Nitrile groups are suitable precursors for amino functions to which various other groups can be attached. Thus, the reaction of the 1,3-bis(3-bromopropyl ether) or 1,3-bis(4-bromobutyl ether) of tert-butylcalix [4]arene with methylmalononitrile has been studied in order to attach four nitrile groups to the narrow rim. Bimacrocyclic dinitriles were formed in 70% yield with malononitrile, and tetranitriles in 85% yield with methylmalononitrile. Subsequent alkylation of the tetranitriles with cobalt dioxane-bis(dicarbollide) gave dianionic calix[4]arenes, isolated as the cesium salts in 68% and 89% yield. Two compounds were characterized by single crystal X-ray analysis.Calixarenes in general and, due to their well-defined conformations, calix[4]arenes in particular have been frequently used as platforms for the assembly of various functional groups. 1 The attachment of four carbamoylmethylphosphinoxide (CMPO) groups to their wide rim 1 2 or narrow rim 2 3 (Figure 1), usually achieved by acylation of the respective tetramines, led to extractants for lanthanides and actinides with drastically increased efficiency compared to a single CMPO molecule [e.g., the technically used (N,N-diisobutylcarbamoylmethyl)phenyloctylphosphine oxide]. The required tetraaminocalix[4]arenes for the synthesis of narrow-rim CMPOs are usually obtained by reduction of cyanoalkyl ethers or by hydrazinolysis of phthalimido residues. Recently, we could show that the combination of two CMPO functions with two anionic cobalt bis(dicarbollide)(1 -) (Cosan) groups (i.e., calixarenes 3) leads to a further drastic increase in the extraction ability; 4,5 Am 3+ is removed from a 5 × 10 -8 M aqueous solution to more than 99% by a single extraction step with a 3 × 10 -6 M solution of 3 (n = 4) in hexyl methyl ketone-dodecane. On the other hand, the extraction ability strongly decreases (by two to three orders of magnitude) when the number of CMPO groups in 1 is reduced from four to three. 6 We therefore aimed to combine four CMPO functions with two Cosan groups on a calixarene platform. Since we excluded a calix[6]arene due to its conformational mobility, we envisaged the binding of two malononitrile units via carbon linkers 7 to the distal oxygen functions of a calix[4]arene, leaving the other two hydroxy groups for the usual attachment of the anionic Cosan groups.
Figure 1At a first glance, the covalent connection of malononitrile to the distal oxygen functions of a calix[4]arene might seem complicated. It requires a bifunctional linker, e.g. an a,w-dibromoalkane, while malononitrile itself is bifunctional under C-alkylation conditions and four oxygen functions can be alkylated in the calix[4]arene (see Scheme 1); however, the 1,3-di-O-alkylation is easily possible 8 with an excess of 1,4-dibromobutane in refluxing acetonitrile. Potassium carbonate as base clearly prevents further O-alkylation of 5b, which was obtained in 65% yield. Even better yields (up to 90%) were obtained for 5a with 3-bromopropanol under Mitsunobu conditions.Unexpectedly, howeve...