Kilo-scale synthesis and purification of 4,4′-[di-t-butyldibenzo]-18-crown-6 and its catalytic reduction to 4,4′-[di-t-butyldicyclohexano]-18-crown-6

Abstract

4,4′-[Di-t-butyldibenzo]-18-crown-6 (DTBDB18C6) and 4,4′-[di-t-butyldicyclohexano]-18-crown-6 (DTBDCH18C6) are valued for their ability to selectively extract metal ions, which has made them useful materials for applications like nuclear waste treatment, sensor development and isotope separations. This paper elucidates a novel method for synthesis and purification of DTBDB18C6, followed by heterogeneous catalytic hydrogenation of DTBDB18C6 to DTBDCH18C6. Synthesis of DTBDB18C6 was carried out in a 5 kg scale, through the slow addition of a dilute solution of the building block (t-butyl catechol) to a concentrated solution of a cyclising agent (dichloroethyl ether), resulting in an improved yield of 60%. The components of the reaction mixture were identified and the desired product was purified (>97% pure) using column chromatography followed by selective washing. The purification process was developed at an analytical scale and was scaled up to a 300 g scale. The effect of column diameter on eluent flow-rate and elution time was studied. The purified DTBDB18C6 was catalytically hydrogenated using a rhodium on alumina catalyst to DTBDCH18C6 in a 500 g scale. The reaction yield was enhanced to 75% by the addition of K⁺ ions in the reaction mixture. The synthesized DTBDCH18C6 showed high values of distribution coefficient for strontium extraction.