Calix[4]arene with a stiff upper rim bridge: spontaneous macrocyclization, structure, and dynamic behaviour

Macrocyclisation of the upper-rim distal disubstituted bromomethylene calix[4]arene derivative proceeds spontaneously on a silica gel due to the spatial proximity of two reaction centers, affording the formation of a bridged calixarene derivative. Due to the short span of the bridge, the product represents an extreme example of calix[4]arenes fixed in the pinched cone conformation with two aromatic rings steeply inclined into the center of the calixarene cavity. Dynamic NMR spectroscopy was used to investigate the temperature-dependent rocking motion of the bridgehead and thermochemical parameters of this motion were determined experimentally, as well as estimated using computational methods. Mass spectrometry was employed to study the upper rim macrocyclization reaction in the gas phase. Macrocyclization of the upper rim disubstituted calix[4]arene is facilitated by the proximity of the reaction centers, imposed by the calixarene scaffold, affording the calix[4]arene with a short upper rim bridge and distorted backbone.