Diphosphabicyclobutane

Diphosphabicyclobutanes Two isomeric diphosphabicyclobutanes (5,11) are obtained by treatment of the phosphaalkenes X‐P = C(Ph)Tms (1a) and X ‐ P = CTms2 (1b) (X = halogen Tms = trimethylsilyl) with 2, Mg, or Na. Both syntheses proceed by coupling of two phosphaalkene units affording, however, two different diphospha‐butadiene intermediates. 4 and 10 readily undergo isomerization reactions to form the diphosphabicyclobutanes. Irradiation of the phosphaalkenes Cp* ‐ P = C(Ph)Tms (12a) and Cp* ‐ P = CTms2 (12b) (Cp* = C5Me5) gives rise to a different reaction pathway. Dimerization of 12a, followed by homolytic cleavage of the Cp* ‐ phosphorus bond and intramolecular recombination produces the butterfly compound 5, whereas homolytic cleavage of the Cp* ‐ element bond in 12 b, followed by rearrangement and intermolecular recombination, affords the diphosphabicyclobutane 11.