Olefin hydroformylation catalyst precursor

Table 2 Hydroformylation of various olefins in SCCO2 and toluene using [(cod)Rh(hfacac)] as catalyst precursor ...

A lthough the hydroformylation of olefins has been known since 1938, the first successful attempts to synthesize optically active aldehydes by hydroformylation using optically active catalysts have been published only recently (I, 2, 3, 4). All the three possibilities to prepare optically active aldehydes (Scheme 1) have been successfully explored (5) using Co(R -Sal)2 or [Co(CO)4]2 and R -SalH (R -SalH = (S)-N-a-methylbenzylsalicylaldimine) as catalyst precursor, but the optical yields obtained were very poor. Much better results have been obtained... 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

Rhodium carbonyl complexes are used as catalyst precursors in the hydroformyla-tion reaction Since rhodium catalysts are two to four orders of magnitude more efficient than cobalt-based ones, they are used to carry out hydroformylation of kineti-cally inert olefins, such as styrene and nitroalkenes. ... 

The last contribution quoted in this section is a phosphine-free hydroformylation process based on a liquid triphasic system consisting of isooctane, water and trioctylmethylammonium chloride (TOMAC). The hydroformylation of model olefins required neat RhCls only as catalyst precursor. In the triphasic system, the catalyst is confined in the TOMAC phase, likely in the form of an ion pair. Products are obtained in excellent yields (> 90% at 80 °C) and high regioselectivity (>98%) in favour of the branched aldehyde in the case of styrene, while the exo isomer was obtained in >90% selectivity in the case of norbornene. The products were easily removed and the catalyst was recycled several times, with no leaching of rhodium into the organic phase. 

In analogy to hydroformylation, alkenes react with SO2 and H2 to give a so-called hydrosulftnation product, sulfinic acids [116]. Cationic Pd(II) and Pt(II) complexes bearing bidentate phosphine ligands are effective catalyst precursors. A plausible mechanism for the hydrosulfination involves formation of alkyl intermediates by olefin insertion into metal hydrides, subsequent insertion of SO2, and reformation of the hydrides with the release of sulfinic acids (Scheme 7.19). However, ahphatic sulfinic acids readily undergo disproportionation to give thiosulfinic acid esters, sulfonic acids, and water at the reaction temperature. The unstable sulfinic acids can be conveniently converted into y-oxo sulfones by addition of a,-unsaturated carbonyl compounds as Michael acceptors to the reaction mixtine (Eq. 7.23) [117]. 

The kinetics of hydroformylation of 1-octene using [Rh(COD)Cl]2 as a catalyst precursor with TPPTS as a water-soluble ligand and ethanol as a co-solvent was further studied by Deshpande et al. [15] at different pH values. The rate increased by two- to fivefold when the pH increased from 7 to 10, while the dependence of the rate was found to be linear with olefin and hydrogen concentrations at both pH values. The rate of hydroformylation was foimd to be inhibited at higher catalyst concentrations at pH 7, in contrast to linear dependence at pH 10 (Figure 5). The effect of the concentration of carbon monoxide was linear at pH 7, in contrast to the usual negative-order dependence. At pH 10, substrate-inhibited kinetics was observed with respect to CO (Figure 6). 

Typical hydroformylation activity is illustrated in Table XII for a model, internal, linear-backbone, olefin mixture (2-, 3- and 4-octenes) using as catalyst precursor, a dispersion of ruthenium(IV) oxide, hydrate in tetrabutylphosphonium bromide (expt. 1). Optionally, a cobalt carbonyl-tertiary phosphine cocatalyst may be added (expt. 

Pino P, Oldani F, Consiglio G (1983) On hydrogen activation in the hydroformylation of olefins with Rh4(CO)i2 or Co2(CO)g as catalyst precursors. J Organomet Chem 250 491-497... 

Multiply substituted and higher olefins could be successfully hydroformy-lated in PEG-containing organic-organic biphasic systems using the [Rh(PEG)J catalyst precursor. This compoiuid is also active in aqueous-organic systems, and its properties are discussed in the subsection on Hydroformylation under Aqueous-Organic Biphasic Catalysis. 

It should be noted that the catalytically active, coordinative unsaturated species in situ forms from the catalyst s precursor at oxo-conditions by losing a CO ligand, see the section The Mechanism of Olefin Hydroformylation with Other Catalysts. 

Triphenylphosphine-Modified Ruthenium Catalyst. The mechanism of olefin hydroformylation using Ru(CO)3(P(C6H5)3)2 as the catalyst precursor has been explained by the classical hydride-, alkyl-, and acyl-complex sequence involving Ru(H)2(CO)(P(C6H5)3) as the principal active catalytic species (125). 

The carbonyls or hydrocarbonyls of cobalt which are used as catalysts are generally formed in situ by feeding cobalt to the reactor in the form of metal oxide, hydroxide or salt of an organic or inorganic acid, either in solution or suspension in olefin, high boiling distillation residues or water. However, the carbonyls may also be formed in a small carbonyl generating reactor which is fed by the same catalyst precursors before they enter the hydroformylation reactor [161]. 

To improve the productivity, several basic approaches are utilized. The most straightforward solution is to add a cosolvent like lower alcohols (MeOH, EtOH) miscible with water, thus improving the solubility of olefins in the aqueous phase. A certain additional effect may be obtained by varying the nature of the catalyst precursors, though, under the conditions used for hydroformylation, rhodium complexes are rapidly transformed into [HRh(CO)L3] thus masking the differences [128,129]. 

The water-soluble pyrazolato complex (Rh(p-Pz)(CO)(TPPTS)]2 was used as precursor for olefin hydroformylation in an aqueous heptane solvent system 9]. Without additional ligand, olefin isomerization dominated hydroformylation at 7 bar CO/H2 (1 1) giving large amounts of 2-hexene. Isomerization was suppressed most effectively at 49.8 bar, leading to aldehyde chemoselectivities >90% and a 2.4-3.S njiso ratio. It was demonstrated that aerobic recycling of the aqueous catalyst phase by... 

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