19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

NMR spectral analysis of strongly second-order 6-, 8-, 9- and 10- spin-systems (1 H─19 F, 19 F─19 F, and 13 C─19 F) in perfluorotoluyl- and tetrafluoro-pyridyl-aromatics using the lineshape method ANATOLIA

A simple to use nuclear magnetic resonance analysis method has been tested on complex ¹ H, ¹⁹ F, and ¹³ C multiplets. This open-source line-shape analysis method analysis of total lineshape (ANATOLIA)¹ provides some significant advantages over traditional assign-iterate methods of NMR spectral analysis by avoiding false minima and progressing optimisation to the global minimum. The target molecules are 1-perfluorotol-4-yl-2-perfluorotol-4-yl-oxymethyl-1H-benzimidazole (molecule-I) and 1-tetrafluoropyrid-4-yl-2-tetrafluoropyrid-4-yl-thio-1H-benzimidazole (molecule-II) which were produced as part of a family of fluorinated drug scaffolds prepared for anticancer and antiparasitic screening. Spectra display significant second-order effects with ¹ H Δδ = 3.68 and 4.67 Hz for the aromatic hydrogen "triplets", with ¹⁹ F ⁴ J_AA' , ⁴ J_BB' , ⁴ J_XX' , and ⁴ J_YY' coupling constants range from +4.8 to -14.0 Hz and for ¹³ C-isotopomers ¹⁹ F Δδ of up to 111.56 Hz. A spin-system of six coupling nuclei (H_a H_b H_c H_d F_Y F_Y' ) was analysed in 12 s, a spin-system of nine coupling fluorine nuclei (AA'BB'CCC-YY') was analysed within 2 min, and 10 coupling nuclei (XX'YY'ZZZ-BB'-H_d ) was optimised in 6 min using a laptop computer. ANATOLIA was also robust enough to be able to yield accurate spectral values from inaccurate input values. In both compounds, a fluorine-fluorine coupling constant was identified between the two fluoro-aromatic rings (F_BB' and F_YY' ) of +4.05 and +4.67 Hz and attributed to a through-space interaction. Ab initio structure optimisations and coupling constant calculations provided useful input data for spectral analysis. A modern ¹⁹ F nuclear magnetic resonance spectrum of perfluorotoluene (octafluorotoluene) and analysis from 1975 was used as a test data set to assess ANATOLIA.

Prediction of 19F NMR Chemical Shifts for Fluorinated Aromatic Compounds

Scaling factors are reported for use in predicting ¹⁹F NMR chemical shifts for fluorinated (hetero)aromatic compounds with relatively low levels of theory. Our recommended scaling factors were developed using a curated data set of 52 compounds, with 100 individual ¹⁹F shifts spanning a range of 153 ppm. With a maximum deviation of 6.5 ppm between experimental and computed shifts, or 4% of the range tested, these scaling factors allow for the assignment of chemical shifts to specific fluorines in multifluorinated aromatics. The utility of this approach is highlighted by several structural reassignments.

Conformational analysis of small molecules: NMR and quantum mechanics calculations

This review deals with conformational analysis in small organic molecules, and describes the stereoelectronic interactions responsible for conformational stability. Conformational analysis is usually performed using NMR spectroscopy through measurement of coupling constants at room or low temperature in different solvents to determine the populations of conformers in solution. Quantum mechanical calculations are used to address the interactions responsible for conformer stability. The conformational analysis of a large number of small molecules is described, using coupling constant measurements in different solvents and at low temperature, as well as recent applications of through-space and through-hydrogen bond coupling constants JFH as tools for the conformational analysis of fluorinated molecules. Besides NMR parameters, stereoelectronic interactions such as conjugative, hyperconjugative, steric and intramolecular hydrogen bond interactions involved in conformational preferences are discussed.

Through-space (19) F-(15) N couplings for the assignment of stereochemistry in flubenzimine

Through-space (19) F-(15) N couplings revealed the configuration of flubenzimine, with the CF3 group on N4 pointing towards the lone pair of N5. The (19) F-(15) N coupling constants were measured at natural abundance using a spin-state selective indirect-detection pulse sequence. As (15) N-labelled proteins are routinely synthesized for NMR studies, through-space (19) F-(15) N couplings have the potential to probe the stereochemistry of these proteins by (19) F labelling of some amino acids or can reveal the site of docking of fluorine-containing drugs. Copyright © 2016 John Wiley & Sons, Ltd.

Atropisomerism of a monosubstituted perfluoro[2.2]paracyclophane. A combined synthetic, kinetic, spectroscopic and computational study

The product of S(N)Ar addition of the enolate of ethyl acetoacetate to perfluoro[2.2]para-cyclophane exists entirely as its enol tautomer 5. This enol exhibits two NMR signals for its enolic proton, and these signals were shown to derive from the presence of two, equal energy conformations that were observable as distinct, stable conformations at room temperature, but which when heated, interconverted with an energy barrier of 23.5 kcal mol(-1). These atropisomers were characterized by NMR, with details of this analysis being provided. Computational work corroborated the NMR conclusions, and provided additional insight into all structural, thermodynamic and kinetic results. Enol product 5 was cyclized, under basic conditions, to form a benzofuran product 6. Its structure was confirmed by NMR, with further structural and mechanistic insights being provided by calculations.

Destination state screening of active spaces in spin dynamics simulations

We propose a novel avenue for state space reduction in time domain Liouville space spin dynamics simulations, using detectability as a selection criterion--only those states that evolve into or affect other detectable states are kept in the simulation. This basis reduction procedure (referred to as destination state screening) is formally exact and can be applied on top of the existing state space restriction techniques. As demonstrated below, in many cases this results in further reduction of matrix dimension, leading to considerable acceleration of many spin dynamics simulation types. Destination state screening is implemented in the latest version of the Spinach library (http://spindynamics.org).