Rhodium compounds, polymer-immobilized

Butadiene is polymerized by rhodium compounds in aqueous or alcoholic solution [178]. It is generally accepted that the active species is a TT-allyl rhodium complex of low valency [28, 179] which is not rapidly terminated by reaction with water or alcohol. No clear kinetic pattern was observed in the earlier papers but a recent investigation [180] has shown the rate and molecular weight data to be accommodated by a scheme involving monomer transfer and physical immobilization of the active centres in precipitated polymer. In the initial stages the polymerization is first order in rhodium and, at constant monomer concentration, is (pseudo) zero order E = 14.8 kcal mole" ). This is followed by a declining rate which is almost independent of temperature. Molecular weights rise slowly to a maximum value with time (ca. 4000 after 22 h at 70°C). 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

Polymer-supported, carbon dioxide-protected NHCs were also prepared for catalysis reactions [150]. These supports were prepared using DMN-H6 with 3-(bicyclo[2.2.1]hept-2-ene-5-ylmethyl)-l-(2-propyl)-3,4,5,6-tetrahydropyrimidin-l-ium-2-carboxylate and a Schrock catalyst. They were later used for the trimerization reaction of isocyanates and for the cyanosilylation of carbonyl-containing compounds. Various metals could be immobilized through the NHC hgands including rhodium(I), iridium(l), and palladium(II). The resulting monoUths polymerized phenylacetylene and were successful catalysts for Heck-type couplings [150]. 

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