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Abstract Photolyses oftrans‐Fe(CO)₃(P((CH₂)_n)₃P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe₃provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH₂)_n)₃P (1). These are isolated as mixtures ofin,in/out,outisomers that equilibrate with degeneratein,out/out,inisomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT³¹P NMR data establish or boundK_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((CH₂)_n)₃EX with any type of bridgehead. Isomeric diborane adducts1 a,d ⋅ 2BH₃are also characterized. Crystal structures ofout,out‐1 aandin,in‐1 a ⋅ 2BH₃aid isomer assignments and reveal unusual cage conformations.