Ring-opening reactions of cyclopropanes. Part 7. Selenenylation and cyanoselenenylation of ethyl 2,2-dimethoxycyclopropanecarboxylates

Abstract

The uncatalyzed reaction of C3-substituted cyclopropanes 1b–d with benzeneselenenyl chloride 3 leads to 1-ethyl 4-methyl 2-(phenylseleno)butanedioates 5b–d while the unsubstituted 1a gives the 3-phenylseleno derivative 6a. Formation of 5b–d occurs regio- and stereoselectively via electrophilic ring-opening of 1 while 6a results from the selenenylation of the alkene 2a formed via isomerization of 1a. The presence of TiCl₄ influences the stereochemistry of the reaction with chloride 3 while it is essential for reaction of the less reactive PhSeCN 4. With this electrophile the reactions lead to α,α-dimethoxycarbonitriles 13a–d while only 1a and 1b give also the expected cyanoselenenyl derivatives 12a and 12b. Moreover, from 1b compound syn- 5b is obtained, and 1a gives, via the alkene 2a, esters 14a and 6a. A mechanistic interpretation suggests the intermediacy of well-stabilized dipolar species 9 which should be formed by coordination of the Lewis acid to the carbonyl oxygen of 1 and subsequent ring opening.