The Intramolecular Edge‐to‐Face Interactions of an Aryl CH Bond and of a Pyridine Nitrogen Lone‐Pair with Aromatic and Fluoroaromatic Systems in Some [3,3]Metaparacyclophanes: A Combined Computational and NMR Study

Abstract

Simple model systems based on the [3,3]metaparacyclophane skeleton have been designed to study the effect of fluorination of the "face" ring on the edge-to-face (EtF) interactions with the C(Ar)-H bond of a phenyl ring or the nitrogen lone-pair of pyridine. Calculations established that in their more stable conformation the model systems adopt a tilted EtF disposition with the rim of the meta-substituted ring pointing towards the face of the para-substituted ring. Topomerization occurs by flipping of the meta-substituted ring, a process that involves the formation of an intermediate featuring an orthogonal EtF disposition of the arenes, which is less stable than the tilted arrangement. The energy barriers to the isomerization were determined by variable-temperature NMR spectroscopy and were well reproduced by DFT calculations. The variation in the energy barrier as a function of the substitution of the para-substituted ring could be rationalized well by a polar interpretation of the EtF interaction in the cases of model systems presenting the PyN...pi interaction but not in the cases of models featuring the C(Ar)-H...pi interaction.

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

“…If the temperature has to be decreased well below -100°C, the samples have to be prepared in a more sophisticated way, with use of liquefied gases such as Me 2 O, [15] vinyl chloride, [16] propane, [17] propene, [18] Freons ® or mixtures of these (see Table 1). CHF 2 Cl/CHFCl 2 , 3:1 -182 [22] CHF 2 Cl/CHFCl 2 , 5:1 -190 [23] CHF 2 Cl/CHFCl 2 /CF 2 Cl 2 , 3:1:1 -170 [24] CHF 2 Cl/CHFCl 2 /CHF 3 , 5:1:1 -188 [25] CHF 2 Cl/CHFCl 2 /CHF 3 , 3:1:1 -182 [26] CCl 2 F 2 /CBrF 3 , 4:1 -157 [27] CF 2 Cl 2 /CBrF 3 , 2:1 -166 [28] CHFCl 2 /CCl 2 F 2 , 1:1 -165 [29] CHF 2 Cl/CHFCl 2 , 1:1 -174 [30] CHCl 2 F/CH 2 =CHCl, 6:1 -160 [31] Me 2 O/THF, 3:1 -140 [32] THF/Et 2 O, 3:2 -135 [32] CHF 2 Cl/CDFCl 2 , 3:1 -171 [33] CH 2 =CHCl/CS 2 , 4:1 -131 [34] CH 2 =CHCl/CS 2 , 3:2 -132 [34] CH 2 =CHCl/CHFCl 2 , 5:2 -152 [35] CD 2 Cl 2 /[D 8 ]toluene, 1:1 -120 [36] The use of hydrogen-containing solvents is often impractical because of their very strong signals, which can overlap with the signals of the dilute solute. Freons ® , in contrast, either do not show any signal in the proton spectra (CBrF 3 , CF 2 Cl 2 ), or only give a signal in the aromatic region of the spectrum (CHFCl 2 , CHF 2 Cl, CHF 3 ) and are also quite good solvents even at very low temperatures.…”

Molecular Structures of THF‐Solvated Alkali‐Metal 2,2,6,6‐Tetramethylpiperidides Finally Revealed: X‐ray Crystallographic, DFT, and NMR (including DOSY) Spectroscopic Studies

“…The flip barrier20,21 of 2,6‐ ansa ‐pyridines is, in the case of a heptamethylene chain, only moderately smaller than that of the carba‐analogous biphenyl (9.0 vs. 11.5 kcal/mol for Z = N and CH, respectively; Table 1). 22,23 Also, in the case of some metaparacyclophanes containing two CH 2 SCH 2 links between the two aryl rings, the differences are quite small 24,25…”

Direct Synthesis of Fe3C‐Functionalized Graphene by High Temperature Autoclave Pyrolysis for Oxygen Reduction

“…The simplicity of such spectra allows the assumption that molecules 5a and 5b adopt, as preferred, an almost orthogonal edge‐to‐face (EtF) disposition (Figure 1),2d with the presence of a symmetry C 2 ‐axis perpendicular to the (1,4)‐bridged aromatic ring and bisecting protons in the 15‐ and 18‐positions of the meta ‐disubstituted benzene. Variable‐temperature NMR (VTNMR) experiments, performed on a sample of 5a (see the Supporting Information) support this assumption, but the possibility of flipping of the (1,3)‐bridged aromatic ring from one side to the other of the (1,4)‐bridged aromatic ring on the NMR time‐scale, cannot be ruled out 2e…”