Diels ionizable substrates

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

In the present context, the term electron rich alkenes refers primarily to enol ethers, enol sulfides, and A-vinylamides or A-vinylamines. Such alkenes are typically much more readily ionizable than are simple alkenes. The conversion of these substrates to the corresponding (highly electron deficient) cation radicals represents a sharp Umpolung. The Diels-Alder additions of tra j -anethole, phenyl vinyl ether, phenyl vinyl sulfide, 1,3-dioxole, and A-methylindole to 1,3-cyclohexadiene have been reported (Scheme 22) [49, 52]. 

Diels-Alder reactions. Enones substituted at the a -position with an acetal group form cyclic vinyloxocarbenium ions. The enhanced dienophilic reactivity of such species is synthetically useful, and diasteroselective processes have been developed in which the ionization is mediated by fluoroboric acid. Scandium(III) triflate provides lower diastereoselectivity, and BFj OEtj is useless, as extensive decomposition of the substrates occurs. 

Another important aspect is the question of the reaction mechanisms. Since the sonochemical activation of Diels-Alder systems seems to have no direct link with the volatility of the substrates, the reactions should not take place in the "microreactor" bubble, and a purely thermal interpretation based on the hot-spot theory then seems inadequate. Alternative explanations could be based on a redox reaction between the two partners, or an ionization of one reactant. The existence of intermediate charge transfer complexes in Diels-Alder chemistry has been postulated for a long time, but most authors consider that no complete electron exchange is actually involved. l With the strong electron-accepting tetracyano-ethylene, a partial transfer occurs, but not at all with acrylic compounds. 

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