Dioxaphosphabicyclooctanes: small caged phosphines from tris(hydroxymethyl)phosphine.

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 P chemical shifts (-59 to -70 ppm), small cone angles (121-140°) and a similar electronic parameter to PPh. Reaction of L1 with [Rh(acac)(CO)] gave the complex [Rh(acac)(CO)(L1)] with a of 1981.5 cm, whereas reaction L1 with [Rh(CO)Cl] gave [Rh(CO)(L1)Cl] with a of 1979.9 cm, remarkably similar to the CO stretching frequencies reported for analogous PPh complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to -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)Cl] forming square planar -[Pd(L)Cl] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)] which was applied as a catalyst in Suzuki-Miyaura couplings, furnishing the C-C coupled products in moderate yields.