Reaktive E=C(p – p)π-Systeme, XX [1] / Reactive E=C(p–p)π-Systems, XX [1] : Synthese und Reaktivität der konstitutionsisomeren Perfluorphosphaalkene F3CP=C(F)CF3 und F5C2P=CF2 / Synthesis and Reactivity of the Isomeric Perfluorophosphaalkenes F3CP=C(F)CF3 and F5C2P=CF2

The vapour phase pyrolysis of the stannylphosphane Me₃SnP(CF3)C₂F₅ (5) at 330 °C leads to 1,2-elimination of Me₃SnF yielding a mixture of the two isomeric perfluorophosphaalkenes F₃CP=C(F)CF₃ (3) and F₅C₂P=CF₂ (4) in a 3:1 molar ratio. 3 is more labile than 4 and proves to be similar to the perfluoro-3-phosphapent-2-ene F₅C₂P=C(F)CF₃ (2) with respect to NMR data and chemical properties. On the other hand 4 resembles the perfluoro-2-phosphapropene F₃CP=CF₂ (1). These results have been deduced from the following reactivity studies: (i) The rate of dimerization being higher for 3 than for 4. (ii) Reaction of the mixture with diethylamine yielding the aminophosphane derivative F₃CP(NEt₂)CF(H)CF₃ (13) as a product of 3 and the C-aminophosphaalkene F₅C₂P=C(F)NEt₂ (14) as the derivative of 4. (iii) Reaction of the mixture with methanol giving the methoxyphosphane F₃CP(OMe)CF(H)CF₃ (15) from 3 and the secondary phosphane F₅C₂P(H)CF₂OMe (16) from 4 as precursors. 3 and 4 prove to be effective dienophiles in reactions with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene, respectively, producing the corresponding Diels-Alder adducts 9 to 12. For these preparations 5 can be used as an equivalent of 3 in a one-pot procedure at 70 °C. On the other hand the phosphane HP(CF₃)C₂F₅ (8) is suited as precursor for 3 in preparing HX addition products.