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Study of 19F-1H couplings in nuclear magnetic resonance. Jones, David Llewellyn

Abstract

A new method of synthesizing organic fluorides stereospecifically was developed; the elements 'BrF' and 'IF' were added quantitatively across the double bond of simple olefins such as cyclohexene, using elemental halogen and silver fluoride at room temperature in benzene. Application of this reaction to the activated olefin, acenaphthylene, resulted in a variety of products, depending on solvent; with pure acetonitrile solvent, quantitative 'BrF' addition occurs, but with an increasing proportion of benzene various side reactions take place, including difluorination and dimerization. Another series of compounds was synthesized from fluoro- acenaphthylene by various addition and substitution reactions, and all together fifteen acenaphthene fluorides were made, including thirteen new compounds. The ¹H and ¹⁹F N.M.R. spectra of these substances were measured and a set of ³JHF values obtained for a wide variation in the electronegativity of the substituents, and for two values of dihedral angle (0°and 100°). This data showed that the dependence of ³JHF upon electronegativity was far from linear, as had hitherto been supposed, but rather exponential, which is in retrospect more logical. Application of this data on ³JHF to some flexible systems, derivatives of indene and benzofuran, allowed their conformations to be determined. In all cases the pucker of the five-membered ring is small (∼25°), except in the cis-bromofluoride of benzofuran where it is much larger, (45-50°). A separate study was made of the conformational inversion of trans-1-fluoro-2-iodocyclohexane (I) and its specifically deuterated analogue, derived from 3,3,6,6-tetradeutero-cyclohexene. Several methods were employed to measure the position of the conformational equilibrium and this permitted a comparison of the methods themselves. It was found that all chemical shifts and coupling constants were temperature dependent, those involving ¹⁹F being particularly susceptible. The only reliable methods were found to be area measurements (at -90°C), linewidth measurements (at -45°C), and temperature-corrected ¹⁹F chemical shift measurements; thus the compound in question was found to have a preference for the diequatorial form, with ∆G° = 0.650 ± 0.025 kcal./mole (at -90°C to 0°C), and a near zero entropy, ∆S° = 0.0 ± 0.5 e.u. This value of ∆G° is only 0.1 kcal./mole smaller than the sum of the A-values for iodine and fluorine, and indicates that the repulsion between these two halogens in the diequatorial form is quite small, about one tenth that between iodine and chlorine, or between chlorine and chlorine, etc. A kinetic study of I was made by applying the general lineshape equation of Gutowsky and Holm for two-site exchange; calculated line-widths and positions were matched to the experimental ¹⁹F spectra (with all ¹H couplings removed by irradiation) at various temperatures, and thus an Arrhenius plot was made and activation parameters derived. ∆FƗ = 10.38 kcal./mole; ∆SƗ = 2.1 e.u.

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