Computational studies stepwise Diels-Alder reactions

The rearrangement of 2-phenyl-3-butyn-2-ol to 3-phenybut-2-en-l-al was examined computationally. The carbocation PhMeC+-C=CH is considered an intermediate or, under some conditions, a transition state. The stepwise Diels-Alder reaction of 1-trimethylsiloxy-1,3-butadiene and 4,6-dinitrobenzofuroxan was studied by computational methods. A stepwise mechanism with a persistent intermediate is supported. However, this intermediate is suggested to be a heterocyclic adduct and not the zwitterionic allyl cation/furoxan anion adduct proposed in the previous studies. 

The theoretical principles of cycloaddition reactions are well understood and there have been many computational studies (see Pericyclic Reactions The Diels-Alder Reaction). Often the hetero-cycloaddition reaction shows similar characteristics to the carbocyclic analog, but a number of special features have been noted. In heterocyclic chemistry the cycloaddition reactions are often dipolar computational studies show that a concerted mechanism is followed in the gas phase. However, a number of studies have noted that these dipolar cycloaddition reactions become stepwise when solvent effects are included (via the reaction field method), with a consequent loss of stereospecificity." Other characteristics of hetero-cycloaddition reactions which have been studied include the endo/exo selectivity" and the regiose-lectivity (for example, [2-1-2] vs. [2-1-4])." High levels of electron correlation are generally required in order to establish these selectivities. 

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