PhP=CPh 2 and Related Phosphaalkenes: A Solution Equilibrium between a Phosphaalkene and a 1,2‐Diphosphetane

The synthesis of phosphaalkenes, ArP=C(R)Ph ( 1 , a : Ar = Ph, R = Ph; b : Ar = o ‐Tol, R = Ph; c : Ar = Mes, R = H), bearing sterically less hindered substituents is reported. Phosphaalkenes 1a – b were prepared by treating Ph 2 C=O with ArP(Li)SiMe 3 , whereas 1c was accessed from the AlCl 3 ‐mediated reaction of ArP(SiMe 3 ) 2 and PhC(O)H. Both 1a and 1b dimerize to afford their respective 1,2‐diphosphetanes ( 2a and 2b ). Compound 2a was characterized by X‐ray crystallography. Dissolution of pure 2a in THF resulted in a temperature dependent equilibrium with monomer 1a (Δ H o = –94.6 ± 14.6 kJ mol –1 ; Δ S o = –284 ± 48 J mol –1  K –1 ). Although monomer E/Z ‐ 1c was identified in solution by its characteristic downfield 31 P NMR chemical shift ( δ = 247.1, 231.5), it was accompanied by the formation of what we believe are oligomers ( δ = –25 to 10 ppm, br.). Attempts to trap phosphaalkenes 1a and 1b by treatment with W(CO) 5 (MeCN) afforded mixtures of complexes W(CO) 5 ( 1a – b ) ( 4a – b ) and W(CO) 4 ( 1a – b ) 2 ( 5a – b ), the disubstituted species being subjected to X‐ray crystallographic characterization ( 5a ).