| Abstract
| - Mechanisms of palladium-catalyzed cycloadditionreactions of methylenecyclopropane with olefins and thoseofoxatrimethylenemethane (OTMM) and azatrimethylenemethane (ATMM)complexes with olefins have been studiedby applying the ab initio molecular orbital method.Assuming an intramolecular coupling mechanism, wehaveexamined some model complexes,Pd(η2-methylenecyclopropane)(ethylene)(PH3)and Pd(η3-OTMM orη3-ATMM)(ethylene)(PH3), to determine the transition statestructures. The coupling of methylenecyclopropane,OTMM,and ATMM with ethylene on the metal center takes place in two steps.In the reaction of methylenecyclopropane,the first step involves an opening of the cyclopropane ring promoted byan attack of ethylene. Themethylenecyclopropane moiety has a π-allylic form at the firsttransition state which leads to a metallacyclicintermediate. The first transition state of the reactions of OTMMand ATMM complexes looks very similar tothat of the methylenecyclopropane complex, having π-allyliccoordinations. The two paths are separated in thesecond step. A [3 + 2] addition product is obtained by areductive elimination from the intermediate metallacycle,whereas a prototropic shift followed by a reductive elimination affordsa [2 + 1] addition product. It is theenergetics of the second stage starting from the metallacycles thatdifferentiates the reactions of the OTMM andATMM complexes from the reaction of the methylenecyclopropane complex.The relative stabilities of variousisomeric forms of these complexes have also been studied.
- Abinitio molecular orbital calculations were carried out on thepalladium catalyzed [3 + 2] coupling of methylenecyclopropane withethylene and [2 + 1] couplings of oxa- and azatrimethylenemethanewith ethylene. The mechanistic difference was clarified byexamining the potential surfaces of thesereactions.
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