The Origin of Regioselectivity in a-Cleavage Reactions of Cyclopropenethiones: Potential Role of Pseudo-Jahn-Teller Effect in Substituted Cyclopropenyl Systems

G. Usha, B. Nageswer Rao, J. Chandrasekhar, V. Ramamurthy

Research output: Contribution to journalArticle

9 Scopus citations

Abstract

Arylalkylcyclopropenethiones undergo highly regioselective photochemical α-cleavage via thioketene carbene intermediates, giving rise to products derived from the less stabilized carbene. UHF MINDO/3 calculations provide an insight into this unexpected regioselectivity. The nπ* triplet of cyclopropenethione is calculated to have a highly unsymmetrical geometry with an elongated C-C bond, a delocalized thiaallyl fragment, and a pyramidal radicaloid carbon (which eventually becomes the carbene center). From this molecular electronic structure, aryl group stabilization is expected to be more effective at the thiaallyl group rather than at the pyramidal radical center. Thus, the stability of the substituted triplet thione rather than that of the thioketene carbene determines the preferred regiochemistry of cleavage. The unusual structure of the cyclopropenethione triplet is suggested to be related to one of the Jahn-Teller distorted forms of the cyclopropenyl radical. An alternative symmetrical structure is adopted by the corresponding triplet of cyclopropenone, partly accounting for its differing photobehavior. A similar structural dichotomy is demonstrated for the corresponding radical anions as well.

Original languageEnglish (US)
Pages (from-to)3630-3635
Number of pages6
JournalJournal of Organic Chemistry
Volume51
Issue number19
DOIs
StatePublished - Jan 1 1986
Externally publishedYes

ASJC Scopus subject areas

  • Organic Chemistry

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