Supercritical hydroformylation

In the research on a supercritical hydroformylation process, all the important dynamic and equilibrium steps in the catalytic system had to be measured for comparisons with conventional media. The hydrogenation of dicobaltocta-carbonyl... 

A recent example where Co2(CO)8 serves as a precatalyst is in the preparation of linear and branched aldehydes via propylene hydroformylation in supercritical C02 (93-186 bar 66-108 °C). Cyclohexane carbaldehyde is produced from cyclohexene using Co2(CO)8 and an acid RCOOH, or else is successful with another established Co catalyst, Co(OOCR)2, assumed to form in situ in the former case. Oligomerization of aldehydes such as n-butanal is achieved with Co2(CO)6L2 as catalyst (L = CO, PR3).1364... 

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

Another solution to the problem of ionic liquid loss to the organic phase is to extract the product from the ionic liquid using a supercritical fluid (See Chapter 8, Section 8.2.2.3). It has been demonstrated that this can be done continuously for a variety of reactions including the hydroformylation of long chain alkenes [20], and that neither the ionic liquid nor the catalyst are leached to significant extents. The only problem here is the high pressures involved (see section 9.8). 

J. W. Rathke, R. J Klingler, T. R. Krausem, Propylene Hydroformylation in Supercritical Carbon Dioxide , Organometallics 1991,10,1350-1355. 

P. G. Jessop, T. Ikariya, R. Noyori, Selectivity for Hydrogenation or Hydroformylation of Olefins by Hydrido-pentacarbonylmanganese(I) in Supercritical Carbon Dioxide , Organometallics 1995,14,1510-1513. 

D. Koch, W. Leitner, Rhodium-Catalyzed Hydroformylation in Supercritical Carbon Dioxide ,/ Am. Chem. Soc 1998,120,13398. 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Another way of getting around the problem of the separation of the catalyst from the substrate is via use of a flow reactor [38], Supercritical carbon dioxide has been used successfully as a medium for the hydroformylation of 1-octene using an immobilized rhodium catalyst. The catalyst is covalently fixed to silica through the modifying ligand A-(3-trimethoxysilyl-n-propyl)-4,5-bis(diphenylphosphino)phenoxazine (Figure 8.13). Selectivity was found to be... 

Keywords Supercritical carbon dioxide Solubility measurement Homogeneous catalysis Multi-phase catalysis Hydroformylation... 

Webb, P.B.and Sellin, M.F. and Kimene, T.E. and Williamson, S. and Slawin, A.M.Z. and Cole-Hamilton, D.J. (2003). Continuous Flow Hydroformylation of Alkenes in Supercritical Fluid-Ionic Liquid Biphasic System. J. Am. Chem. Soc., 125, 15577-15588. 

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

Promising results were observed in Friedel-Crafts alkylation77 and epoxidation.78 Higher rates or better selectivities were found for hydroformylations in supercritical C02.79-84 Simple trialkyl phosphines, for examples, were shown to provide highly active Rh catalysts.81 Hydroboration showed enhanced regioselec-tivity.85 The Wacker reaction performed in alcohol-supercritical C02 exhibits high reaction rates and markedly increased selectivity toward methyl ketone.86... 

Supercritical hydrogenation is just one example of continuous reactions which can be carried out in SCCO2 solution. Other reactions which have been carried out successfully include Friedel-Crafts alkylation of aromatics by alcohols [64], the dehydration of alcohols to form ethers [65] (using acid catalysts), and the hydroformylation of alkenes [52] (using rhodium catalysts immobilized on Si02). In each of these reactions, it is possible to obtain a selectivity which is at least as good, and often better, than with conventional solvents. However, the precise role of the scCC>2 in these reactions is not as obvious as in supercritical hydrogenation. 

As a unique medium for asymmetric hydroformylation, supercritical carbon dioxide has recently been examined, which can be carried out in an extremely low catalyst concentration. The reactions of styrene (16a) and pentafluorostyrene (16e) catalyzed by Rh-BINAPHOS appear to give mixed results that are highly dependent on the reaction conditions [77,78], Enantioselectivity up to 92-95% ee for 16a or 85 % ee for 16e has been observed [78]. A biphasic reaction system has also been examined for the reaction using Rh(acac)(CO)2 with a sulfonated diphosphine ligand BINAS [79], The reaction proceeds smoothly at 40°C and 100 atm in high conversion with excellent branched aldehyde selectivity (95%), but enantioselectivity is very low (18% ee). The use of these newer reaction conditions is still in the very early stage and further development is expected in the next decade. 

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