Aldehydes rhodium-catalyzed hydroformylation

Another route to the diol monomer is provided by hydroformylation of allyl alcohol or allyl acetate. Allyl acetate can be produced easily by the palladium-catalyzed oxidation of propylene in the presence of acetic acid in a process similar to commercial vinyl acetate production. Both cobalt-and rhodium-catalyzed hydroformylations have received much attention in recent patent literature (83-86). Hydroformylation with cobalt carbonyl at 140°C and 180-200 atm H2/CO (83) gave a mixture of three aldehydes in 85-99% total yield. 

Although early catalysts were based on cobalt, nowadays, rhodium catalysts are preferred because they require lower pressure and afford higher chemo- and regioselectivity [1,2]. In recent years, extensive research into the production of only linear aldehydes has provided impressive results. The application of phosphines with a wide bite angle in the rhodium catalyzed hydroformylation of terminal alkenes enable the regioselectivity to be almost totally controlled [3]. Branched selective hydroformylation, al-... 

For example, rhodium catalyzed hydroformylation of 2-formyl-N-allyl-pyrrol gives an approx. 1 1 mixture of iso- and u-aldehydes. The latter cyclizes immediately in an aldol reaction followed by dehydration giving 7-formyl-5,6-indolizine in up to 46% (Scheme 29) [83]. Since here only one of the aldehyde groups can act as the enolate nucleophile this cyclization proceed with high regioselectivity (Scheme 29). 

Hydroformylation of hetero olefins such as carbonyl compounds is not known to proceed with significant levels of efficiency, whereas the hydroformylation of olefins has been developed to a sophisticated stage. Generally, aldehydes resultant from the latter process exhibit a low propensity to undergo further hydroformylation, with the exception of some activated aldehydes. The rhodium-catalyzed hydroformylation of formaldehyde is the key step in the synthesis of ethyleneglycol from synthesis gas. Chan et al. found... 

In the first set of experiments O. Nuyken et al. studied the rhodium-catalyzed hydroformylation of 1-octene to its corresponding -aldehyde (Scheme 6.5) [51-53]. The active catalyst species was formed in situ by mixing the appropriate amount of polymeric macroligand, Rh(CO)2acac and 1-octene in water. The results are summarized in Tab. 6.3. 

Initial studies showed that the encapsulated palladium catalyst based on the assembly outperformed its non-encapsulated analogue by far in the Heck coupling of iodobenzene with styrene [7]. This was attributed to the fact that the active species consist of a monophosphine-palladium complex. The product distribution was not changed by encapsulation of the catalyst. A similar rate enhancement was observed in the rhodium-catalyzed hydroformylation of 1-octene (Scheme 8.1). At room temperature, the catalyst was 10 times more active. For this reaction a completely different product distribution was observed. The encapsulated rhodium catalyst formed preferentially the branched aldehyde (L/B ratio 0.6), whereas usually the linear aldehyde is formed as the main product (L/B > 2 in control experiments). These effects are partly attributed to geometry around the metal complex monophosphine coordinated rhodium complexes are the active species, which was also confirmed by high-pressure IR and NMR techniques. 

Scheme 8.1 Rhodium-catalyzed hydroformylation of alkenes leading to linear (L) and branched (B) aldehydes and isomerized (IS) olefins.

Assemblies based on 8 and pyridine phosphorus ligands 5-7 were used as supramolecular ligands in the rhodium-catalyzed hydroformylation and showed typical bidentate behavior. The chelating bidentate assembly exhibited lower activities (a factor of three) than the monodentate analogue. Only a slightly higher selectivity for the linear aldehyde was observed. The chiral ligand assemblies based... 

Union Carbide invented the industrial use of highly active ligand-modified rhodium complexes.90-93 [RhH(CO)(PPh3)3], the most widely used catalyst, operates under mild reaction conditions (90-120°C, 10-50 atm). This process, therefore, is also called low-pressure oxo process. Important features of the rhodium-catalyzed hydroformylation are the high selectivity to n-aldehydes (about 92%) and the formation of very low amounts of alcohols and alkanes. Purification of the reactants, however, is necessary because of low catalyst concentrations. 

Casey, C.P., Whiteker, G.T., Melville, M.G., Petrovich, L.M., Gavney, J.A., Jr. and Powell, D.R. (1992) Diphosphines with natural bite angles near 120 Deg increase selectivity for n-aldehyde formation in rhodium-catalyzed hydroformylation. J. Am. Chem. Soc.,... 

Scheme 5 Mechanism of rhodium-catalyzed hydroformylation (formation of the n-aldehyde)...

Regioselective hydroformylation. The rhodium-catalyzed hydroformylation of (Z)-f-butyldiphenylsilylalkenes is an excellent route to p-silyl aldehydes (equation I). A bulky silyl group is essential for this regiocontrol. 

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

C. P. Casey, G. T. Whiteker, M. G. Melville, L. M. Petrovich, J. A. Gavney, and D. R. Powell, J. Am. Chem. Soc., 114, 5535 (1992). Diphosphines with Natural Bite Angles near 120° Increase Selectivity for -Aldehyde Formation in Rhodium-Catalyzed Hydroformylation. 

An enantiomerically pure aldehyde, (lR,2R,3R)-2,7,7-trimethylbicyclo[3.1.1]hep-tane-2-aldehyde, is produced from a-pinene by rhodium-catalyzed hydroformylation [79, 80]. Initially, reaction with ferrocene under acidic conditions leads to a 1 1 mixture of diastereoisomeric cations, but on standing for a few hours at room temperature, isomerization by rotation around the ferrocene — cationic carbon bond to the thermodynamically more stable cation (with configuration (R) at the cationic center) occurs (Fig. 4-11). An enantiomerically pure amine is available by trapping of this cation by azide and reduction [75]. Analogously, the isomeric aldehyde with the bicyclo [2.2.1] heptane structure is formed by hydroformylation of a-pinene with cobalt catalysts [79, 80] and was used as the starting material for an isomeric series of chiral amines [75]. 

The aldehydes are usually fed into the oxidation reaction as distilled products, but in the rhodium-catalyzed hydroformylation of 2,2,4-trimethyl-1-pentene the resulting crude aldehyde can be converted directly to the corresponding Cg acid at temperatures of 40-60 °C without loss of activity of the Rh catalyst [10]. 

---