Heterofunctionalised phosphites built on a calix[4]arene scaffold and their use in 1-octene hydroformylation. Formation of 12-membered P,O-chelate rings

Abstract

The calixarene phosphites L¹–L⁴ were obtained in high yield through reaction of PCl₃/NEt₃ with the monofunctionalised cone-calixarenes p-tert-butylcalix[4]arene(OH)₃OR, in which the R substituents bear an oxygen donor ligand [R = CH₂P(O)Ph₂ (L¹), CH₂CO₂Et (L²), CH₂C(O)NEt₂ (L³), CH₂CH₂OMe (L⁴)]. The calixarene core of the four ligands adopts a cone conformation and, hence, the phosphites become potential P,O-chelating systems. Phosphite L¹ is remarkably stable towards aqueous NaOH, but the presence of slightly acidic water results in phosphonate formation. Slow oxidation of L¹ in air afforded the corresponding mixed phosphine oxide–phosphate. In the complexes [RuCl₂(p-cymene)L¹], [cis-PtCl₂(L¹)₂] (9), trans-[PdCl₂(L¹)₂], [Pd(8-mq)Cl(Lⁿ)] (8-mqH = 8-methylquinoline, n = 1–3), [Pd(dmba)Cl(L¹)] (dmbaH = N,N-dimethylbenzylamine), [Pd(η³-C₄H₇)Cl(L²)], [Rh(acac)(CO)Lⁿ] (n = 1–3) and [RhCl(CO)(L¹)₂], the phosphites behave as a monodentate phosphorus donor ligands. Owing to their steric crowding, the two cis-disposed ligands of complex 9 cannot freely rotate about their coordination axis. In the solid state, the calixarene backbones of complex 9 display a so-called ‘up-up-out-up’ conformation. Chelating phosphite behaviour was found in the cationic complexes [Pd(8-mq)Lⁿ]BF₄ (n = 1–3). In solution, the large, chelating P,O-loop of the latter complexes swings from one side of the metal plane to the other, the dynamics possibly being facilitated by the flexibility of the calixarene backbone. The four oxo-functionalised phosphites were tested as catalysts for 1-octene hydroformylation. The observed reaction rates lie in the range reported for other medium-bulky phosphites. Furthermore, the hydroformylation rate decreases as the donor strength of the side group increases, suggesting binding of the O-donor during catalysis. The L/B ratios lie in the range 1.4–3.6, the highest linear aldehyde selectivity being observed with the phosphite ester L³.