Ab initio studies of pericyclic reactions involving phosphorus substituted alkenes and alkynes
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The Diels-Alder Reaction between 1,3,5-triphosphabenzene and phosphaacetylene, forming tetraphosphabarrelene, was examined. This reaction was then compared to its carbon analogue (benzene + acetylene). The phosphorus reaction gave a smaller activation energy than the carbon system, as well as producing a higher degree of exothermicity. These results are consistent with the previous work done on phosphorus substituted systems. All structures were optimized to the MP4/6-31G*//MP2/6-31G* level. Transition states, as well as ground states, were verified by analytical frequency calculations. Topological electron density analysis was performed on all species, and showed that both reactions are synchronous and concerted. Secondly, the [2+2] cycloaddition (self-addition) of 1H, 3Himinomethylidene phosphine was studied. Resulting products were a variety of 1,2- and 1,3- diphosphetane isomers. Our investigation reveals that formation of the 1,2-diphosphetanes proceeds in a single step, while the 1,3-diphosphetane is formed via a two step mechanism. Formation of the 1,3- product is kinetically favored, and is usually the observed product. The 1,2- product is thermodynamically favored, but is only observed under certain conditions.