Conformational studies of dibenzo-16-crown-5 lariat ethers

Conformations of sym-(R)(OR')dibenzo-16-crown-5 compounds with R = decyl and R' = H, CH3, CH2CO2H, and CH2CO2CH3, as well as R = alkyl, fluoroalkyl, alkenyl, and alkynyl and R' = CH2CO2H, in solution have been probed by NMR spectroscopy. The difference in chemical shifts of the diastereotopic hydrogens for the methylene groups adjacent to the geminally substituted ring carbon is utilized to assess the ease of inversion of the three-carbon bridges. For lariat ethers with R = C10H21, the chemical shift difference is much larger when R' = CH2CO2H and CH2CO2CH3 than H and CH3. In non-hydrogen bonding solvents, the chemical shift difference for lariat ethers with R = C10H21 is much larger when R' = CH2CO2H than CH2CO2CH3 which demonstrates intramolecular hydrogen bonding of the carboxylic acid-containing side arm with a crown ether oxygen.