CYCLIZATION EFFECTS ON THE GAS-PHASE BASICITIES OF ESTERS AND ETHERS - AN EXPERIMENTAL AND MO STUDY

Ab initio calculations, at different levels of accuracy, have been performed for different cyclic oxygen bases (2-oxetanone, gamma-butyrolactone, oxetane, and tetrahydrofuran), for the corresponding acyclic counterparts (methyl acetate, ethyl acetate, methyl propionate, methyl ethyl ether, dimethyl ether, and methyl propyl ether), and for their protonated species. The gas-phase proton affinities for some of these species were reinvestigated by FT-ICR techniques. Our results show that beta-propiolactone, contrary to larger lactones, is less basic (6 kcal/mol) than the corresponding aliphatic ester. This is a consequence of substantial hybridization changes upon cyclization, which for five-membered (or larger) systems are quite small. Cyclization effects on ethers are smaller than those on esters and are in the opposite direction. Accordingly, oxetane is a stronger base than methyl ethyl ether. A topological analysis of the electronic charge density shows that protonation at the carbonyl oxygen atom of lactones produces significant activation of the alkyl C-O bond while the acyl C-O linkage becomes reinforced. Quite on the contrary, protonation at the ether-like oxygen atom activates both C-O bonds, but preferentially the acyl one, which in the particular case of beta-propiolactone breaks apart. As a consequence, while gamma-butyrolactone protonates at the carbonyl oxygen atom, for beta-propiolactone both protonated species (carbonyl-protonated and ether-protonated) are equally stable.