Dioxaphosphabicyclooctanes: small caged phosphines from tris(hydroxymethyl)phosphine

Abstract

Dioxaphosphabicyclo[2.2.2]octanes (L1–L4) have been prepared in a one-pot reaction from tris(hydroxymethyl)phosphine and various α,β-unsaturated ketones. The non-volatile phosphines oxidise very slowly in air. They possess highly upfield ³¹P chemical shifts (−59 to −70 ppm), small cone angles (121–140°) and a similar electronic parameter to PPh₃. Reaction of L1 with [Rh(acac)(CO)₂] gave the complex [Rh(acac)(CO)(L1)] with a ν_(CO) of 1981.5 cm⁻¹, whereas reaction L1 with [Rh(CO)₂Cl]₂ gave [Rh(CO)(L1)₂Cl] with a ν_(CO) of 1979.9 cm⁻¹, remarkably similar to the CO stretching frequencies reported for analogous PPh₃ complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to n-nonanal of 68%, regardless of the substituents. However the ligand substituents had a significant effect on the catalyst activity, with increased steric bulk around the coordination environment giving a three-fold increase in aldehyde yield. The phosphines undergo ligand subsitution with [Pd(MeCN)₂Cl₂] forming square planar trans-[Pd(L)₂Cl₂] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)₄] which was applied as a catalyst in Suzuki–Miyaura couplings, furnishing the C–C coupled products in moderate yields.