Trans-4-octene, isomerization/hydroformylation

Using mild reaction conditions (10 bar, 125 °C), a high conversion of trans-4-octene and a high selectivity to n-nonanal can be obtained with toluene as the solvent. Cyclic carbonates like propylene carbonate (PC) are also suitable solvents for the isomerizing hydroformylation of trans-4-octene. Furthermore, the selectivity to -nonanal is increased up to 95% when PC is used in a single phase. The product n-nonanal can be extracted with n-dodecane or with a mixture of dodecane isomers. 

Propylene carbonate is a good solvent of the rhodium precursor [Rh(acac) (00)2] and the phosphite ligand BIPHEPHOS and can thus be used as the catalyst phase in the investigation of the isomerizing hydroformylation of trans-4-octene to n-nonanal in a biphasic system [24]. As already mentioned, the reaction products can be extracted with the hydrocarbon dodecane. Instead of an additional extraction after the catalytic reaction, we carried out in-situ extraction experiments, where the products are separated from the catalytic propylene carbonate phase while the reaction is still in progress. Conversion of 96% and selectivity of 72% was achieved under comparably mild conditions (p(CO/H2) = 10 bar, T = 125 °C, 4 h, substrate/Rh = 200 1). 

With increasing concentration of methylated /1-cyclodextrin the selectivity to n-nonanal increases from 64% to 72%, while the conversion of the olefin is constantly as high as 97%. Obviously the addition of the methylated /i-cyclodextrin has only a moderate influence on the isomerizing hydroformylation of trans-4-octene to n-nonanal. The addition of only 0.2 mol.-% of methylated /3-cyclodextrin lowers the isomerization rate which results in the formation of slightly more branched aldehydes. In pharmacy j6-cyclodextrins are established as solvation mediators between polar and less polar solvents. This is one possible explanation for the rise in selectivity to n-nonanal with an increasing j6-cyclodextrin concentration. At higher con-... 

To elucidate the use of TMS systems for the isomerizing hydroformylation, PC was chosen as the solvent for the rhodium catalyst, because the best selectivity to n-nonanal of 95% with a conversion on trans-4-octene of also 95% was achieved in this solvent under single-phase conditions. Dodecane was used as a non-polar solvent for the extraction of the product and p-xylene served as the mediator between the catalyst and the product phase [24]. Appropriate operation points for the reaction within this solvent system were determined by cloud titrations. 

Fig. 7 Isomerizing hydroformylation of trans-4-octene in the IMS system PC/dodecane/ p-xylene...

The consecutive isomerization-hydroformylation reaction of trans-4-octene yields high conversion and high selectivity of n-nonanal in the polar solvent propene carbonate [Eq. (8)] [33]. 

A famous example of a ligand structure that promotes the isomeriza-tion-hydroformylation reaction sequence in a highly selective manner is the BIPHEPHOS ligand (Scheme 6.14.6). BIPHEPHOS has been demonstrated to convert trans-4-octene into 1-nonanal with a remarkably high selectivity of 89% (given the complex reaction scheme) (Behr et al., 2003). However, the Rh-BIPHEPHOS hydroformylation system for trans-4-octene is relatively slow (TOF = 46h ), leaving room for further ligand optimization to make combined isomerization/hydroformylation processes more efficient. 

Scheme 6.14.6 Isomerization/hydroformylation of trans-4-octene using a Rh-BIPHEPHOS catalyst. Adapted from Behr etal. (2003).

Behr, A., Obst, D., Schulte, C., and Schosser, T. (2003) Highly selective tandem isomerization-hydroformylation reaction of trans-4-octene to n-nonanal with rhodium-BIPHEPHOS catalysis. J, Mol, Catal, A-Chem, 206, 179-184. 

The hydroformylation of trflns-3-octene at room temperature using the (non-encapsulated) rhodium catalyst based on tris(weta-pyridyl)phosphine afforded 2-ethylheptana] and 2-propylhexanal in exactly a 1 1 ratio. The encapsulated catalyst provided an unprecedented selectivity for 2-propylhexanal of 75% (Scheme 8.3). Again the selectivity is largely retained at 40 °C whereas at 80 °C the isomerization side reaction prohibits the selective formation of aldehydes. Similar regioselectivities were obtained in the hydroformylation of frflns-2-hexene, trans-2-nonene and trans-3-nonene at 25 °C. 

Various papers describe the aqueous biphasic hydroformylation for simple olefins as well as for functionalized olefins or dienes [154-174] (cf. the Section 6.1). In recent work [175], the synthesis of n-nonanal by consecutive isomerization and hydroformylation reactions of trans-4-octene has been described. The catalyst used was the in situ combination of Rh(acac)(CO)2 and the chelate phosphite BIPHE-PHOS. Performing the reaction in propylene carbonate the selectivity to n-nonanal could be raised up to 95%. If after the reaction the product is extracted with dodec-... 

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