Structural Aspects of Cyclopropyl Homoconjugation: Experimental Studies and Ab Initio Calculations

Abstract

AbstractLow‐temperature crystal structural studies of a series of saturated and unsaturated bicyclo[2.2.1]heptadiene, heptene and heptane compounds with 7‐spirocyclopropyl substitution reveal significant differences in the bond lengths of the three‐membered rings and in the CC single bonds of the bicyclic fragment. A complex interplay of strain and different types of conjugation influence the molecular structure of the bicycloheptadiene derivative 1, where the difference in the length of the three‐membered ring bonds is 0.040 Å and all CC single bonds in the bicyclic fragment are lengthened significantly. Ab initio calculations at the HF/6‐31G(d) (to a minor extent) and MP2/6‐31G(d) levels are in good agreement with the experimental data. Calculated charge distributions and dipole moments further support the relevance of cyclopropyl homoconjugation in the investigated prototype of Walsh and through‐space π‐orbital interaction. Static difference electron density maps have been derived from the experimental data by multipole refinements which showed exocyclic shifts of electron density in the planes of the three‐membered rings and significant bond ellipticities at the CC single bonds in the unsaturated bicyclic units.