Bengt Thulin

NMR studies of conformations and dynamic processes—III : Cyclophanes with unsaturated bridges

Abstract Three cyclophanes, each displaying a different type of dynamic process, have been studied by NMR methods. The barriers to these processes are attributed mainly to the decrease in π-electron overlap between the benzene rings and adjacent double bonds which occurs in the transition state for each process. In [5 2 ] paracyclophanetetraene, two successive flippings of the benzene rings interconvert the two hydrogens in the methylene groups (Scheme 1). In tetramethyl [2 4 ] paracyclophanetetraene, the passage of one methyl group through the central cavity of the molecule interconverts two conformations of similar, but not equal, free energy (Scheme 2). In [2 6 ] orthoparacyclophanehexaene, the orthosubstituted rings change sides by passing through the centre of the cyclophane (Scheme 3).

NMR studies of conformations and dynamic processes—II : [26]Bicyclophanes with ethylene bridges

Abstract The conformations and dynamic processes in two bicyclophanes have been analysed on the basis of temperature-dependent 1H NMR spectra. Both bicyclophanes are suggested to have a lowest-energy conformation of D3 symmetry in which the substituents at all ethylene bridges are gauche+ (or gauche−) oriented. The interconversion of the mirror image conformers of each bicyclophane equilibrates the two hydrogens in each methylene group, the barriers being ca 36 and 37 kJ mol−1, respectively, as determined by line-shape analysis.

Electrochemical formation of dianions from unsaturated cyclophanes

A series of [2.4]cyclophanetetraenes and closely related compounds has been shown to undergo reversible two-electron reduction at a mercury electrode in dimethylformamide solution. The cyclophanes which contain a conjugated periphery of 4nπ-electrons, thus being formally antiaromatic hydrocarbons, form dianions of pronounced stability. A considerable degree of structural and geometrical variation is tolerated.

[2n]Para[44–n]paracyclophanes; synthesis, structure, and cyclic voltammetry

Three new unsaturated [2n]para[44–n]paracyclophanes (n= 1, 2, or 3) have been prepared by a new multiple Witting reaction. The structures and electron delocalisation of the neutral compounds and the anions are discussed. The latter were obtained by cyclic voltammetry at a mercury drop. Unsaturated cyclophanes with electron conjugation along the perimeter form stable dianions if the number of π-electrons is 4n+ 2 in the dianions.