Cyclopropyl anion, conrotatory ring opening

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... 

The generally slow conrotatory ring-opening reaction of cyclopropyl anions as compared to the fast isomerization of the corresponding allyl anions has been supported by means of MO calculations with the parent anions (Table 31) . 

Table 1 gives the results of several MO calculations for the conrotatory ring-opening of the cyclopropyl anion to give the allyl anion, and of the isomerization of the allyl anion 13). 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

Fig. 9. Conversion of oyclopropyl species to allyl species. Conrotatory changes are on the left disrotatory changes are on the right. Upper section alternative modes of ring opening and orbital correlation of cyclopropyl species. Middle section symmetry state correlation of radical. Bottom section symmetry state correlation of anion.

These data demonstrate that the forbidden disrotatory ring opening of 15 is much slower than the ring-opening reactions of the cyclopropyl anions 10 and 14 which are not prevented from occurring in the predicted conrotatory mode. This kinetical criterium for the thermal conrotatory cyclopropylallyl anion transformation has been published independently by Ford and coworkers 20). 

Thermolysis or photolysis of suitably substituted aziridines is a very convenient method for the generation of azomethine ylides. Thermal ring opening of aziridines involves a conrotatory pathway like that of a cyclopropyl anion with which it is isoelectronic. As predicted by the orbital symmetry rules, the ring opening of aziridines follows disrotatory mode under the photochemical conditions. The stereochemical consequences of these two modes of ring opening result in the formation of tra/is-azomethine ylide on thermolysis of cw-dicarboxylic ester and of cis-azomethine ylide on photolysis (Scheme 5.17). The rra j-dicarboxylic acid ester behaves in the expected manner. 

Figure 15.9 defined suprafacial and antarafacial interactions of a lone pair in a p orbital (called an uj component). Using these definitions, predict if the ring-opening of the cyclopropyl anion shown below will occur in a conrotatory or disrotatory fashion. What will be the stereochemistry of the product ... 

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