Cyclopropyl-allyl anion transformation

It was only after Woodward and Hoffmann in 1965 had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation that a new interest developed in this reaction. By means of the iso-7c-electronic aziridine 310 Huisgen and coworkers succeeded in demonstrating that the thermal recation gave a conrotatory formation of azomethine ylid (311) and that the light-induced reaction resulted in a disrotation to give 312. 

The necessity of both prerequisites is illustrated by the following examples which, for different reasons have not been useful in determining the stereochemistry of the cyclopropyl-allyl anion transformation. 

These results have been attributed to an increased ionicity of the carbon-lithium bond in the case of 331 and 340 as compared to 342 and 343 . This conclusion is supported by electrochemical measurements and MNDO calculations A similar conclusion had been reached earlier by Boche and Martens for thermal cyclopropyl-allyl anion transformations. 

Photochemical cyclopropyl-allyl anion transformations have been observed by Newcomb and Ford 20c) and by Fox 22) for example, the photochemical disrotation of 10 to give 12. 

A reexamination of the reactions of 93a-d with different bases and ET reagents led to the following results and conclusions 74). The cyclopropyl anion 94b transforms completely and in a disrotatory manner into the allyl anion 95b, even at —75 °C within 1 h 95 b gives 96 b on protonation. This strongly suggests a similar pathway in the reactions of 93a with base. Indeed, 93a is completely transformed into 96a after... 

Under the category of kn, k = 4q, electrons system, we can cite the ring closures of allyl anion to cyclopropyl anion, 1,3- and 1,4-pentadienyl cation to cyclopen-tenyl cation, and 1,3,5,7-octatetraene to 1,3,5-cyclooctatetraene. Under the category kn, k = 4q + 2, we can quote the ring closures of allyl cation to cyclopropyl cation and 1,3- and 1,4-pentadienyl anion to cyclopentenyl anion. The chemical equations for these transformations are given below. 

This section covers the formation of cyclopropanes via cyclization of reactive allylic intermediates (cations, anions, radicals). Included are those transformations of allylic functional derivatives (e.g. allylic halides, alcohols, aldehydes, ketones, acids, esters, boronates, Grignard reagents) to cyclopropyl derivatives that do not actually proceed via allylic reactive intermediates, but which are not covered by other sections of this volume. Additionally, this section will cover methods for the formation of cyclopropanes by pericyclic reactions. 

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