Allylic Proton–Proton Coupling in Stereodefined Alkylidenecyclobutanes and Alkylidenecycopentanes^*

William F. Bailey^1* and Timo V. Ovaska²*

¹ Department of Chemistry, The University of Connecticut, Storrs, Connecticut 06269–3060, USA.^*Responsible author: William.Bailey@uconn.edu

² Department of Chemistry, Connecticut College, New London, Connecticut 06320–4196, USA.^*Responsible author: tvova@conncoll.edu

Received June 28, 2009 ]]> Accepted August 14, 2009

Abstract

Analysis of the ¹H NMR spectra of eight stereodefined alkylidenecycloalkanes allows determination of the magnitude of allylic coupling in these compounds. In contrast to the trend normally observed, the magnitude of transoid–allylic coupling was found to be invariably greater by ~0.5 Hz than the cisoid–allylic coupling. The larger transoid–allylic coupling may be attributed to the slightly smaller dihedral angle relating allylic =C–C–H plane with the C=C–C plane in the E–isomers.

Keywords: Proton NMR, Allylic Coupling, Alkylidenecycloalkanes.

Resumen

El análisis de los espectros de RMN protónica de ocho alquilidéncicloalcanos estereoisoméricos permite determinar la magnitud de los acoplamientos alílicos en estos compuestos. En contraste con la tendencia normal, la magnitud de los acoplamientos en segmentos alílicos transoides es siempre mayor que la encontrada en sistemas alílicos cisoides, por aproximadamente 0.5 Hz. Los valores mas grandes en las constantes de acoplamiento para los sistemas transoides pudiera explicarse con base en el ángulo diedro existente entre el plano alílico =C–C–H y el plano en el segmento C=C–C presente en los isómeros E, que es ligeramente mas pequeño.

Keywords: RMN de protones, acoplamientos alílicos, alquilidéncicloalcanos.

Acknowledgment

This work was supported by the Connecticut Department of Economic Development. We thank the editorial board of this journal, and particularly Prof. Eusebio Juaristi, for arranging this memorial issue dedicated to Ernest Eliel.

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^*Dedicated to Professor Ernest L. Eliel: a giant in the field of stereochemistry and to us a mentor and friend to whom much is owed for his generous support and encouragement.