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

✍ Scribed by William Adcock; Alexander R. Krstic

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 102 KB
• Volume: 38
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(200002)38:2<115::aid-mrc598>3.0.co;2-9

✦ Synopsis

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 4substituted(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 ( ) in 1. The shift pattern for 3 is also more consistent with a dominant substituent electronegativity influence than an electrostatic field effect ( F ). Most importantly, the signs of the 19 F SCS of 1 and 4 are diametrically opposed. The origin of the effect in 1 is ascribed to hyperconjugative and homohyperconjugative interactions governing the degree of electron population of the Ł orbital of the C-F bond. By contrast, the perturbation of the fluorine -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.