Aldehydes by hydroformylation

A lthough the hydroformylation of olefins has been known since 1938, the first successful attempts to synthesize optically active aldehydes by hydroformylation using optically active catalysts have been published only recently (I, 2, 3, 4). All the three possibilities to prepare optically active aldehydes (Scheme 1) have been successfully explored (5) using Co(R -Sal)2 or [Co(CO)4]2 and R -SalH (R -SalH = (S)-N-a-methylbenzylsalicylaldimine) as catalyst precursor, but the optical yields obtained were very poor. Much better results have been obtained... 

In this review the synthetic aspects of asymmetric hydroformylation will be discussed first the experimental data relevant to attempt a rationalization of the results will then be considered. The closely related synthesis of optically active aldehydes by hydroformylation of optically active olefinic substrates in the presence of achiral catalysts7,8 and the different asymmetric hydrocarbonylation reactions, such as the synthesis of esters from olefins, carbon monoxide and alcohols in the presence of optically active catalysts9 , are beyond the scope of this review and will not be discussed here. 

The synthesis of aldehydes by hydroformylation of alkenes is an important industrial process discovered in 1938. The use of biphasic catalysis, which is a well-established method for the separation of the product and the recovery of the catalyst, was developed for this reaction in the Ruhrchemie-Rhdne-Poulenc process a water-soluble rhodium complex is used as catalyst - the reaction proceeding in water - and the organic layer is formed simply by the alkene reagent and the aldehyde products. Butanal is manufactured this way, but owing to the low solubility of longer olefins in water, this process is limited to C2-C5 olefins. To overcome this limitation, ionic liquids have been studied as a reaction solvent for this biphasic reaction. 

This reaction has been extensively modified to various conditions, including a reaction without a catalyst, the generation of aldehydes by hydroformylation of olefin,two-phase reaction, and—more important—the solid phase-supported reactions. ... 

The linear a olefins described m Section 14 15 are starting materials for the preparation of a variety of aldehydes by reaction with carbon monoxide The process is called hydroformylation... 

Excess hydrogen brings about the hydrogenation of the aldehyde and allows the process to be adapted to the preparation of primary alcohols Over 2 X 10 Ib/year of a variety of aldehydes and alcohols is prepared in the United States by hydroformylation... 

The switch from the conventional cobalt complex catalyst to a new rhodium-based catalyst represents a technical advance for producing aldehydes by olefin hydroformylation with CO, ie, by the oxo process (qv) (82). A 200 t/yr CSTR pilot plant provided scale-up data for the first industrial,... 

The nickel or cobalt catalyst causes isomerization of the double bond resulting in a mixture of C-19 isomers. The palladium complex catalyst produces only the 9-(10)-carboxystearic acid. The advantage of the hydrocarboxylation over the hydroformylation reaction is it produces the carboxyUc acids in a single step and obviates the oxidation of the aldehydes produced by hydroformylation. 

Breit B (2007) Aldehydes synthesis by hydroformylation of alkenes. In Bruckner R (ed) Science of synthesis, vol 25. Thieme, Stuttgart, pp 277-317... 

The unsubstituted quinazolidine system 5 was constructed from mesylate 173. The key feature in this synthesis is based on a cyclohydrocarbonylation of the protected 4-amino-l,6-heptadiene 169 catalyzed by Rh(acac)(CO)2-BIPHEPHOS. Formation of the hemiamidal-aldehyde 171 took place by hydroformylation of the two olefin moieties and cyclization. Elimination of water gave 172, which, after treatment with NaBFE, subsequent mesylation to 173, and catalytic hydrogenation, afforded 5 (Scheme 29) <1998TL4599>. 

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. 

The catalyst formed in this manner exhibited carbonyl infrared absorptions, as shown in Table XXX. These catalysts were tested by hydroformylation of ethylene or propylene at 100°C and atmospheric pressure. Both were effective, with (A) being better than (B), probably because of the higher surface area. The aldehyde formed from propylene was a mixture of 63% n- and 37% isobutyraldehyde. The rate expression for ethylene hydroformylation was ... 

Catalysts and Catalyst Concentration. The most active catalyst for benzaldehyde reduction appears to be rhodium [Rh6(C0)i6 precursor], but iron [as Fe3(C0)i2] and ruthenium [as Ru3(C0)12] were also examined. The results of these experiments are shown in Table 1. Consistent with earlier results (12), the rhodium catalyst is by far the most active of the metals investigated and the ruthenium catalyst has almost zero activity. The latter is consistent with the fact that ruthenium produces only aldehydes during hydroformylation. Note that a synergistic effect of mixed metals does not appear to be present in aldehyde reduction as contrasted with the noticeable effects observed for the water-gas shift reaction (WGSR) and related reactions (13). 

Hydroformylation is the most successful application of a homogeneous catalytic reaction to industrial processes [1], Aldehydes are conveniently produced by hydroformylation, in which CO and hydrogen are simultaneously added to an alkene (Figure 1). 

