Synthesis, structural properties, electrophilic substitution reactions and DFT computational studies of calixbenzofurans†
Calixbenzofurans have been synthesized by a modified TosMIC coupling reaction, followed by acid treatment and an intramolecular cyclization reaction with TMSI (trimethylsilyl iodide); X-ray analysis established the structures of two samples, both showing a cone conformation. 1H NMR spectroscopic analyses of the calixbenzofurans reveal that they can adopt drastically different conformations in solution and undergo very fast conformational changes relative to the NMR time scale. Calixbenzofuran 4a exists as two conformers, namely the cone and saddle forms, in a ratio of 83 : 17 at −50 °C. A series of calixbenzofuran derivatives was synthesized by electrophilic aromatic substitutions, such as bromination, formylation and acylation, to investigate the influence of the substituents on the conformations of the calixbenzofurans. 1H NMR spectral analyses of the acyl derivatives at room temperature indicated that these macrocycles exist as a mixture of two isomers that are slowly interconverted on the 1H NMR timescale. The conformational isomers of the calixbenzofurans and their derivatives obtained from DFT methods (based on the crystal structure analysis results) were used to estimate the total energies of the different conformations.