Study of 5-Endo Cyclization of 5-Oxapenta-2,4-dienoyl Radical and Related Radicals by ab Initio Calculations. Unusual Nonradical Cyclization Mechanism

5- Endo cyclization was shown to be remarkably effective in a 5-oxapenta-2,4-dienoyl radical by theoretical calculations as well as our previous experiments, even though a 5- endo closure is disfavored according to Baldwin's rule. An ab initio calculation suggested that this acyl radical has a flat U-shaped geometry and can be represented as a ketene-substituted α-carbonyl radical. Furthermore, the carbonyl oxygen interacts with the electron-deficient ketene group in the formation of a new σ-bond in which the radical contributes to the stabilization of the transition state as if it were a delocalized cyclopentadienyl-like radical. This nonradical cyclization mechanism was supported by IRC and spin density analyses. On the other hand, the calculated cyclizations of related radicals ( i . e ., penta-2,4-dienoyl, 5-oxapent-4-enoyl, pent-4-enoyl, 5-oxapenta-2,4-dienyl, and penta-2,4-dienyl radicals) were shown to occur from an expected normal intramolecular addition to an olefinic or carbonyl terminus. In these cases, 5- endo cyclization was favored at the olefinic terminus as opposed to the carbonyl terminus. This can be explained by the different polarization of these radical acceptors toward nucleophilic radicals.