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The utility of the PM3 method for predicting the reactivities of cyanoethenes in diels‐alder reactions with pyrrole
Author(s) -
Jursic Branko S.,
Zdravkovski Zoran
Publication year - 1994
Publication title -
journal of heterocyclic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.321
H-Index - 59
eISSN - 1943-5193
pISSN - 0022-152X
DOI - 10.1002/jhet.5570310623
Subject(s) - chemistry , cyclopentadiene , reactivity (psychology) , computational chemistry , homo/lumo , molecular orbital , pyrrole , diels–alder reaction , transition state , ab initio , photochemistry , organic chemistry , molecule , catalysis , medicine , alternative medicine , pathology
The reliability of PM3 for predicting the reactivity of cyanoethenes in Diels‐Alder reactions with pyrrole was tested. Using frontier orbital theory it was predicted that the reaction is LUMO dienophile controlled, and the reaction should be facilitated in comparison to cyclopentadiene additions (pyrrole has higher HOMO energy). The transition structures were generated and compared with similar ones generated with both ab initio and PM3 calculations for cyclopentadiene additions: The geometries of the transition structures are similar, although the new CC forming bonds are 2–6% shorter, and the degree of asynchronicity is higher. The prediction of the activation barriers with the PM3 method failed totally. The preferred exo isomer in each case, indicated that the predicted stereoselectivity was incorrect. The prediction of the relative reactivity was also unreliable. Qualitative analyses of similar series of compounds can be obtained by frontier orbital energy correlations. All conclusions were made on the assumption that the reaction is concerted, which does not exclude the possibility that PM3 might be able to predict reliable reaction barriers if some other mechanisms are considered.