Synthesis and Properties of Conformationally Restricted Cavitand Systems

Abstract

The work described in this thesis is focused on the synthesis, functionalisation and study of conformationally restricted cavitand systems. Cavitands are rigid molecules with a concave π-basic surface, which bind a variety of guests. Larger supramolecular systems have been synthesised by covalently connecting several cavitands together. Two such multi-cavitand arrays are presented in this thesis, namely superbowls and cyclotetra-cavitands.

Chapter 1 is a review of resorcinarenes and pyrogallolarenes. The configurational and conformational possibilities of these molecules is discussed, along with the various synthetic methods developed for their preparation and derivatisation. The reported types of hydrogen-bonded, metal coordinated and covalently-linked multi-resorcinarene and multi-pyrogallolarene structures are described. The host-guest chemistry of these structures is described, as is their use as a catalyst for chemical transformations.

Chapter 2 is an introduction to the chemistry of superbowls and related deep-cavity cavitands. The synthesis of superbowls by the reaction of four ‘wall’ cavitands with one ‘base’ cavitand is described, followed by the methods developed for their derivatisation. The ability of superbowls to act as hosts to tetra-alkyl ammonium salts and aspirin is discussed. Finally, the chemistry of deep-cavity cavitands with introverted functionality is surveyed.

Chapter 3 describes the synthesis of new ortho-functionalised, arylated cavitands. These arylated cavitands exist as two atropisomers due to restricted rotation of the biaryl bond. The two atropisomers are described as either ‘inside’ or ‘outside’, depending on whether the ortho-functionality resides inside or outside of the cavity of the cavitand. Methods for the synthesis and interconversion of these two atropisomers are investigated. By varying the temperature and solvent, either the inside or outside atropisomer could be made the predominant species in solution. The addition of acids to the solution of a cavitand with an amine substituent resulted in significant changes to atropisomer equilibrium ratios.

Chapter 4 describes the synthesis of new superbowls by a novel synthetic route. This route first combines one functionalised ‘wall’ cavitand with the ‘base’ cavitand, followed by the incorporation of three unfunctionalised wall cavitands. This novel approach permits the synthesis of sizeable amounts of previously difficult-to-obtain mono-functionalised superbowls. The first superbowl with introverted functionality is reported. A preliminary study of this superbowl’s inside/outside atropisomerism and host-guest chemistry is described.

Chapter 5 describes how cavitand bowls carrying two distally-positioned terminal alkyne substituents generate cyclotetramers from Glaser coupling reactions. The 1,3-butadiynyl-linked cyclotetra-cavitand exists predominantly in two equivalent saddle shaped conformations. Interchange is rapid on the human timescale at ambient temperature but is modulated through manipulation of bowl rim substituents. Conversion of the four 1,3-butadiynyl-linkers into 2,5-thiophenylene-linkers causes a switch to a square-shaped cyclotetra-cavitand with the four sulfurs directed outside of the newly-formed macrocycle.