Synthesis and Conformational Properties of Partially Alkylated Methylene-Bridged Resorc[4]arenes – Study of the 'Flip-Flop' Inversion

Autor: Jitka Moravcová, Jan Stursa, Hana Dvořáková, Michal Čajan
Rok vydání: 2006
Předmět:
Zdroj: European Journal of Organic Chemistry. 2006:4519-4527
ISSN: 1099-0690
1434-193X
Popis: The deprotection of resorc[4]arene octaisopropyl ether 5 by boron trichloride in dichloromethane has been found to give a parent resorc[4]arene, 1, as well as a mixture of partially alkylated derivatives, 6, 7, 8 and 9, the ratio of which depended on the conditions used. The conformational behaviour of the prepared alkyl ethers was studied with temperature-dependent 1H NMR spectroscopy and quantum chemical calculations. It was found that the compounds 6, 7 and 8 interconvert between two possible identical crown conformations (flip-flop inversion). The activation free energy ΔG‡ values for this process, obtained by NMR, strongly depends on the number of hydroxy groups in the molecule; an increase in their number generally increases the values of ΔG‡. The flip-flop inversion was not observed in the cases of fully deprotected 1 and monoisopropyl resorc[4]arenes 9, which are soluble only in polar solvents, inconvenient for the formation of a hydrogen bonding system. Molecular modelling studies revealed that the flip-flop inversion proceeds via the consecutive rotation of two adjacent rings, leading to a transition state with deformed chair geometry. Subsequent rotation of the remaining two units leads to the formation of the second crown conformer. The calculated heights of the energy barriers correspond well with those obtained by NMR spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
Databáze: OpenAIRE
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