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Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes
Author(s) -
Timmerman Peter,
Vreekamp Remko H.,
Hulst Ron,
Verboom Willem,
Reinhoudt David N.,
Rissanen Kari,
Udachin Konstantin A.,
Ripmeester John
Publication year - 1997
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19970031115
Subject(s) - hydrogen bond , chemistry , steric effects , intramolecular force , antiparallel (mathematics) , crystallography , crystal structure , melamine , proton nmr , non covalent interactions , calixarene , stereochemistry , stereocenter , molecule , organic chemistry , physics , quantum mechanics , magnetic field , enantioselective synthesis , catalysis
Abstract Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well‐defined box‐like assemblies in the presence of two equivalents of 5,5‐diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10 −4 M. This paper reports the first X‐ray crystal structure, and the MALDI TOF mass spectra together with the complete 1 H NMR spectroscopic characterization of these hydrogen‐bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3‐bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1 H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen‐bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. t Bu) can significantly inhibit the formation of the hydrogen‐bonded assembly, but this effect very much depends on the exact positions of the groups.