STRUCTURALLY-SELECTIVE GAS-PHASE ION-MOLECULE REACTIONS OF DIBENZO-16-CROWN-5 COMPOUNDS

Steric and substituent effects can play large roles in influencing the outcomes of organic reactions. In this work, the use of ion-molecule reactions of dibenzo-16-crown-5 compounds (lariat ethers) by tandem ma ss spectrometry to probe the influence of the pendant groups on the se lectivity of their gas-phase reactions was evaluated. Lariat ethers ar e macrocyclic ethers with pendant substituents that have been develope d as new types of hosts for molecular recognition. Dimethyl ether (DME ) was the reactant chosen because of its well characterized reactivity with various organic substrates possessing different functional group s. Only those dibenzo-16-crown-5 compounds with no or at most one subs tituent at the center carbon of the three-carbon bridge form the diagn ostic [M + 13](+) product ion through a methylene substitution process . Dibenzo-16-crown-5 compounds with geminal substituents on the center carbon of the three-carbon bridge form the [M + 45](+) ion, but not t he characteristic [M + 13](+) ion. Causative factors may be steric blo cking of the reaction pathway by the geminal groups or a requirement f or the presence of at least one hydrogen on the center carbon of the t hree-center bridge for formation of the [M + 13](+) ion. CAD, deuteriu m labelling, molecular orbital calculations and comparisons with model compounds provide additional information about the reaction pathways.