Shape-persistent arylene ethynylene macrocycles have attracted much attention in supramolecular chemistry and materials science because of their unique structures and novel properties. In this Review we describe recent examples of macrocycle synthesis by cross-coupling (Sonogashira: aryl acetylene macrocycle or Glaser: aryl diacetylene macrocycle) and dynamic covalent chemistry. The primary disadvantage of the coupling methods is the kinetically determined product distribution, since a significant portion of oligomers grow beyond the length of the cyclic targets ("overshooting"). Better results have been obtained recently by a dynamic covalent approach involving reversible metathesis reactions that afford macrocycles in one step. Mechanistic studies demonstrate that macrocycle formation is thermodynamically controlled by this route. Remaining synthetic challenges include the efficient preparation of site-specifically functionalized structures and larger, more complex two- and three-dimensional molecules.

Shape-persistent macrocycles: structures and synthetic approaches from arylene and ethynylene building blocks.

Molecular Rods. 1. Simple Axial Rods.

Renaissance of annulene chemistry.

Conjugated oligoenynes based on the diethynylethene unit.

The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D9h ground state geometry with alternating C-C (1.36 angstroms) and C identical withC (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C(18) are described. The analysis of laser flash heating experiments on this precursor by time-of-flight mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C(18) as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor.

https://science.sciencemag.org/content/sci/245/4922/1088.full.pdf

A. R. Galbraith

Papers

722. Macrocyclic acetylenic compounds. Part II. 1,2:7,8-Dibenzocyclododeca-1,7-diene-3,5,9,11-tetrayne