Rhodium, 2,4-pentanedionate

Ligand-Modified Rhodium Process. The triphenylphosphine-modified rhodium oxo process, termed the LP Oxo process, is the industry standard for the hydroformylation of ethylene and propylene as of this writing (ca 1995). It employs a triphenylphosphine [603-35-0] (TPP) (1) modified rhodium catalyst. The process operates at low (0.7—3 MPa (100—450 psi)) pressures and low (80—120°C) temperatures. Suitable sources of rhodium are the alkanoate, 2,4-pentanedionate, or nitrate. A low (60—80 kPa (8.7—11.6 psi)) CO partial pressure and high (10—12%) TPP concentration are critical to obtaining a high (eg, 10 1) normal-to-branched aldehyde ratio. The process, first commercialized in 1976 by Union Carbide Corporation in Ponce, Puerto Rico, has been ficensed worldwide by Union Carbide Corporation and Davy Process Technology. 

Thermolysis of (cycloheptatrienylmethyl)carbene complexes 554 [toluene, 1-2 h, 80-100°C MLn = Cr(CO)5, W(CO)5] affords an equilibrium mixture of 4,5-homotropilidenes 555 and 556. According to the NMR data and the results of AMI calculations, the formation of isomer 556 (equation 218) is strongly favored277. This course of events was called intramolecular cyclopropanation , and it was shown that the equilibrium between the 4,5-homotropilidene complexes is significantly different from that of the metal-free ligands. By reaction of the latter (555 and 556) with bis(ethylene)rhodium 1,3-pentanedionate 557, the complexes 558 and 559 of both 4,5-homotropilidenes were obtained in a 1 3 ratio. These complexes are non-fluxional and are configurationally stable at room temperature (equation 219)277. 

Although the halogenated chelates of chromium, cobalt, and rhodium would be difficult to prepare from the sensitive 3-halo-2,4-pentanediones, the copper (II) bromochelate was synthesized both from the bromodiketone and by direct bromination of copper acetylacetonate. The relatively labile copper chelates form much more rapidly than the kinetically stable chelates of chromium, cobalt, and rhodium. 

The Acac complex was usually added to provide a 1% mixture in an appropriate solution of TPP in a previously nitrogenated, stirred, 9 to 1 volume mixture of toluene and perdeuterobenzene. One of the CO ligands was immediately displaced by TPP as indicated by the instant gas evolution. After removing the CO in vacuum, the resulting solutions of the intermediate (IV) were hydrogenated to obtain the tris-TPP complex. The 2,4-pentanedione by-product did not compete in the presence of excess TPP and H2 for coordinating the rhodium. 

A system kinetically very similar to the phosphine-free rhodium carbonyl catalyst is obtained with bulky phosphites (Fig 6.3). At temperatures from 50 to 80°C, and CO and H2 partial pressures ranging from 10 to 70 bar, the rate of aldehyde formation is first order in H2 and approximately minus one order in CO. The reaction rate is independent of the concentration of 1-octene at conversions below 30%. The reaction was found to be first order in rhodium concentration and insensitive to the phosphite/rhodium ratio, provided that the absolute concentration was sufficiently high to generate a hydride complex from the pentanedionate precursor (reaction 9). 

C30H44B2F80 Rh2, Bis[(pentamethylcyclopentadienyl)(2,4-pentanedion-ato)rhodium] tetrafluoroborate, 45B, 983 C3oHn6GeOP2Pt, cis-(Hydroxydiphenylgermyl)phenylbis(triethylphos-phine)platinum(II), 37B, 436... 

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