168649-10-3Relevant academic research and scientific papers
Alkylation Products of a Calix[8]arene Trianion. Effect of Charge Redistribution in Intermediates
Neri, Placido,Consoli, Grazia M. L.,Cunsolo, Francesca,Rocco, Concetta,Piattelli, Mario
, p. 4236 - 4239 (2007/10/03)
The behavior of a well-defined calix[8]arene trianion under alkylation conditions was investigated by reacting the tetraethylammonium salt of p-tert-butylcalix[8]arene trianion with p-methylbenzyl bromide. Chromatography of the reaction mixture afforded the following derivatives in order of decreasing yields: 1,3,5,7-tetrakis-, 1,3-bis-, 1,3,5-tris-, 1,2,4-tris-, and mono(p-methylbenzyl)-p-tert-butylcalix[8]arene, besides trace amounts of 1,4- and 1,5-disubstituted compounds. Comparable results were obtained using p-tert-butylbenzyl bromide. The observed regioselectivity has been explained by assuming that after each alkylation step the left negative charge is redistributed, by exchange equilibria, over the remaining phenolic groups with preferential localization at those positions where contiguous H-bond stabilization is possible and the electrostatic repulsion is minimized. The occurrence of protonation/deprotonation equilibria allows the formation of 1,3,5,7-tetrasubstituted calix[8]arene.
"Alternate Alkylation" of p-tert-Butylcalixarene in the Presence of Weak Bases
Neri, Placido,Geraci, Corrada,Piattelli, Mario
, p. 4126 - 4135 (2007/10/02)
Alkylation of p-tert-butylcalixarene in the presence of weak bases has been investigated using p-methylbenzyl bromide as an electrophile and CsF as a weak base in THF/DMF.Initially, a sequence of monoalkylation steps on progressively more alkylated compounds has been carried out, leading to the isolation and characterization of 13 p-methylbenzyl ethers ranging from mono-to octasubstituted.Successively, the time course of the composition of the reaction mixture has been determined at regular time intervals by chromatographic analysis and products isolation, in order to have a semiquantitative dynamic picture of the whole process.The reaction path goes mainly through the formation of mono-, 1,3-di-, 1,3,5-tri-, 1,3,5,7-tetra-, 1,2,3,5,7-penta-, 1,2,3,4,5,7- and 1,2,3,5,6,7-hexa-, and heptasubstituted derivatives.This result agrees with an "alternate alkylation" mechanism based on the preferential formation of monoanions stabilized by two flanking hydrogen bonds.However, the isolation of appreciable amounts of 1,2,4-tris(p-methylbenzyl) ether indicates that less stable monoanions can also be formed, possibly due to either temporary conformational preferences or the conformational mobility of the calixarene macrocycle, resulting in a less effective hydrogen-bonding stabilization of anionic intermediates.
