Orbital Symmetry Basis for the Stereospecificity of Electrocyclic Reactions

Orbital Symmetry Basis for the Stereospecificity of Electrocyclic Reactions 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 29), we can further generalize the predictions based on the symmetry of the polyene HOMO. The HOMOs of the An systems are like those of 1,3-dienes in having opposite phases at the terminal atoms. The HOMOs of other An + 1 systems are like trienes and have the same phase at the terminal atoms. Systems with An tt electrons will undergo electrocyclic reactions by conrotatory motion, whereas systems with An+ 2 a electrons will react by the disrotatory mode. 

The cyclobutene-butadiene interconversion can serve as an example of the construction of an orbital correlation diagram. For this reaction to occur, the four tt orbitals of butadiene must be converted smoothly into the two t and two a orbitals of the ground state of cyclobutene. The tt orbitals of butadiene are i, 2, 1 3, and 4. For cyclobutene, the four orbitals are ct, tt, ct, and tt, with each of them classified with respect to the symmetry elements that are maintained in the course of the transformation. The relevant symmetry features depend on the structure of the reacting system. The most common elements of symmetry to be considered are planes of symmetry and rotation axes. An orbital is classified as symmetric, S, if it is unchanged by reflection in a plane of symmetry or by rotation about an axis of symmetry. If the orbital changes sign (phase) at each lobe as a result of the symmetry operation, it is called antisymmetric, A. Proper molecular orbitals must be either symmetric or antisymmetric. If an orbital is neither S nor A, it must be adapted by combination with other orbitals to meet this requirement. 

Classification with Respect to plane Respect to axis 

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