Transmission of polar substituent effects across the bicyclo[1.1.1]pentane ring system as monitored by 19F NMR shifts

A series of 3-substituted(X)bicyclo[1.1.1]pent-1-yl fluorides (1) together with a more limited series of 3-substituted(X)bicyclo[1.1.1]pent-1-yl trifluorides (4) were synthesized and their F-19 and C-13 NMR spectra were recorded. Correlation of the F-19 substituent chemical shifts (SCS) of 1 against the F-19 SCS of 4-substituted(X)bicyclo[2.2.2]oct-1-yl fluorides (5), together with a linear multiple regression analysis of the SCS against polar substituent parameters, provide evidence for the importance of strong electronegativity effects (sigma(chi)) in 1. The shift pattern for 3 is also more consistent with a dominant substituent electronegativity influence than an electrostatic field effect (sigma(F)). Most importantly, the signs of the 19F SCS of 1 and 4 are diametrically opposed. The origin of the sigma(chi) effect in 1 is ascribed to hyperconjugative and homohyperconjugative interactions governing the degree of electron population of the sigma* orbital of the C-F bond. By contrast, the perturbation of the fluorine pi-electron density appears to be the pertinent molecular parameter in 4. The (1)J(C,F) values of 1 were not subject to any sensible empirical analysis, Copyright (C) 2000 John Wiley & Sons, Ltd.