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 (NH 3 or n ‐BuLi) yields, in a first step, the semi‐ylide salts 3a – c which, unexpectedly, undergo hydrolytic or ammonolytic cleavage at the ylidic PCHP bridge resulting in formation of the phosphane oxide or phosphane imine salts 5a , c and 6a , respectively. The second deprotonation step, using (C 2 H 5 ) 3 PCHCH 3 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 CH 3 I at one P centre only. The resulting phosphonium salt 10 gives the mono‐ylide 11 on treatment with NaNH 2 .