2-Silyl group effect on the reactivity of cyclopentane-1,3-diyls. Intramolecular ring-closure versus silyl migration

Manabu Abe; Satoshi Kawanami; Chizuko Ishihara; Masatomo Nojima

doi:10.1021/jo049580z

2-Silyl group effect on the reactivity of cyclopentane-1,3-diyls. Intramolecular ring-closure versus silyl migration

J Org Chem. 2004 Aug 20;69(17):5622-6. doi: 10.1021/jo049580z.

Authors

Manabu Abe¹, Satoshi Kawanami, Chizuko Ishihara, Masatomo Nojima

Affiliation

¹ Department of Materials Chemistry, Graduate School of Engineering, Osaka University (HANDAI), Suita 565-0871, Osaka, Japan. abe@che.eng.osaka-u.ac.jp

PMID: 15307731
DOI: 10.1021/jo049580z

Abstract

Generation of singlet and triplet 2-silylcyclopentane-1,3-diyls and their reactivity have been investigated in the thermal and photochemical denitrogenation of 2,3-diaza-7-silylbicyclo[2.2.1]hept-2-ene. 5-Silylcyclopentene (silyl migration product) is quantitatively obtained, while 5-silylbicyclo[2.1.0]pentane (intramolecular ring-closure product) is not detected in the denitrogenation reactions. Deuterium labeling studies clarify that 5-silylcyclopentene is formed by a suprafacial [1,2] silyl migration in singlet 2-silylcyclopentane-1,3-diyl. UDFT calculations closely reproduce the observed reactivity of the singlet diradical: The enthalpic barriers of the intramolecular ring-closure are calculated to be DeltaH++exo468 = 5.8 kcal/mol and DeltaH++endo468 = 6.7 kcal/mol, which are much higher than the energy barrier for the [1,2] silyl migration, DeltaH++468 = 2.7 kcal/mol. The notable effect of the silyl group on raising the energy barrier of the intramolecular cyclization is rationalized by an electronic configuration of the lowest singlet state of 2-silylcyclopentane-1,3-diyls.