Transmission of polar substituent effects in the adamantane ring system as monitored by19F NMR

An extensive series of (E)/(Z)-5-substituted(X)adamant-2-yl fluorides (2 and 3, respectively) and (E)/(Z)-4-substituted(X)adamant-1-yl fluorides (4 and 5, respectively) were synthesized and characterized and their 19F chemical shifts measured in several solvents. Correlation of the 19F substituent chemical shifts (SCS) against polar field parameters (σF) together with comparisons against the 19F SCS of 4-substituted(X)bicyclo[2.2.2]oct-1-yl fluorides (1) provide unequivocal evidence for the importance of electrostatic field and ‘through-three-bond’ electron delocalization (double hyperconjugation) effects as long-range modes of transmission of polar effects in these saturated systems. The former effect is shown to be a function not only of spatial factors (angles and distance) but also the ‘stiffness’ of the C—F σ bond. The latter electronic mechanism is clearly the dominant factor regulating the 19F SCS of 2 and 4 but is switched ‘off’ in 3 and 5 as a result of specific stereoelectronic requirements.