Diels-Alder reactions mechanistic considerations

Styrene, leads to [4 + 2]-adducts exclusively (251-258) 242 is also isolated in varying amounts, With methyl sorbate both [4 + 2]- and [4 + 4]-adducts (259-262) are obtained. 1,4-Dicarbomethoxybutadiene does not add to 138 242 is obtained in 90% yield. All photoadducts (251-262) are photolabile on irradiation (z — 253.7 nm) they are reconverted to 138 and the alkene. Significantly for preparative purposes, the product distributions (stereo-, regio-, and peri-selectivity) differ considerably from those obtained in the thermal Diels-Alder reactions (Section IV,B) mechanistic details are given in the original papers. 

For some time the synthetic potential of this reaction as a source of Diels-Alder adducts underwent considerable study. One outcome of these studies was the realization that of the four possible o-xylylenol isomers, only ones with the enolic OH group pointed out (the F-photoenol ) reacted with dienophiles [23]. Mechanistic studies picked up in the 1970s, after Matsuura and Kitaura reported that, in the absence of dienophiles, benzocyclobutenols are formed from 2,6-dialkylphenyl ketones but not from simple o-alkylphenyl ketones [24]. Previously the absence of cyclobutenol products had been quite puzzling and led to suggestions that they were formed from the initial biradical but underwent rapid electrocyclic opening to the o-xylylenols. 

Mechanistic Considerations in Lewis Acid Catalysis of the Diels-Alder Reaction [7]... 

Diels-Alder reactions have been thoroughly studied, theoretically and mechanistically, and both inter- and mtra-variants are of considerable preparative importance. However, modifications of old reactions are always being made and this year has seen the first reported case in which a monosubstituted benzene ring acts as the diene in an intramolecular [4 + 2] cycloaddition. When heated at 100 °C, with or without solvent, the allene carboxanilides (100) isomerize to pyrrolines (101), which may be crystallized from the reaction mixtures. 

Bauld has carried out a large number of mechanistic studies of radical cation mediated Diels—Alder and cyclobutanation reactions, as discussed in detail in two recent reviews. Much of the above discussion concerning the concerted vs. stepwise nature of the dimerization reactions also applies to the addition of an alkene radical cation to a different alkene. Although the addition of alkene radical cations to dienes can lead to both cyclobutane and Diels-Alder products, the latter usually predominate for dienes with at least modest s-cis conformer populations. It is clear that in some cases the Diels-Alder adducts arise via rearrangement of initial divinylcyclobutane products. However, cyclobutanation and Diels-Alder adduct formation have been demonstrated to occur by independent pathways in other systems. There is also considerable experimental and theoretical data in support of a concerted but nonsynchronous mechanism for these reactions. 

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