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 31 P chemical shifts (-59 to -70 ppm), small cone angles (121-140°) and a similar electronic parameter to PPh 3 . Reaction of L1 with [Rh(acac)(CO) 2 ] gave the complex [Rh(acac)(CO)(L1)] with a ν (CO) of 1981.5 cm -1 , whereas reaction L1 with [Rh(CO) 2 Cl] 2 gave [Rh(CO)(L1) 2 Cl] with a ν (CO) of 1979.9 cm -1 , remarkably similar to the CO stretching frequencies reported for analogous PPh 3 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) 2 Cl 2 ] forming square planar trans -[Pd(L) 2 Cl 2 ] 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.