Orbital symmetry allowedness/forbiddenness

Both of these dienes are conformationally flexible as regards the adoption of an s-cis conformation, but the former apparently has a somewhat higher s-cis content and might therefore be expected to preferentially adopt the dienic role. However, the product corresponds to the use of 2,3-dimethyl-l,3-butadiene in the dienic role. Since this latter diene is by far the more difficult to ionize of the two dienes, the observed reaction must be of the allowed [4-1-1] type. The possibility that this empirically observed sense of role selectivity has its origin in orbital symmetry allowedness/forbiddenness was tested in the maimer shown in Scheme 25. 

The real breakthrough in recognizing the role that symmetry plays in determining the course of chemical reactions has occurred only recently, mainly through the activities of Woodward and Hoffmann [5, 6], Fukui [7, 8], Bader [9, 10], Pearson [11], Halevi [12, 13], and others. The main idea in their work is that symmetry phenomena may play as important a role in chemical reactions as they do in the construction of molecular orbitals or in molecular spectroscopy. It is even possible to make certain symmetry based selection rules for the allowedness and forbiddenness of a chemical reaction, just as is done for spectroscopic transitions. 

An approach very closely related to that of Woodward and Hoffmann is the so-called Hiickel-Mobius approach 35> based on the rule An +2 electron systems prefer Hiickel geometries and An electron systems prefer Mobius geometries 36>. When no symmetry exists and there is no cyclic orbital array the allowedness or forbiddenness of a reaction can be determined by following the form of the MO s during the reaction 37>. A detailed quantum mechanical study of the stereochemistry of thermal and photo cyclo-addition reactions has been reported38), and a quantum mechanical discussion of the applicability of the Woodward-Hoffmann rules can be found in a paper by George and Ross 39>. 

In their now classic monograph [1], Wooodward and Hoffmann concentrate on three basic types of no mechanism reaction Electrocyclic reactions -notably polyene cyclizations, cycloadditions, and sigmatropic rearrangements. These three reaction types will be taken up in this and the next two chapters from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) [2, 3, 4], the formalism of which follows naturally from that developed in Chapter 4. The similarities to the original WH-LHA approach [5, 6], and the points at which OCAMS departs from it, will be illustrated. In addition, a few related concepts, such as allowedness and forbiddenness , global vs. local symmetry, and concertedness and synchronicity , will be taken up where appropriate. 

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