Hydroformylation of 1-hexene

In this context, the use of ionic liquids with halogen-free anions may become more and more popular. In 1998, Andersen et al. published a paper describing the use of some phosphonium tosylates (all with melting points >70 °C) in the rhodium-catalyzed hydroformylation of 1-hexene [13]. More recently, in our laboratories, we found that ionic liquids with halogen-free anions and with much lower melting points could be synthesized and used as solvents in transition metal catalysis. [BMIM][n-CgHi7S04] (mp = 35 °C), for example, could be used as catalyst solvent in the rhodium-catalyzed hydroformylation of 1-octene [14]. 

In the rhodium-catalyzed hydroformylation of 1-hexene, it has been demonstrated that there is a correlation between the solubility of 1-hexene in ionic liquids and reaction rates (Figure 5.3-4) [28]. 

The system (23)/SnCl2, an active intermediate in the catalytic hydroformylation of 1-hexene, has been investigated by 31P NMR spectroscopy and two species are observed at low temperature, in equilibrium with the starting Pt complex (23). One is complex (27), and the other is a species which does not show Sn-P coupling and which has been tentatively attributed to a complex having chloride ions bridging the Pt and Sn metal centers. Formation of the complex (27) does not occur when EtOH is added to the CD2C12 or acetone solutions.91... 

Pt(CH2) ( + )-DIOP ] (89), [PtCl2 (+)-DIOP ] (90), or [PtCl2 ( )-DIOP ] (91) were used as catalyst precursors for the hydroformylation of styrene and 1-hexene. A mixture of (89) and (90) hydroformylates styrene with 49% overall yield the product contains 32.5% of linear aldehyde and the optical purity of the (i )-PhCHMeCHO was 28%. The hydroformylation of 1-hexene over the same catalyst gave Me(CH2)sCHO and (i )-Me(CH2)3CMeCHO in 64% overall yield whereby the product contained 86.5% Me(CH2)sCHO and the optical purity of the (i )-Me(CH2)3CH2CHO was 11%. 

Ligand (136), an analog of PPh3 with amphiphilic character, was used for making [Rh(CO) (136)(acac)]. The rhodium-based hydroformylation of 1-hexene using catalysts formed in situ... 

Fig. 9. Effect of solvent variation on hydroformylation of 1-hexene. Reprinted with permission from Ind, Eng. Chem., Prod. Res. Dev. 8, 291 (1969). Copyright by the American Chemical Society.

The catalyst containing 2.0% Rh, insoluble in organic solvent, was used for hydroformylation of 1-hexene at 80°C and 43 atm of 1/1 H2/CO. The catalyst concentration was 1 mmole Rh per mole of olefin. After 4 hours a 41% yield of aldehyde was obtained, with a 2.5 1 isomer ratio. Some isomerization to internal olefins also occurred. A significant feature was the rhodium concentration of 2 ppm in the product. 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

Further information on the reaction intermediates is achieved by in situ NMR experiments. Because the signals in NMR spectra depend upon the concentration of the investigated species, a quantitative treatment is possible. Bianchini and coworkers investigated the hydroformylation of 1-hexene [62], using high-pressure NMR spectroscopy to evaluate the influence of synthesis gas on the equilibria of rhodium triphenylphosphine species. They were able to establish at least four resting states of rhodium (catalyst species that do not participate directly in the reaction). When synthesis gas interacted with... 

In the quest for suitable solvent systems the [Rh(CO)2(SULPHOS)] complex (SULPHOS = 31) was found to catalyze the hydroformylation of 1-hexene in water-methanol/isooctane (1/1/1, v/v/v) yielding heptanal and 2-methylhexanol in a ratio of 2.2 (80 °C, 30 bar syngas) [83]. An important point here is in that the biphasic micture becomes homogeneous above 60 °C, but phase separation occurs again upon cooling to room temperature. This kind of solvent behaviour may lead to fast reactions at higher... 

Similar to the above case, hydroformylation of 1-hexene using a water-soluble rhodium catalyst [RhH(CO)(TPPMS)3] gave lower yields when a-cyclodextrin was added to the biphasic reaction system [14]. Again, the reason was suspected in the interaction between the cyclodextrin and the rhodium catalyst. 

An ionic liquid was fully immobilized, rather than merely supported, on the surface of silica through a multiple-step synthesis as shown in Fig. 15 (97). A ligand tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium) salt (tppti) was prepared so that the catalyst, formed from dicarbonylacetylacetonate rhodium and the ligand (P/Rh = 10), could be soluble in both [BMIMJBFq and [BMIM]PF6. The supported ionic liquid-catalyst systems showed nearly three times higher rate of reaction (rate constant = 65 min ) that a biphasic system for the hydroformylation of 1-hexene at 100°C and 1500 psi in a batch reactor, but the n/i selectivity was nearly constant the same for the two ( 2.4). Unfortunately, both the supported and the biphasic ionic liquid systems exhibited similar metal leaching behavior. 

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