Carbon-13 N.M.R. shifts as a measure of transannular interaction in unsaturated alicyclic systems

Abstract

Through-space transannular interaction between formally non-conjugated methylene and carbonyl n-electrons in conformationally rigid 3,7-disubstituted bicyclo[3.3.1]nonanes (1) has been demonstrated by Senda et al. by comparing the 13C n.m.r. shift values of the sp2 atoms. This procedure has been extended successfully to include conformationally mobile benzo derivatives (2) of bicyclo[3.3.2]decane and 1,5-disubstituted cyclooctanes (4). It also succeeds for 5,7-disubstituted derivatives (3) of 1,2,4-ethanylylidene-1H-cyclobuta[cd]pentalene where through- bond interaction is known to be involved. The method is concluded to be a general and reliable one for detection of transannular π-electron delocalization and to have potential in prediction of intramolecular cyclizations.