Rhodium Phosphite Precatalysts

With sterically hindered monophosphites, such as Alkanox 420, Rh complexes hosting one or two P-ligands are known [118]. The ratio increases with increasing phosphite concentration. Diphosphites form chelate complexes. 

Ortho-metalation of hydroformylation catalysts has been found to occur also during distillation of the reaction mixture of a continuous hydroformylation reaction. Especially in the presence of an excess of olefin and after stripping hydrogen 

---

Rhodium precatalysts with phosphites are mostly prepared by the reaction of Rh(acac)(CO)2 (acac = acetylacetonate) with the phosphite under syngas (Scheme 2.102). The precatalysts formed, such as HRh(CO) P , (with w + w = 4), usually cannot be isolated. They are observable only by spectroscopic methods [114,115]. The progress ofthe precatalyst formation can be investigated by means of UV-vis spectroscopy or, in case of chiral ligands, with UV-vis CD (circular dichroism) spectroscopy [116]. Different high-pressure NMR techniques coupled with IR provide valuable information about the structure of the precatalyst that is formed [92b, 114,117]. 

Scheme 2.104 Preparation of a phosphite-modified rhodium precatalyst from an ortho-metalated catalyst precursor.

---