Computational Studies of the Reactivity, Regio-Selectivity and Stereo-Selectivity of Pericyclic Diels-Alder Reactions of Substituted Cyclobutenones

Abstract

The power of the Diels-Alder reaction was expanded recently through the discovery by Li and Danishefsky that cyclobutenone is an unusually reactive dienophile and that the adducts formed can be converted to products that are formally the Diels-Alder adducts of unreactive dienophiles. However, the effects of substituents on the reactivity as well as the region - and stereo-selectivity of the Diels-Alder reactions of cyclobutenone have not been clearly elucidated yet. This paper reports the results of a computational study at the MP2/6-31G* level of theory into the effects of substituents on the reactivity, regio-selectivity and stereo-selectivity of the Diels-Alder reactions of some substituted cyclobutenones with cyclic and acyclic dienes. It was found that the Diels-Alder reaction of maleic anhydride is far more feasible kinetically than the reaction of cyclobutenone, the activation barrier of the former being more than thrice that of the latter, indicating that maleic anhydride is a far better dienophile than cyclobutenone, which in turn implies that for cyclic dienophiles ring strain is not the dominant factor controlling the kinetics of the Diels-Alder reaction as has been suggested elsewhere. The Diels - Alder reactions of cyclobutenones were all found to follow an asynchronous concerted reaction pathway. In the reactions of the parent (unsubstituted) cyclobutenone with 1,3-butadiene and cyclopentadiene, the endo pathway is the most preferred kinetically, by 2.24 and 1.64 kcal/mol respectively. However, in the reactions of the 4,4-disubstituted cyclobutenones the exo pathway becomes the most preferred in the reactions with both 1,3-buadiene and cyclopentadiene, except for the CN-substituted cyclobutenone where the endo pathway is still the most preferred pathway. In the reactions of the 4-monosubstituted cyclobutenone with 1,3-butadiene, the anti-positions are preferred over the syn positions. The endo-anti position gives the most reactive dienophile kinetically. In the reactions of trans-piperylene with substituted cyclobutenones, the meta-endo position is the most preferred kinetically. In the reactions of isoprene with substituted cyclobutenones, the para-endo substitution gives the lowest activation barriers and therefore the most favorable reaction kinetics. In all the reactions considered in this work, the CN-substituted species have the lowest activation barriers and the most stable products. In the reactions of 4,4-disubstituted cyclobutenones with 1,3-butadiene and cyclopentadiene, the order of activation barriers is CN OH>Cl>CH3.