¹⁹F nuclear magnetic resonance studies of aromatic compounds. Part II. The¹⁹F chemical shifts in meta- and para-substituted fluorobenzenes, and 4-substituted 3′- and 4′-fluoro-trans-stilbenes

Abstract

A detailed study has been made of the effects of substituents, X, upon the shielding of lgF nuclei in series of meta and para-X-fluorobenzenes, 4-X-4'-fluoro-trans-stilbenes, and 3'-fluoro-4-X-trans-stilbenes. The correlation of substituent chemical shifts for lSF nuclei in fluoroaromatic compounds with reactivity parameters is examined, and a reassessment is made of the Taft and Swain and Lupton separation of polar substituent effects into field/inductive and resonance components. Science and Technology, London SW7 2AYTHE study of substituent effects upon 19F shielding in aromatic fluorine-containing molecules is of interest .1-3 Many attempts have been made to interpret such effects in terms of local electron density changes caused by the ring substituents, by use of a variety of molecular orbital methods ranging from the simple Huckel MO theory to VESCF method^.^ The approach underlying most of this work has been to assume that in para-substituted fluorobenzenes the interacting groups are sufficiently separated for o bond effects to be negligible and to relate the observed shielding changes to x-electronic density variation upon the carbon atom a t o fluorine. A growing amount of experimental evidence t Part I, I. R. hger and L. Phillips, preceding paper.

Stereoselective addition reactions of allylic sulfides to acetylenic esters: (E)/(Z) stereochemical variations by Lewis acid

“…The stereochemistry of nucleophilic additions to acetylenic compounds is an object of current attention.1 The theoretical studies by Houk et al2 and Dykstra et al3 have predicted on the basis of ab initio calculations that nucleophilic additions to unactivated acetylenes proceed via a single transition state to give anti products, and this is observed experimentally.4 In contrast, nucleophilic additions to activated acetylenes are known to give variable stereochemical results, i.e., syn and anti adducts (eq 1), R-OOR . HNu depending on the substituents, nucleophiles, and reaction media.1,4-8 These are also compatible with the recent ab initio calculations by Caramella and Houk,9 which indicate that the vinyl anions formed by nucleophilic addition to acetylenes are bent but the barrier to inversion is considerably lowered by electron-withdrawing substituents. On the other hand, less attention has been paid to the stereochemical effect of the Lewis acids, whereas the Lewis acids have been often employed to activate the acetylenic esters in their addition reactions.10,11…”

Correlation of nmr Chemical Shifts with Hammett σ Values and Analogous Parameters

“…In contrast, the corresponding data for fluoro compounds, 8 It is noteworthy that these expressions use parameters, UI and u~, which are now defined essentially by kinetic and equilibrium data, and hence criticism of (S.20) and (S.21) does not necessarily undermine the validity of the dual-parameter approach itself. ' A major criticism of equation (S.lS) has been the lack of any resonance dependence, and Phillips and his co-workers 64 claim that 8(Fm) tFor the statistical evaluation of the quality of correlations Taft and his group prefer the quantity f (the ratio of the standard deviation to the rms of the data) to the more usual correlation coefficient, r.…”

“…79 Good correlations were found with (TI and (T~ for most systems in all solvents, but when Z was a neutral group results were not notably worse for correlations with ~ and ffi. With charged groups in the bridging position, data were correlated best with the appropriate inductive and modified resonance parameters ( 64 (subsequently modified 66 ) that changes in 8(Fp) for the stilbenes are wholly attributable to resonance interactions, is unreasonable. Some contribution from field effects must be involved, and, on the whole, Taft's dual-parameter approach is the more satisfactory.…”

Synthesis and biological evaluation of some stilbene-based analogues

“…1 H NMR spectra were recorded on a 'Bruker-300' instrument with tetramethylsilane (TMS) as the internal standard in CDCl 3 as a solvent. Stilbene analogues namely 4a (Bevington et al, 1995), 4b (Ager et al, 1972), 4e and 4h (Kvaran et al, 2000), 4f (Bunce et al, 1990), 4i (Traylor and Stewart, 1986), 4k (Siegrist et al, 1969), 4l (Gupta et al, 1984), 4m (Murata et al, 2002) and 4n (Prukala, 2006) were prepared according to literature.…”