Diastereoselective Reactions of Sulfur- and Selenium-Bridged Bisphosphaalkenes with Tetrachloro-o-benzoquinone

2,4-Diphospha-3-thia- and 3-selenapentadienes [(Me 3 Si) 2 C = P] 2 E ( 1a : E = S; 1b : Se) react as bifunctional phosphaalkenes with two equivalents of cyclopentadiene and of tetrachloro-o-benzoquinone (TOB), furnishing 2a and 2b from double [2+4] cycloaddition reactions of the diene and 3a and 3b from the reactions of TOB with the P = C double bonds. The phosphanorbornane-related chalcogenophosphinous anhydrides 2a and 2b are obtained as pairs of isomers, whereas the reactions with TOB proceed diastereoselectively. X-ray crystallographic data confirm that bis-dioxaphospholene-related 3b consists of a mixture of (RR) and (SS) enantiomers. A third equivalent of TOB can be added oxidatively to one phosphorus atom of 3a and 3b , furnishing the spirocyclic compounds 6a and 6b with P(σ λ 5 5 ) connectivity. 3a and 6a are configurationally stable at room temperature, whereas the selenium derivatives 3b and 6b undergo slow isomerisation in solution.