Determination of the Group Electronegativity of CF3Group in 3′-O-CF3-Thymidine by1-NMR

Publisher Website
Google Scholar
Abstract
The interplay of enthalpy of the gauche effect (ΔH°GE) of the [X3′-C3′-C4′-O4′] fragment in various 3′-substituted (X) 2′,3′-dideoxythymidine derivatives 1–7 and the inherent anomeric effect drives the two-state North ⇄ South equilibrium in the constituent sugar moiety. The group electronegativity of 3′-OCF3 substituent in Marriott's, Inamoto's and Mullay's scales has been determined from simple calibration graphs correlating the group electronegativity of various 3′-substituents (X) in 2′,3′-dideoxythymidine derivatives 1–7 with the experimental strength (ΔH°GE) of the [X3′-C3′-C4′-O4′] gauche effect. ΔH°GE has been experimentally determined from pseudorotational analyses of temperature-dependent 3JHH coupling constants, and can be used as an unambiguous tool for direct experimental estimation of the group electronegativity of a specific substituent covalently attached to 3′-carbon of 2′,3′-dideoxythymidine, which can be compared, in turn, with the theoretical estimation carried out according to Marriott's or Inamoto's procedure. Inconsistency found between theoretical values in Marriott's and Inamoto's scales, on the one hand, and between our experimental estimate and the theoretical value in Marriott's scale, on the other, have been solved by refining the electronegativity scale using our experimental data for 1–7.