Substituent chemical shifts in NMR. Part 4—1H SCS in some 2‐substituted norbornanes and bornanes

Abstract

AbstractThe 1H chemical shifts and substituent chemical shifts (SCS) were recorded for several monosubstituted norbornanes and bornanes. The observed substituent effects are generally similar for the two ring systems, but difter considerably from those observed for the cyclohexanoid systems in steroids. A consistent trend for the eclipsed C‐2—C‐3 fragment in this ring system is that all the substituents produce the largest SCS on the trans oriented proton, rather than the spatially nearer cis oriented proton. Indeed, for the OH substituent these SCS are opposite in sign, that for the cis oriented proton being negative (i.e. to high field). This trend is not observed in the cyclo‐hexane system, in which the distance dependence of the vicinal SCS is as expected. The major cross‐ring SCS are of the 2‐endo substituents with the C‐6 protons, giving a large positive SCS at the 6n proton and a small negative SCS at the 6x proton. This trend is very similar to that observed in the cyclohexane ring (axial‐axial SCS), and suggests a similar electric field mechanism.