Synthesis and ¹³C NMR Analysis of a Series of Bridgehead-Substituted Polycycloalkanes: ¹³C-Labeled Methyl as the Substituent

The synthesis of eight polycycloalkanes substituted at the bridgehead with a ¹³C-labeled methyl group is described. Their ¹³C nuclear magnetic resonance spectra have been measured and the various NMR parameters are reported. It is found that there is no linear correlation between coupling involving directly bonded carbon atoms and the product of the s characters of the bonding orbitals. The experimentally determined values of ³J(CC) are in close agreement with values calculated by using the self-consistent perturbation theory at the indo level of approximation. In several strained systems, nonbonded interactions provide significant contributions to the vicinal carbon-carbon coupling constants; MO results demonstrate that these interactions oppose through-bond effects. Very good agreement is also noted between the observed ³J(CC) values and those determined from a Karplus relationship; this includes the substrates in which through-space effects are especially important.