THEORETICAL PREDICTION AND EXPERIMENTAL TESTS OF CONFORMATIONAL SWITCHES IN TRANSITION-STATES OF DIELS-ALDER AND 1,3-DIPOLAR CYCLOADDITIONSTO ENOL ETHERS

Transition structures for the cycloadditions of butadiene, acrolein, n itrosoethylene, and methylenenitrone to 1-butene, silyl vinyl ether, a nd methyl vinyl ether have been located using ab initio RHF theory wit h the 3-21G basis set and with density functional theory using the Bec ke3LYP functional and the 6-31G basis set. The computational results show that there is a snitch in the conformation of the enol ether from syn (COCC = 0 degrees), which is favored by 2.3 kcal/mol in the react ant, to anti (COCC = 180 degrees), which is favored by 1.2-6.6 kcal/mo l in the various transition structures studied here. The results are c onsistent with the experimental stereoselectivities in reactions of ch iral enol ethers observed by Denmark and Reissig. The preference of th e anti conformation in the transition structures is due primarily to e lectrostatic effects and, to a lesser extent, steric effects. The pref erence is predicted to be influenced significantly by polar solvents. The magnitude of this preference was calculated theoretically and meas ured experimentally: the rates of cycloadditions of conformationally f ixed enol ethers, 2,3-dihydrofuran and 2-methylenetetrahydrofuran, wit h 1-nitroso-1-phenylethylene and with C-benzoyl-N-phenylnitrone were s tudied. Observed relative rates are in good agreement with prediction and confirm that enol ethers adopt the s-trans conformation in transit ion states of cycloadditions.