Investigation of the electronic and thermal rearrangement properties of SPIRO[2,4]HEPTA-4,6-DIENES

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Title: Investigation of the electronic and thermal rearrangement properties of SPIRO[2,4]HEPTA-4,6-DIENES
Author: Crandall, Raymond K.
Abstract: The mechanism of thermally induced hydrogen and alkyl migrations in suitably substituted cyclopentadienes have been extensively studied. Although the course and mechanism of alkyl shifts in cyclopentadienes have been investigated, there is currently a lack of agreement on the precise mechanisms of alkyl migrations. It is apparent that the mechanism is largely dependent on the nature of both the migrating substituent and upon other substituents in the molecule and, in some cases, the shift may occur by more than one competing mechanism. Alkyl shifts in spiro substituted cyclopentadienes have been clearly shown to proceed via a conceited pathway. However, the participation of a biradical process has not been excluded. The purpose of this research was to clarify the exact nature of the alkyl shift in appropriately substituted spiro[2.4]hepta-4,6-dienes as to concerted or biradical. Previous work has shown that 4- methylspiro[2.4]hepta-4,6-diene rearranged to an equilibrium mixture of approximately equal amounts of 4 and 5-methylspiro[2.4]hepta-4,6-dienes. The slightly positive entropy of activation of the dual alkyl migration as well as other previous similar work suggested a possible biradical component to the rearrangement. The effects of ring strain in the intermediate as well as stabilization resulting from alkyl substitution were of particular interest. The appropriate molecule for study of the rearrangement would have a geometric label on the five membered ring to detect alkyl migration and a stereochemical label on the three membered ring to detect biradical formation during the rearrangement. The cz>2,3,4-trimethylspiro[2.4]hepta-4,6-diene molecule was prepared by photolysis of the methyldiazocyclopentadiene in cz>2-butene solution and purified by gas liquid chromatography. The static pyrolysis results showed that the products of the approximate first order thermal rearrangements were consistent with a biradical pathway. The products derived from the equilibrium mixture gave no evidence for a significant concerted mechanism operating. It is impossible to exclude the simultaneous operation of a slower concerted pathway with the results of this research.
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Date: 1982-08

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