Relative thermodynamic stabilities of isomeric vinyl ethers derived from 2-acetyl- and 2-isobutyryltetrahydrofurans. An experimental and DFT study

The relative thermodynamic stabilities of four isomeric vinyl ethers obtain ed by acid-catalyzed cleavage of MeOH from the dimethyl acetals of both 2-a cetyltetrahydrofuran and 2-isobutyryltetrahydrofuran have been determined b y chemical equilibration in cyclohexane solution. In addition, the structur es and relative energies of many of these compounds have also been studied theoretically by DFT calculations. In each series of compounds, the endocyc lic isomers, 5-(1-methoxyethyl)-2,3-dihydrofuran and 5-(1-methoxy-2-methylp ropyl)-2,3-dihydrofuran, respectively, show the highest thermodynamic stabi lities. This is particularly prominent for the vinyl ethers derived from 2- isobutyryltetrahydrofuran in which the equilibrium mixture consists of 90% of the endocyclic isomer at room temperature. The geometrical (exocyclic) i somers of the latter series of compounds show unexpected relative thermodyn amic stabilities, but on the basis of the geometry-optimized structures pro vided by the DFT calculations, a reasonable explanation can be found.