Electron transfer catalyzed reactions .6. Cation radical catalyzed Diels-Alder reaction of electron-rich allenes

The cation radical catalyzed cycloaddition of electron-rich arylallenes 1a-e with 1,2,3,4,5-pentamethylcyclopentadiene (2a) afforded the Diels-Alder products 3 and 4 at 0 degrees C in 5 min with a high chemoselectivity, facial selectivity, and stereoselectivity. From the results of various mechanistic tests, it is inferred that the electron transfer induced reaction proceeds via a [3 + 2] pathway by cycloaddition of the diene cation radical to a neutral allene 1 exhibiting a rather short chain length. However, with electron-withdrawing substituents at the remote end of the allene functionality, the cycloaddition is only a minor reaction path. With diarylallene 1f and diene 2a, or 2b, no cycloaddition but formation of products 7 and d(1)-7 occurred. It is concluded that distonic cation radicals may be intermediates in a stepwise formal Diels-Alder reaction induced by electron transfer.