ELECTRON-TRANSFER CATALYZED-REACTIONS .3. KETENE DIENE [4 + 2] CYCLOADDITION PRODUCTS VIA CATION RADICAL INITIATED DIELS-ALDER REACTION OR VINYLCYCLOBUTANONE REARRANGEMENT

Whereas the thermal reaction of the aryl methyl ketenes 1a and 1b with 2 resulted, as anticipated, in the periselective formation of the vinylcyclobutanones 3-6, the aminium ion salt initiated cycloaddition afforded selectively the Diels-Alder products 10 and 12. The potential role of Bronsted and Lewis acids, the suitability of different one-electron oxidants, and the effect of reactant concentration, reaction time, and added neutral amines was tested. From the results it is deduced that in the aminium ion salt initiated reaction the diene cation radical reacts with a neutral ketene in a [3 + 2] cycloaddition. Additionally, a second approach to the all-carbon ketene/diene Diels-Alder products via the cation radical vinylcyclobutanone rearrangement was developed. Thus, 3a-6a could be smoothly rearranged to the norbornenones 10a-12a. Importantly, the stereochemical outcome depends on the individual isomer, covering the whole range from 100% retention to 100% inversion of configuration. With regard to the mechanism it is concluded that the rearrangement proceeds via a ring-opened distonic cation radical. The charge distribution in the intermediate cation radical and the energetics of the product cation radicals control the stereochemical outcome. The monocyclic vinylcyclobutanone 9 could be rearranged with 100% retention to the formal Diels-Alder product 14. No rearrangement occurred with 7. Although mechanistic results propose that the correct bond is cleaved in 7.+, formation of the corresponding norbornenone cation radical is presumably too slow because of energetic reasons.