Woodward-Hoffmann rules for

Roald Hoffmann (1937—) was born in Zloczow, Poland, just prior to World War II. As a boy, he survived the Holocaust by hiding in the attic of a village schoolhouse. In 1949, he immigrated to the United States, where he received an undergraduate degree at Columbia University and a Ph.D. at Harvard University in 1962. During a further 3-year stay at Harvard as Junior Fellow, he began the collaboration with R. B. Woodward that led to the development of the Woodward-Hoffmann rules for pericyclic reactions. In 1965, he moved to Cornell University, where he remains as professor. He received the 1981 Nobel Prize in chemistry. 

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Fukui at Kyoto Imperial University in Japan introduced a simplified version. According to Fukui, we need to consider only two molecular orbitals, called the frontier orbitals. These frontier orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In ground-state 1,3,5-hexa-triene, for example, 1//3 is the HOMO and excited-stale 1,3,5-hexatriene, however, 5 is the LUMO. 

A part of the chemical consequences of the cyclic orbital interactions in the cyclic conjngation is well known as the Hueckel rule for aromaticity and the Woodward-Hoffmann rule for the stereoselection of organic reactions [14]. In this section, we describe the basis for the rnles very briefly and other rules derived from or related to the orbital phase theory. The rules include kinetic stability (electron-donating and accepting abilities) of cyclic conjugate molecules (Sect. 2.2.2) and discontinnity of cyclic conjngation or inapplicability of the Hueckel rule to a certain class of conjngate molecnles (Sect. 2.2.3). Further applications are described in Sect. 4. 

The orbital phase theory can be applied to cyclically interacting systems which may be molecules at the equilibrium geometries or transition structures of reactions. The orbital phase continuity underlies the Hueckel rule for the aromaticity and the Woodward-Hoffmann rule for the stereoselection of organic reactions. 

Table 5.3. Woodward-Hoffmann Rules for Electrocyclic Reactions...

Although the photochemistry of the thietane ring has been studied to some degree in the gas phase, the study of its decomposition in solution or in glassy matrices has not been extensive. Biradicals are postulated as intermediates in order to account for the differences in stereochemistry, abiding by the Woodward-Hoffmann rules for concerted reactions. Biradi-... 

The Woodward-Hoffmann rule for thermal pericyclic reactions ... 

Table 7.1 Woodward-Hoffmann rules for cycloaddition reactions...

On the other hand, Takacs and coworkers added organometallic reducing agents to the reaction mixture and promoted the formation of low-valent iron(O) bipyridine complexes. The mechanism of the low-valent iron-catalyzed Alder-ene reaction involves coordination of the two starting materials within the ligand sphere of the iron, which makes the Woodward-Hoffmann rules for such reactions obsolete [11]. Thereby, the scope of the reactions was broadened so that alkenes and 1,3-dienes could also be used as educts in a formal [4 + 4]-cycloisomerization (Scheme 9.3) [12]. Intriguingly, the diastereoselectivity of the cydopentane products can be influenced by either the application of the 2Z-isomer 3 or the 2 E-isomer 4. Especially the E-isomers 4 gave almost exclusive cis selectivity [13]. 

The unsubstituted benzo-TA 263 that was generated in situ from 2-ethoxy-2,3-dihydrobenzo-TA 262 reacts with 1,3-dienes to give the Diels-Alder adducts 264 and 265 (73JHC149) possessing a cis-configuration according to the Woodward-Hoffmann rules for [2 + 4]-cycloaddition (Scheme 103). 

Group Transfer Reactions. There are so few of these reactions that a fully general rule for them can wait until the next section, where we see the final form of the Woodward-Hoffmann rules. For now, we can content ourselves with a simplified rule which covers almost all known group transfer reactions. When the total number of electrons is a (4 +2) number, group transfer reactions are allowed with all-suprafacial stereochemistry. 

The conrotatory ring opening predicted by correlation diagrams based on the Woodward-Hoffmann rules for cyclopropyl anions have their conterpart in the isoelectronic aziridines. The interconversions outlined below were investigated by Huisgen et and photochemical ring opening was shown to follow a... 

Fig. 12.4 shows two possible ways for this to happen the HOMO of the diene can combine with the LUMO of the dienophile or the LUMO of the diene can combine with the HOMO of the dienophile. The thermal reaction with this six-electron transition state is allowed, but the corresponding photochemical mechanism is forbidden. More generally, the Woodward-Hoffmann rule for concerted cycloaddition reactions can be stated If the number of electrons in the transition state equals An [An + 2], then thephotochemical [thermal] reaction will be allowed, but the thermal [photochemical] reaction will be forbidden. 

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Pukui... 

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