Collect. Czech. Chem. Commun. 2011, 76, 1199-1222
Published online 2011-09-22 09:09:26

Problems with a conformation assignment of aryl-substituted resorc[4]arenes

Jakub Kaminskýa, Hana Dvořákováb, Jan Štursaa,c and Jitka Moravcovác,*

a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo nám. 2, 166 10 Prague 6, Czech Republic
b Laboratory of NMR Spectroscopy, Prague Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic
c Department of Chemistry of Natural Compounds, Prague Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic


Acid-catalyzed condensation of resorcinol with 3,5-diisopropoxybenzaldehyde and 3,5-dihydroxybenzaldehyde afforded aryl substituted resorc[4]arenes 1a and 1b, respectively. All 16 hydroxyls in 1b were acetylated providing resorc[4]arene 1c. The conformational behaviour of 1a, 1b and 1c was studied by NMR spectroscopy and quantum chemical calculations. It was found that the stabilization of their conformations is an effect of competing π-π and OH-π interactions, hydrogen bonding and steric features, respectively. As a result, C2 symmetrical boat conformations 1a, 1b and 1c with aryls in axial positions were identified in all cases. In case of 1c also the formation of C2 symmetrical conformation with aryls in equatorial positions (boat-eq) was identified. Moreover, compounds 1a and 1b being able to create hydrogen bonds, adopt also symmetrical C4 crown conformations. For 1c(boat-ax), the boat-boat conversion with energy barrier of 80 kJ/mol was observed, while the 1c(boat-eq) was found to be rigid in the whole accessible temperature range. Both conformers of 1c exhibit also second dynamic process – rotation of bridge aryl rings (ΔG‡ = 66 kJ/mol).

Keywords: Aryl-substituted resorc[4]arenes; Conformational analysis; Density functional theory; NOE.

References: 34 live references.