the atoms in an electrocyclic reaction independent of the equilibrium constant for the reaction. The position of an equilibrium is controlled by AG°. Both a conjugated diene and a triene can undergo thermal electrocyclic reactions. In both cases, the cyclic compounds have more O bonds and fewer Jt bonds. However, AG° is favorable only for the triene. The strain of the four-membered ring of the diene effectively reverses the relative stability of the reactants and products in the cyclization of the diene. 

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

Dienes may be involved in electrocyclization reactions as well. Two well-documented examples are the cyclobutene ring opening244 and the 1,3-cyclohexadiene formation245 reactions. Predictions regarding the stereochemical outcome of these rearrangements can be made applying the orbital symmetry rules. The thermally... 

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,... 

Let s begin by considering the simplest electrocyclic reaction, the thermally induced interconversion of a diene and a cyclobutene. As illustrated in the following example, the reaction is remarkably stereospecific, occurring only by a conrotatory motion ... 

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). 

A common type of electrocyclic reaction is the ring-opening of a cyclobutene to a butadiene. The stereochemistry of the new alkene(s) in the diene can be interpreted on the basis of the Woodward-Hoffmann rules. For a four electron component, thermal ring-opening occurs by a conrotatory process (both terminal p-orbitals moving clockwise or anticlockwise), whereas the photochemical reaction... 

Two examples of electrocyclic reactions for synthesis of carbocycles were reported. Diels-Alder reaction of D-glucose-derived diene 20 with quinone 21 gave 22, which epimerized on base catalysis to the trans-fused analogue. Levoglucosenone-derived enediene 23, was elaborated to triene 25 (via 24), thermal [3+3] rearrangement giving cyclohexadiene 26, then converted to 27, an... 

The electrocyclic reaction in which unsaturated substituents participate at C-2 of cyclobutenedione provides a somewhat different cyclization mode. The thermal rearrangements of 2-dienylcyclobutenones 88 and 3-(o-vinylphenyl)-cyclobutenediones 92 imderwent well-precedented 47r-67r electrocycUc reactions, but within the diene moiety to phenolic intermediates 90 and 94. These were allowed to react intramolecularly to give benzofurans 91 and naphthofu-... 

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