Hydroformylation with unmodified rhodium catalysts

W Leitner, D Koch. Hydroformylation with unmodified rhodium catalysts in supercritical carbon dioxide. PCT Patent No. WO 99/03810, 1999. 

Hydroformylation with Unmodified Rhodium Catalysts in scCC>2... 

In general, rhodium-catalyzed hydroformylation of alkynes proceeds much slower than the reaction with olefins. It should be remembered that homogeneously catalyzed hydroformylation of olefins with unmodified rhodium catalysts can be irreversibly poisoned by the presence of even trace quantities of alkynes. As Liu and Garland [94, 95] found by means of in situ IR spectroscopy, the reason is likely the formation of dinuclear rhodium-carbonyl complexes I, which are stable even in the presence of hydrogen (Scheme 4.17). Therefore, alternative pathways for the production of a,P-unsaturated aldehydes have been suggested, consisting of Ni-catalyzed hydrocyanation followed by chemoselective hydrogenation [96]. 

A comparison between experimental and MO data on regioselectivity concerning the hydroformylation of several vinyl substrates (propene, 2-methylpropene, 1-hexene, 3,3-dimethylbutene, fluoroethene, 3,3,3-trifluoropropene, vinyl methyl ether, allyl methyl ether, styrene) with unmodified rhodium catalysts was reported. The activation energies for the alkyl rhodium intermediate formation, computed at either level along the pathways to branched or linear aldehydes, allowed one to predict the regioselectivity ratios. Steric effects may be less important for the unmodified carbonyl complexes and electronic factors dominate, assuming that alkene insertion in Rh-H is irreversible. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Both modified and unmodified rhodium catalysts have shown good activity and selectivity for the hydroformylation of 1-octene in SCCO2. With respect to the unmodified catalyst, higher reaction rates can be achieved compared to organic solvents or liquid CO2 under similar conditions. These modified systems exhibit higher regioselectivities compared to conventional solvent systems. 

Highly active unmodified rhodium catalysts for the hydroformylation of various olefins in SCCO2 are formed under mild conditions from [(cod)Rh(hfa-cac)] (8 cod = cis,cis-l,5-cyclooctadiene) and a number of other simple rhodium precursors [24]. Especially for internal olefins, the rate of hydroformylation is considerably higher than using the same catalysts in conventional liquid solvents under otherwise identical conditions. A detailed study of the hydroformylation of 1-octene (Scheme 6) using the online GC setup shown in Fig. 3 revealed a network of competing isomerization and hydroformylation when 8 was used without additional modifiers. As a result, the regioselectivity for the desired linear n-aldehyde varied considerably with conversion. At 60% conversion, the product aldehydes contained almost 80% of nonanal, whereas only 58 % linear aldehyde were present in the final product mixture. 

It is to remark that in the hydroformylation of styrene, the most investigated vinyl aromatic substrate, the predominance of the branched aldehyde at room temperature is higher with unmodified rhodium precursors than with phosphine-modified ones [4, 7c, 28]. In this context, when hydroformylation of styrene with chiral phosphines occurs without asymmetric induction and with a large prevalence of the branched aldehyde (> 96%), it is likely that unmodified rhodium-catalysts are also present in the reaction mbcture [14, 29, 30]. 

Bearing in mind the greater atomic radius of Rh, it becomes apparent why an unmodified rhodium catalyst generates a greater amount of branched aldehydes in comparison to the cobalt congener. For example, in the hydroformylation of 1-pentene, an Hb ratio of only 1.6 1 was found, while with the cobalt complex a ratio of 4 1 resulted. A similar correlation has been qualitatively deduced from reactions mediated by the metal clusters Rug(CO)j 2> 0 3(00)22, and 4(00)22. Because of the larger atomic radii of the metals, in hydroformylation these catalysts produce more branched aldehydes than observed in the reaction with Co2(CO)g. Unfortunately, most of these results were achieved under different reaction conditions or are difficult to interpret because of low reaction rates and are therefore not strictly comparable. 

In preliminary attempts, hydroformylation was carried out with an unmodified rhodium catalyst [72]. Later on, Rh complexes based on monodentate [73] or bidentate phosphorus ligands [74] became the catalysts of choice in order to produce the corresponding 4-hydroxy aldehydes, which are in equilibrium with cyclic hemi-acetals. This transformation can be used to synthesize substituted dihydrofurans by elimination of water in a final step [75]. 