An excess of ligand, including CO, will often inhibit isomerisation. HCo(CO)4, an unstable hydrido-carbonyl complex, belongs to the examples of catalysts also active in an atmosphere of CO. This is the only homogeneous catalyst being commercially applied, albeit primarily for its hydroformylation activity. Higher alkenes are available as their terminal isomers or as mixtures of internal isomers and the latter, the cheaper product, is mainly converted to aldehydes/alcohols by hydroformylation technology. Later we will see that the isomerisation reaction also plays a pivotal role in this system. Since 1990 several catalysts based on rhodium, platinum and palladium have been discovered that will also hydroformylate internal products to terminal aldehydes. 

Oxo reaction. Reacting an olefin with synthesis gas (CO and H2) to produce an aldehyde (called hydroformylation) followed by hydrogenation (addition of hydrogen), producing an alcohol containing one more carbon than the original olefin. 

Hydroformylation (0.xo process). Production of aldehydes by catalytic reaction of an olefm with carbon monoxide and hydrogen. Cobalt catalysts are often used. 

The asymmetric reactions discussed in this chapter may be divided into three different types of reaction, as (1) hydrometallation of olefins followed by the C—C bond formation, (2) two C C bond formations on a formally divalent carbon atom, and (3) nucleophilic addition of cyanide or isocyanide anion to a carbonyl or its analogs (Scheme 4.1). For reaction type 1, here described are hydrocarbonyla-tion represented by hydroformylation and hydrocyanation. As for type 2, Pauson-Khand reaction and olefin/CO copolymerization are mentioned. Several nucleophilic additions to aldehydes and imines (or iminiums) are described as type 3. 

The linear olefins are used in the production of biodegradable detergents. The Cq to cuts are fed to alkylation units where the olefins react with benzene to form alkylbenzenes which are subsequently sulphonated. The paraffins present in the feed pass through unconverted. These are then chlorinated and used as plasticizers. The linear olefins can also be converted to linear aldehydes and alcohols by hydroformylation. 

The hydroformylation reaction, also known as the oxo reaction, is used extensively in commercial processes for the preparation of aldehydes by the reaction of one mole of an olefin with one mole each of hydrogen and carbon monoxide. The most extensive use of the reaction is in the preparation of normal- and iso-butyraldehyde from propylene. The ratio of the amount of the normal aldehyde product to the amount of the iso aldehyde product typically is referred to as the normal to iso (N I) or the normal to branched (N B) ratio. In the case of propylene, the normal- and iso-butyraldehydes obtained from propylene are in turn converted into many commercially-valuable chemical products such as n-butanol, 2-ethyl-hexanol, trimethylol propane, polyvinylbutyral, n-butyric acid, iso-butanol, neo-pentyl glycol,... 

CH3(CH2)vCH(CH3)CHO, C11H22O, Mr 170.29, i pgg.skPa 119-120 °C, df 0.8948, 1.4205, is not reported to have been found in nature. It is a colorless liquid with an aldehydic, citrus-peel-like, waxy-green odor. 2-Methyldecanal is obtained as a by-product in the manufacture of 2-methylundecanal by hydroformylation of 1-decene (see 2-methylundecanal). It is used in perfumery to refresh green and citrus nuances. 

Monoacetals of substituted succinaldehydes (162), readily prepared by hydroformylation of optically active a,(3-unsaturated aldehyde acetals, were used to synthesize 3-substituted thiophenes having an optically active substituent (163). In these cases, while the use of hydrogen sulfide and HC1 in methanol at room temperature was more convenient, comparison with formation of (163) by the Paal synthesis from an appropriately substituted succinic acid salt gave products having about the same amount of racemization (73JOC2361). 

The isomerization of the olefin prior to its hydroformylation has been the explanation of this question (3) and the formation of isomeric aldehydes was related to the presence of isomeric free olefins during the hydroformylation. This explanation, however, is being questioned in the literature. The formation of (+) (S) -4-methylhexanal with an optical yield of more than 98% by hydroformylation of (+) (S)-3-methyl-l-pentene (2, 6) is inconsistent with the olefin isomerization explanation. Another inconsistency has been the constance of the hydroformylation product composition and the contemporary absence of isomeric olefins throughout the whole reaction in hydroformylation experiments carried out with 4-methyl-1-pentene and 1-pentene under high carbon monoxide partial pressure. The data reported in Ref. 4 on the isomeric composition of the hydroformylation products of 1-pentene under high carbon monoxide pressure at different olefin conversions have recently been checked. The ratio of n-hexanal 2-methylpentanal 2-ethylbutanal was constant throughout the reaction and equal to 82 15.5 2.5 at 100°C and 90 atm carbon monoxide. 

The aldehyde synthesis by hydroformylation of alkenes described in the preceding section can be achieved indirectly using boron hydrides. An oversimplified expression of this reaction is... 

Phosphine-modified rhodium catalysts hydroformylate alkynes to saturated aldehydes.1 The reaction most likely proceeds by a rapid hydrogenation to yield the alkene, followed by hydroformylation. 

The relative position of L with respect to S and of H with respect to Z can be decided, for instance, by hydroformylation of (Z)-2-butene, which yields only one aldehyde that is chiral. When the catalyst is optically active, the predominating antipode in the reaction product indicates the face of the unsaturated carbon atoms preferentially attacked by CO and therefore the more stable transition state (Fig. 8) (that is, on the assumption that the difference in the free energy of the transition state mainly depends on steric interactions, the transition state in which such steric interactions are smaller). 

---