Woodward-Hoffmann neutral

The Woodward-Hoffmann orbital symmetry rules are not limited in application to the neutral polyene systems that have been discussed up to this point. They also apply to charged systems, just as the Htickel aromaticity rule can be applied to charged ring systems. The conversion of a cyclopropyl cation to an allyl cation is the simplest... 

Such correlation diagrams, like the Woodward-Hoffmann rules of organic chemistry, only indicate which reactions are symmetry allowed and are therefore unlikely to have large energies of activation if they are exothermic or thermo-neutral. These diagrams also indicate which reactions are symmetry forbidden and are likely to have either large energy barriers or small frequency factors because they cannot take place adia-batically. 

The Woodward-Hoffmann ru are not limited m application to the neutral systems that have been discussed up roTHis point. They also apply to charged systems. The conversion of cyclopropyl cation to allyl cation has been thoroughly studied and represents the simplest possible case of an electrocyclic transformation, since it involves only two tt-electrons. Because of the restrictions imposed on the internuc-lear angles in cyclopropyl rings, carbonium ions do not form readily, and cyclopropyl halides and arenesulfonates are quite unreactive under ordinary solvolytic conditions. For example, it was found that temperatures of 180°C were necessary for cyclopropyl tosylate to react in acetic acid, and the product was allyl acetate, rather than cyclopropyl acetate. A mechanism was considered in which cyclopropyl cation was formed in the rate-determining step, followed by rapid conversion to allyl... 

The thermal Diels-Alder reaction of electron-rich 1,3-dienes with electron-deficient alkynes affords the corresponding cyclohexa-1,4-dienes, aromatization of which gives the corresponding benzenes. As the thermal Diels-Alder reaction proceeds in a concerted fashion that is best described by the Woodward-Hoffmann rules, it is difficult to use electronically neutral substrates. On the other hand, as the transition metal-catalyzed Diels-Alder reaction may proceed in a stepwise fashion, electronically neutral substrates can be employed. For the transition metal-catalyzed Diels-Alder reaction, cobalt-based complexes are the most frequently employed catalysts [38], although some rhodium-based complexes have also been used [39]. 

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