Molecules that Turn Themselves Inside‐Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrobicyclic Dibridgehead Diphosphines P((CH2)n)3P Newly Accessible by Earth‐Abundant‐Metal Templates

Abstract

AbstractPhotolyses of trans‐Fe(CO)3(P((CH2)n)3P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe3 provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH2)n)3P (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 31P NMR data establish or bound Keq, 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((CH2)n)3EX with any type of bridgehead. Isomeric diborane adducts 1 a,d ⋅ 2BH3 are also characterized. Crystal structures of out,out‐1 a and in,in‐1 a ⋅ 2BH3 aid isomer assignments and reveal unusual cage conformations.