Selection rules Woodward-Hoffmann

Photochromic reactions of the heterocyclic fulgides are also in accordance with the Woodward-Hoffmann selection rules. The molecular structures have dramatic effects on the quantum yields of photoinduced ring-closure and ring-opening reactions of fulgides. The photochemical reactions of furyl fulgides are shown in Scheme 35 and the quantum yields are summarized in Table 4.29.44 45 104... 

The so-called aromaticity rules are chosen for comparison, as they provide a beautiful correspondence with the symmetry-based Woodward-Hoffmann rules. A detailed analysis [92] showed the equivalence of the generalized Woodward-Hoffmann selection rules and the aromaticity-based selection rules for pericyclic reactions. Zimmermann [93] and Dewar [94] have made especially important contributions in this field. 

The rules based on the Hiickel-Mobius concept have their counterpart among the Woodward-Hoffmann selection rules. There was a marked difference between the suprafacial and antarafacial arrangements in the application of the Woodward-Hoffmann treatment of cycloadditions. The disrotatory and conrotatory processes in elec-trocyclic reactions presented similar differences. The suprafacial arrangement in both of the reacting molecules in the cycloaddition as well as the disrotatory ring closure in Figure 7-25 correspond to... 

This type of condensation is of great interest in connection with the Woodward-Hoffmann selection rules for symmetry-allowed concerted suprafacial and antarafacial cycloaddition reactions.284 The generalized rules for cycloaddition of an m- to an n-electron system predict that the concerted supra-supra or antara-antara dimerization is allowed in the excited state (i.e., photochemically) when m + n = 4q, and in the ground state (i.e., thermally) when to + n = 4q + 2, where to and n are the numbers... 

The Woodward-Hoffmann selection rules for sigmatropic rearrangements are summarized in Table 8.2. 

Having learnt about the concerted reactions, we can now undertake the theory of these reactions. The development of the theory of concerted reactions has been due chiefly to the work of R.B. Woodward and R. Hoffmann. They have taken the basic ideas of molecular orbital theory and used them, mainly in a qualitative way, to derive selection rules which predict the stereochemical course of various types of concerted reactions. These rules are best understood in terms of symmetries of interacting molecular orbitals. Here are will see some of the most important theoretical approaches and see how they are interrelated. 

The symmetry-based selection rules for cyclizations and other rearrangements and reactions of oiganic molecules developed by Woodward and Hoffmann are covered in detail in their book ... 

The Generalized Woodward Hoffmann Pericyclic Selection Rules... 

COMPARISON OF THE WOODWARD-HOFFMANN AND DEWAR-ZIMMERMAN PERICYCLIC SELECTION RULES... 

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