When 2,6-dimethylhepta-1,5-diene or 2,6-dimethylocta-l,5-diene was subjected to the hydroformylation with an unmodified rhodium catalyst under rather severe conditions, racemic citronellal (R = Me) was formed with a yield of 74% (Scheme 6.22) [96]. Citronellal confers citronella oil its distinctive lemon scent. On the industrial scale, (S)-citronellal is prepared as intermediate of the synthesis... 

BASF claimed the hydroformylation of numerous a-branched styrenes (Scheme 6.60) [164]. For example, in a 3 kg scale, 2-(4-isopropylphenyl)-prop-l-ene was reacted in the presence of an unmodified rhodium catalyst to give, in accordance with Keulemans rule, exclusively the terminal aldehydes. 

Scheme 6.106 Hydroformylation of ethylene oxide with an unmodified rhodium catalyst. 6.3.2...

Koch and Leitner [73] submitted several olefins to the rhodium-catalyzed hydroformylation in SCCO2 (Scheme 7.12). Most remarkably, in the reaction with the less reactive olefin trafis-3-hexene with an unmodified rhodium catalyst, the superiority of condensed CO2 in comparison to toluene as solvent becomes obvious. Even by optimum agitation, the conversion in the organic solvent was far from that observed under supercritical conditions. 

January 1999 This invention describes the hydroformylation of substrates with C=C double bonds using unmodified rhodium catalysts in an SCCO2 reaction mixture to preferentially form the branched isomeric products and the separation of the product and catalyst from the SCF mixture. 

Although the overall reaction mechanisms (catalytic cycles) written for hydroformylation reactions with an unmodified cobalt catalyst (Scheme 1) and the rhodium catalyst (Scheme 2) serve as working models for the reaction, the details of many of the steps are missing and there are many aspects of the reaction that are not well understood. 

A similar in situ spectroscopic strategy was undertaken to determine if a Heck and Breslow binuclear elimination mechanism might be present in the unmodified rhodium-catalysed hydroformylation of alkenes. In one study alone, over 15 substrates were investigated where the acyl species RCORh(CO)4 was always observed, but under the conditions used, only the linear term was statistically supported [49], However, hydroformylation of two substrates, namely, cyclohexene and cyclooctene, exhibited outlier behaviour. With these two substrates, full conversion of the catalyst precursor Rh4(CO)i2 was never observed. Further detailed study of cyclohexene could not verify a statistically supported quadratic contribution [50, 51], The in situ study of cyclooctene at much lower CO partial pressure was more fruitful, as RCORh(CO>4, HRh(CO>4 and Rh2(CO)g were all observed simultaneously [52]. At the mean reaction conditions used in this study, 40% of product formation arose from the quadratic term k2[Rh]. Therefore, it appears that... 

Homogeneous unmodified or ligand-modified rhodium catalysts are predominantly utilized for the transformation of olefins with a chain length major advantages of rhodium catalysis are the reduced syngas pressure and lower reaction temperatures. These features have also been recognized by the chemical industry. Thus, in 1980 less than 10% of hydroformylation was conducted with rhodium, and by 1995 this had been increased to about 80% [3]. In some cases, a combination of Co and Rh can be advantageous [4]. 

Sometimes, also polynuclear clusters such as Rh4(CO)j2 or Rh6(CO)26 were submitted to the formation of rhodium catalysts [18]. Metallic rhodium embedded in inorganic materials (carbon, AI2O3) was tested for mini-plant manufacturing. In this context, the frequently phosphorus ligands [PPhj, P(OPh)3] were added with the intention to detach rhodium from the heterogeneous layer (activated rhodium catalyst = ARC) [19, 20] More recently, ligand (Xantphos, PPhj, BIPHEPHOS)-modified or unmodified rhodium(O) nanoparticles were used as catalyst precursors for solventless hydroformylation [21]. It is assumed that under the reaction conditions these metal nanoparticles decompose and merge into soluble mononuclear Rh species, which in turn catalyze the hydroformylation. 

It should be noted that some metal catalysts can initiate 1,2-sigmatropic rearrangement, which may lead to further modification of the olefin serving as a substrate of the hydroformylation. A frequently cited example is the isomerization-hydroformylation of a-pinene with an unmodified cobalt catalyst (Scheme 5.9) [65]. In strong contrast to rhodium, the cobalt catalyst produced 2-formylbornane. The 1,2-sigmatropic rearrangement was explained by the acidic nature of HCo(CO)4. 

---