Hexatriene photochemical cyclization

For the thermal and photochemical cyclization of hexatriene, the controlling HOMO S are Vo and respectively. As shown in Fig. 3.5.9, the allowed... 

A similar reasoning can be applied to hexatriene to be converted into cyclohexadiene. The only significant change is that the number of tt electrons becomes six corresponding to six conjugated C atoms, and so there is one more node in the frontier orbitals. Accordingly, the symmetries of the HOMO and LUMO at the terminal C atoms are different from those for butadiene, and cyclization occurs by the disrotatory mode or by the conrotatory mode, respectively, for a thermally or a photochemically controlled reaction. The general rule is that the thermal cyclization reactions of a A 7r-electron system will be conrotatory for A =4 and disrotatory for A =4g-f-2 (g = 0,1,2,...). For photochemical cyclizations these relationships are reversed. 

The product of the photochemical cyclization of a polyene has a different stereochemistry from the product of the thermal cyclization process. This difference results from the motion of the terminal 2p orbitals of a different frontier molecular orbital. Ultraviolet radiation promotes an electron from the HOMO to the LUMO of the molecule. The LUMO of a 1,3,5-hexatriene is 71 it is antisymmetric. Because the signs of the terminal 2p orbitals on the same side of the molecule are opposite, the required motion of a bond formation is conrotatory. Let s compare the result of this motion for (2 , 4Z,6 )-octatriene with the thermal reaction. Conrotatory motion of the orbitals causes simultaneous conrotatory motion of the C-1 and C-8 methyl groups and yields rra 77r-5,6-dimethyl-1,3-cyclohexadiene (Figure 25.6b). 

Introduction of a double bond in the acyl portion of Ar-acylenamines or A-acylanilides forms a six -electron conjugated system and therefore, an electrocyclic cyclization under photochemical conditions is expected, as in the case of photocyclization of 1,3,5-hexatrienes and stilbenes. 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

In both systems, the photochemical forward reaction entails the cyclization of 1,3,5-hexatriene to 1,3-cyclohexadiene ... 

Among other examples, they quote the thermal isomerization of cyclobutenes as conrotatory, and the cyclization of hexatrienes to cyclo-hexadienes, which is disrotatory if obtained thermally and conrotatory when obtained photochemically. 

Applying this rule in the case of 1,3—butadiene, which has four electrons in the tt system, predicts that cyclization would occur as a result of con-rotation as was deduced earlier. The cyclization of 1,3,5—hexatriene (which has six 7T electrons) occurs by disrotation. It must be remembered that these predictions apply to thermally induced cycHzations and that photochemical excitation produces an excited state that causes the rules stated previously to be reversed for that type of reaction. 

Consider the thermal reaction 1,3,5-hexatriene — 1,3-cyclohexadiene. (a) Use the symmetry of the polyene HOMO to predict whether the reaction path is conrotatory or disrota-tory. (The HMOs need not be found explicitly all that is needed is the signs of the AOs in the HOMO and these can be found by noting that the HMO nodal pattern is the same as the FE MO nodal pattern.) (b) Do the same as in (a) when the reaction occurs photochemically. (c) State the general rules for the cyclization of the polyene (16.9) with n ir electrons. 

Hexatrienes can also cyclize photochemically through anti-Hiickel... 

FMO Explanation In ground state HOMO of 1, 3, 5-hexatriene is f3 having m-plane symmetry. Hence, concerted - cyclization should be disrotatory. In first excited state HOMO, is y4 with C2-axis of symmetry, hence, conrotatory ring closure is S3anmetry allowed under photochemical conditions. 

The different stereochemical features of thermal and photochemical [(,2 + 4] processes are nicely summarized in the reactions of I l6]-annulene, (Equation 6.31). The cyclizations occur at the tennini of the trans,cis,trans-hexatriene units of the [ 16]-annulene structure. 

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