A facile synthesis of 1,3-diiodocalixarene 6 has been achieved by copper-catalyzed iodination of the 1,3-bistriflate ester 2a of p-tert-butylcalixarene. After protection of the hydroxy groups with iodomethane, diiodide 6 is subjected to halogen-lithium exchange with butyllithium, followed by carbonation with CO2 or formylation with N-formylpiperidine and subsequent deprotection of the hydroxy groups to give novel dicarboxylic acid 11 or dialdehyde 16 in practical yields. The iodo groups of diiodide 6 pass through the calixarene macrocycle; the activation free energy for the conversion of the more stable syn conformer 6syn to the less stable anti conformer 6anti is ΔG‡ = 104 kJ mol-1 at 298 K. Dialdehyde 16 shows fast self-exchange between two equivalent species with a cone conformation, ΔG‡, being 63.2 kJ mol-1. Dicarboxylic acid 11 adopts a cone conformation and forms a dimer in solution as suggested by 1H NMR and X-ray crystallographic analyses. The arrangement of the iodide groups of compound 6 can be fixed predominantly to anti (17a and 17b) by introducing bulky alkyl groups (e.g., propyl groups) onto the hydroxy groups. The stereospecific alkylation of the iodo groups of the resulting di-O-alkylated anti-1,3-diiodides provides access to the anti-1,3-dialkylcalixarenes 19, which is otherwise difficult to obtain.
ASJC Scopus subject areas
- Organic Chemistry