Abstract
Calixarenes are cyclic oligomers obtained by condensation of suitable p-functionalised phenols with formaldehyde, usually allowing for the synthesis of the well known small calixarenes (including up to eight phenolic subunits). We report here the discovery of much larger members of this family, exhibiting sizes up to 90 phenolic subunits: the giant calixarenes. These macrocycles are obtained according to simple, easily scalable processes, in yields up to 65%. We show that the formation of these giant macrocycles is favored by an oxygen-containing-group at the para-position of the starting phenol, high concentrations of heavy alkaline bases (rubidium or cesium hydroxides) and long reaction times. A mechanism is proposed to rationalize these observations. These giant macrocycles can also be obtained in the quasi-solid state, opening interesting perspectives in the field of calixarenes chemistry. Along with their intrinsic fundamental interest, these objects are also opening interesting applicative potentialities.
Highlights
Calixarenes are cyclic oligomers obtained by condensation of suitable p-functionalised phenols with formaldehyde, usually allowing for the synthesis of the well known small calixarenes
We show here that both the one-step and two-steps processes, used in combination with some specific p-functionalized phenols, lead to a surprisingly high yield of very large cyclic oligomers, the giant calixarenes
During an attempted synthesis of p-(benzyloxy)calix[6]arene, we refluxed a mixture of p-(benzyloxy)phenol, formaldehyde and RbOH (0.3 equivalent vs. phenol) in xylene
Summary
Calixarenes are cyclic oligomers obtained by condensation of suitable p-functionalised phenols with formaldehyde, usually allowing for the synthesis of the well known small calixarenes (including up to eight phenolic subunits). P-(tBu)phenol is to date the only starting product allowing for calixarenes of various sizes to be obtained in the pure state on a large scale, thanks to the considerable work of Gutsche and collaborators during the 80’s1,38–40 The size of these macrocycles usually ranges from 4 to 8 phenolic repeating units (PRUs), with only very few studies being devoted to larger members (up to 20 PRU41–46). We show here that both the one-step and two-steps processes, used in combination with some specific p-functionalized phenols, lead to a surprisingly high yield (up to 65%) of very large cyclic oligomers, the giant calixarenes This strongly contrasts with the well-known p-(tBu)phenol/
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