Carbodiphosphorane Isomers Based on a 1,3‐Diphosphaindane Skeleton, and Their Precursors

In a search for ring‐strained carbodiphosphoranes , the diastereomeric bis‐phosphanes 1a(RR, SS, RS) and the symmetrical analogue 1c have been converted into the corresponding cyclic methylene‐bridged bis‐phosphonium bromides 2a, c and hexafluorophosphates 2b. 2a(RR; SS) and 2a(RS) were separated by fractional crystallization. Treatment of 2a–c with base (NH₃ or n‐BuLi) yields, in a first step, the semi‐ylide salts 3a–c which, unexpectedly, undergo hydrolytic or ammonolytic cleavage at the ylidic PCHP bridge resulting in formation of the phosphane oxide or phosphane imine salts 5a, c and 6a, respectively. The second deprotonation step, using (C₂H₅)₃PCHCH₃ as a transylidating agent, affords the conjugated bis‐ylide 7a in the case of 2a, 3a(RS), but gives the carbodiphosphorane 4c in the case of the methyl‐free 2c, 3c. With n‐BuLi, 2a, (RS) also yields the analogous species 4a(RS), probably as an Li^⊕ adduct, as proven by its conversion into the symmetrical methylated product 8a(RS). – 1c is methylated with CH₃I at one P centre only. The resulting phosphonium salt 10 gives the mono‐ylide 11 on treatment with NaNH₂.