Molecules that Turn Themselves Inside-Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrobicyclic Dibridgehead Diphosphines P((CH 2 ) n ) 3 P Newly Accessible by Earth-Abundant-Metal Templates.

Photolyses of trans-Fe(CO) ₃ (P((CH ₂ ) _n ) ₃ P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe ₃ provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH ₂ ) _n ) ₃ P (1). These are isolated as mixtures of in,in/out,out isomers that equilibrate with degenerate in,out/out,in isomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT ³¹ P NMR data establish or bound K _eq , rates, and activation parameters for a variety of phenomena, many of which involve homeomorphic isomerizations, topological processes by which certain molecules can turn themselves inside out (e. g., in,in⇌out,out). This provides the first detailed mapping of such trends in homologous series of aliphatic bicyclic compounds XE((CH ₂ ) _n ) ₃ EX with any type of bridgehead. Isomeric diborane adducts 1 a,d ⋅ 2BH ₃ are also characterized. Crystal structures of out,out-1 a and in,in-1 a ⋅ 2BH ₃ aid isomer assignments and reveal unusual cage conformations.
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