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Transmission of polar substituent effects across the bicyclo[1.1.1]pentane ring system as monitored by 19 F NMR shifts
Author(s) -
Adcock William,
Krstic Alexander R.
Publication year - 2000
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(200002)38:2<115::aid-mrc598>3.0.co;2-9
Subject(s) - chemistry , substituent , electronegativity , bicyclic molecule , polar , chemical shift , computational chemistry , fluorine 19 nmr , stereochemistry , crystallography , nuclear magnetic resonance spectroscopy , organic chemistry , physics , astronomy
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 19 F and 13 C NMR spectra were recorded. Correlation of the 19 F substituent chemical shifts (SCS) of 1 against the 19 F 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 (σ χ ) in 1 . The shift pattern for 3 is also more consistent with a dominant substituent electronegativity influence than an electrostatic field effect (σ F ). Most importantly, the signs of the 19 F SCS of 1 and 4 are diametrically opposed. The origin of the σ χ effect in 1 is ascribed to hyperconjugative and homohyperconjugative interactions governing the degree of electron population of the σ * orbital of the C– F bond. By contrast, the perturbation of the fluorine π‐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 © 2000 John Wiley & Sons, Ltd.