Organophosphorus compounds, 137 - On the unusual dienophilicity of trimethylsilylphosphaacetylene

The diphosphatricyclooctenes 8a, b are obtained from reactions of the phosp haacetylene 2 with the buta-1,3-dienes 6a, b through a reaction sequence in volving Diels-Alder, ene, and intramolecular [4 + 2] cycloaddition reaction s with the intermediates 7 and 9. An ab initio MO investigation at the MP2/ 6-31+G(.) level of theory has provided reasons why the postulated intermedi ates cannot be detected. The polycyclic products 8a, b were characterized i n the form of the corresponding metal carbonyl complexes 10a, b, and 11. Re action of the silylated phosphaalkyne 2 with cyclohexa-1,4-diene (12) furni shes the monophosphatricyclooctene 14; this reaction has also been studied by means of ab initio calculations. The result of the reaction of 2 with cy clopentadiene (15), however, is a complete surprise: the triphosphaspiropol ycyclic system 17 is formed, apparently through Diels-Alder, homo-Diels-Ald er, and homo [4 + 1] cycloaddition reactions with the intermediates 16 and 18.