A Nontemplated Route to Macrocyclic Dibridgehead Diphosphorus Compounds: Crystallographic Characterization of a “Crossed‐Chain” Variant of in/out Stereoisomers

Reactions of (O=)PH(OCH₂CH₃)₂ and BrMg(CH₂)_mCH=CH₂ (4.9–3.2 equiv; m=4 ( a ), 5 ( b ), 6 ( c )) give the dialkylphosphine oxides (O=)PH[(CH₂)_mCH=CH₂]₂ ( 2 a – c ; 77–81 % after workup), which are treated with NaH and then α,ω‐dibromides Br(CH₂)_nBr (0.49–0.32 equiv; n=8 ( a′ ), 10 ( b′ ), 12 ( c′ ), 14 ( d′ )) to yield the bis(trialkylphosphine oxides) [H₂C=CH(CH₂)_m]₂P(=O)(CH₂)_n(O=)P[(CH₂)_mCH=CH₂]₂ ( 3 ab′ , 3 bc′ , 3 cd′ , 3 ca′ ; 79–84 %). Reactions of 3 bc′ and 3 ca′ with Grubbs’ first‐generation catalyst and then H₂/PtO₂ afford the dibridgehead diphosphine dioxides ( 4 bc′ , 4 ca′ ; 14–19 %, n′=2m+2); ³¹P NMR spectra show two stereoisomeric species (ca. 70:30). Crystal structures of two isomers of the latter are obtained, out,out‐ 4 ca′ and a conformer of in,out‐ 4 ca′ that features crossed chains, such that the (O=)P vectors appear out,out. Whereas 4 bc′ resists crystallization, a byproduct derived from an alternative metathesis mode, (CH₂)₁₂P (=O)(CH₂)₁₂(O=)P(C H₂)₁₂, as well as 3 ab′ and 3 bc′ , are structurally characterized. The efficiencies of other routes to dibridgehead diphosphorus compounds are compared.