X‐Ray crystallographic study of a 1,3,2‐dioxaphosphorinane dimer containing five‐coordinated phosphorus in the 12‐membered ring

Attempted preparation of phosphorane 9 with a 1,3,2-dioxaphosphorinane ring attached diequatorially to phosphorus led to a mixture of products from which was isolated 12-membered ring phosphorane 10, formally the dimer of 9, in low yield (22%). An X-ray crystal structure of 10 revealed a highly symmetrical molecule with trigonal bipyramidal geometry at both phosphorus atoms. The 12-membered ring is attached to phosphorus diequatorially. The ring is strongly puckered about phosphorus with an equatorial O-P-O angle of 110°. The chemical shift equivalence of the CH2O resonances in the 1H NMR spectrum of 10 shows the 12-membered ring to be highly mobile at ambient probe temperature. At −89°C, these resonances are fully decoalesced. The value of ΔG‡ for the process that averages these peaks was found to be 10.1 kcal/mol at coalescence (−42.0°C).

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