Wood ward-Hoffmann rule

In view of the enormous impact which symmetry-based rules concerning the stereochemistry of concerted addition and cyclization reactions (Wood-ward-Hoffmann rules) have had in recent years a detailed introduction to this subject has been added. 

J. P. Freeman and W. H. Graham, J. Amer. Chem. Soc., 89, 1761 (1967). Note that in this reference the interpretation of the results as a violation of the Wood ward-Hoffmann rules presupposes the linear (suprafacial-suprafacial) path the idea of the nonlinear path, which brings the results and theory into agreement, developed later [see note 1(a), p. 626],... 

The number of photoreactions that proceed in agreement with the Wood-ward-Hoffmann rules (Table 7.3) is very large. Some examples are collected in Scheme 26. 

Frontier orbital analysis is a powerful theory that aids our understanding of a great number of organic reactions. Its early development is attributed to Professor Kenichi Fukui of Kyoto University, Japan. The application of frontier orbital methods to Diels-Alder reactions represents one part of what organic chemists refer to as the Wood-ward-Hoffmann rules, a beautifully simple analysis of organic reactions by Professor R. B. Woodward of Harvard University and Professor Roald Hoffmann of Cornell University. Professors Fukui and Hoffmann were corecipients of the 1981 Nobel Prize in chemistry for their work. 

Refer to the molecular orbital diagrams of allyl cation (Figure 10.12) and those presented earlier in this chapter for ethylene and 1,3-butadiene (Figures 10.8 and 10.9) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Wood-ward-Hoffmann rules. 

The rules of organic chemistry are ordering principles, creating order where chemists had previously believed only disorder was to be seen. At all times, chemists have all too easily become accustomed to coming to terms with an apparent de facto lack of order time and again, pioneers have proven that, beneath the surface, a form of order does indeed prevail. The Wood-ward-Hoffmann rules here are star witnesses ). 

Pericyclic reactions are often treated using FRONTIER-ORBITAL theory or the WOOD-WARD-HOFFMANN RULES. 

Fig. 14.20l Itvo equivalent schemes for the cycluaddition reaction of ethylene. l vu ethylene molecules, after excitation to the triplet state, dimerize forming cydobutane (a), because everything is prepared for election pairing and formation of the new bonds (see text). We obtain the same from the Wood-ward-Hoffmann rules (Fig. (b), (c), (d)). According to these rules we assume that the ethylene molecules preserve two planes of symmetiy and 2 during all stages of the reaction. We concentrate on...

Theoretical Aspects, Calcidatioiis and Physical Data.—Probably the most significant paper on theory this year is the one in which Day puts the flesh on the bones of the tacit assumptions that many chemists have been making, namely that the Wood-ward-Hoffmann rules for pericyclic reactions and the Dewar-Zimmerman rules for... 

Consider Fig. 3.i6 it is seen that k + 2) electrons are involved in the reaction, and further that two electrons are delivered from the hybrid (sp") orbital of Xyz. This orbital, considered alone, can be acted upon in the supra-facial or antarafacial senses, and so can the k 7r-electron olefin. Therefore, there are the four usual combinations, namely supra-supra, antara-antara, antam-supra, and supra-antara (respectively Fig. 3.16(a) (i), (b) (ii), (a) (ii), and (b) (i)). When (/c + 2) - (4 + 2) electrons it will be expected, because of the general Wood ward-Hoffmann rule, that the supra-supra or antara-antara interactions will occur in the thermal cheletropic reactions. These respectively correspond to a linear cheletropic reaction with disrotatory cleavage, and to a non-linear cheletropic reaction with conrotatory cleavage. The alternative pathways are reserved for the cases (/ -I- 2) = An electrons. In the photochemical reactions the usual cross-over relationship should apply. 

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