Resorcinarene

Resorcinarenes (also Calixresorcinarene ) are macrocyclic compounds derived from resorcinol (1,3- dihydroxybenzene ). The resorcinol are respectively connected to two methylene bridges with one another so that a cyclic oligomer of four aromatic resorcinol produced. They are closely related to calixarenes, Pillararenes and Pyrogalloarenen and structurally reminiscent of cyclodextrins. With Adolf von Baeyer and Donald J. Cram, two Nobel laureates employed, inter alia, with the chemistry of resorcinares.

Synthesis

Resorcinarenes are usually represented by the condensation of an aldehyde with resorcinol under Bronsted acid catalysis in a suitable solvent or solvent mixture under reflux. The product can be obtained by recrystallization or chromatography, in pure form, and often in high yield.

In a reversible -type Friedel-Crafts aromatic substitution of activated by the acid attacks the aldehyde resorcinol in the ortho -and para- position. The resulting alcohol can again under acid catalysis to attack a molecule of resorcinol, water being released. In this reaction, there is no plastic material, as in the closely related synthesis of phenolic resins, since the reaction is reversible ( retro Friedel- Crafts), is performed under thermodynamic control, and since the resulting resorcinarene is the more thermodynamically stable product.

As an alternative to the drastic reaction conditions mentioned above resorcinarenes can also be displayed under solvent-free conditions or, alternatively, by Lewis acid -catalyzed protocols, starting either from resorcinol / aldehydes or of benzyl alcohols or of cinnamic acid esters.

Structure

Resorcinarenes each consist of four Resorcinmolekülen (1,3- dihydroxybenzene ), which bridges methyl 4 - are connected to each other, and 6-position. Depending on the aldehyde used in the synthesis, the methylene bridges each carrying an aliphatic or aromatic radical, which may comprise additional functionality. The configuration of the stereocenters at the methylene bridges is (mostly) uniform.

Resorcinarene can exist in various conformations, one of which vases is shaped the most attention, since it can be considered as a hemisphere with its concave surface. This also bowl or crown-like conformation has called stabilized by hydrogen bonds upper edge and a lower edge. The upper edge of which is decorated with eight hydroxy group which may be covalently bridged to force or to stabilize the Vasenkonformation. Alternatively, the hydroxyl groups can be functionalized to build Cavitands or carcerands, which are characterized by a relatively large internal volume, can be incorporated guest molecules in which.

Chemically closely related macrocycles are the phenol ( hydroxybenzene ) derived calixarenes and Pillararene, and from pyrogallol ( 1,2,3- trihydroxy ) derived Pyrogalloarene.

History

Already Adolf von Baeyer was reacted aldehydes with resorcinol, the resultant resinous product, however, could not be characterized with the help of that analysis. Since then, other researchers isolated in similar reactions crystalline products, but these could only poorly characterized. Dutch and bird struck a half century after Baeyer before the tetrameric cylische structure for these products, which was finally confirmed until almost 50 years later ( see below). Building on the work done by Erdtman and colleagues synthesized Högberg resorcinarenes and pointed their structure by NMR spectroscopy. Last doubts about the tetrameric structure of the resorcinarenes could be overcome later by Rötgenstrukturanalyse. One of the first recognized, inter alia, Aoyama and colleagues the potential of resorcinarenes for host-guest systems. It is now known that self-assemble the unmodified concave resorcinarenes under certain conditions hexameric capsules in the interior and catalytic reactions are again possible under certain conditions. Popular but resorcinarenes are mainly as the basis of synthetic receptors ( see below).

Use

In supramolecular chemistry resorcinarenes be used as a basis for the construction of molecular containers ( synthetic receptors) as cavitands or carcerands, the guest molecules can store in its interior. They are also used as filling material for chromatography columns.