Hydroformylation branched aldehyde formation

Figure 6.33. Relationship between isomerisation and branched aldehyde formation during hydroformylation...

A chapter written in 1996 covers hydroformylation catalyzed by organometallic complexes in detail,219 whereas a review written 5 years later gives a summary of the advances on hydroformylation with respect to synthetic applications.220 A selection of papers in a special journal issue has been devoted to carbonylation reactions.221 A major area of the research has been the development of fluorous biphasic catalysis and the design of new catalysts for aqueous/organic biphasic catalysis to achieve high activity and regioselectivity of linear or branched aldehyde formation. 

The catalytic hydroformylation of alkenes has been extensively studied. The selective formation of linear versus branched aldehydes is of capital relevance, and this selectivity is influenced by many factors such as the configuration of the ligands in the metallic catalysts, i.e., its bite angle, flexibility, and electronic properties [152,153]. A series of phosphinous amide ligands have been developed for influencing the direction of approach of the substrate to the active catalyst and, therefore, on the selectivity of the reaction. The use of Rh(I) catalysts bearing the ligands in Scheme 34, that is the phosphinous amides 37 (R ... 

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-... 

Steric effects in the alkene structure also affect linearity. As a result, quaternary carbon atoms are rarely formed in hydroformylation45 In contrast, electronic effects in hydroformylation of arylalkenes often result in the predominant formation of the branched aldehyde.6 40 43 46- 8 Styrene has a marked tendency to form 2-phenylpropanal when hydroformylated in the presence of rhodium catalysts. Rhodium complexes modified by biphosphine49 or mixed amino phosphine oxide ligands50 were shown to give the branched aldehyde with high reactivity and selectivity (iso normal ratios <61.5). 

Mitsubishi Kasei introduced a process to manufacture isononyl alcohol, an important PVC (polyvinyl chloride) plasticizer, via the hydroformylation of octenes (a mixture of isomers produced by dimerization of the C4 cut of naphtha cracker or FCC processes).95 First a nonmodified rhodium complex exhibiting high activity and selectivity in the formation of the branched aldehyde is used. After the oxo reaction, before separation of the catalyst, triphenylphosphine is added to the reaction mixture and the recovered rhodium-triphenylphosphine is oxidized under controlled conditions. The resulting rhodium-triphenylphosphine oxide with an activity and selectivity similar to those of the original complex, is recycled and used again to produce isononanal. 

The asymmetric hydroformylation of 1-alkenes suffers from low regioselectivity for the formation of branched aldehydes, although the enantioselectivity has exceeded 80% ee (entries 7,9,11-15) [80], Introduction of bulky substituent(s) at the C-3 position increases the regioselectivity for branched aldehydes to some extent, and enantioselectivity has reached up to >99% ee (entries 13, 15) [80]. This reaction needs much improvement in regioselectivity to be useful as synthetic method. 

As shown by reaction 9.8, in an asymmetric hydroformylation reaction only the branched aldehyde product can have optical isomers. The linear aldehyde (shown within the brackets) is an undesirable byproduct. Successful asymmetric hydroformylation reaction thus needs to be chemo-, regio-, and enantioselec-tive. Lack of chemoselectivity leads to hydrogenation rather than hydroformylation, and lack of regioselectivity results in the formation of the linear rather than the branched isomer. 

In the BASF process the 1,2-diacetate is the substrate for the hydroformylation step. It can be prepared either directly via oxidative acetoxylation of butadiene using a selenium catalyst or via PtCl4-catalyzed isomerization of the 1,4-diacetate (see above). The latter reaction affords the 1,2-diacetate in 95% yield. The hydroformylation step is carried out with a rhodium catalyst without phosphine ligands since the branched aldehyde is the desired product (phosphine ligands promote the formation of linear aldehydes). Relatively high pressures and temperatures are used and the desired branched aldehyde predominates. The product mixture is then treated with sodium acetate in acetic acid to effect selective elimination of acetic acid from the branched aldehyde, giving the desired C5 aldehyde. 

The hydroformylation of 1-hexene catalyzed by rhodium carbonyl has recently been studied by Lazzaroni and coworkers [21]. They were particularly interested in the influence of reaction parameters on the regioselectivity and the chemoselectivity (to aldehyde and 2-hexene). To understand their results we have to extend Scheme 6.1 by taking account of the formation of linear and branched aldehydes, as well as of isomerization. This is shown in Scheme 6.2. 

The use of the [Rh(BIPHOS)(COD)] complex for the hydroformylation of styrene gave excellent regioselectivity, favoring the formation of the branched aldehyde (98%) <2001EJI2385>. The same complex very effectively... 

Both developments opened up a new era of asymmetric hydroformylation. The results are promising and research is now focused on the synthesis of structurally related ligands. Other ligands, such as the P-N ligand 10, are also showing very high selectivities. Faraone and co-workers, in the hydroformylation of vinyl-naphthalene, reported the exclusive formation of the branched aldehyde while a rhodium/10 catalyst was used (conversion 100%) [84], The enantiomeric excess obtained was 78 % for the / -enantiomer. With methylacrylates an ee of 92 % was observed. For further informations see Sections 2.9 and 3.3.1. 

The triaryl phosphine seems to have the right combination of steric (to induce the formation of linear product at the 1,2-insertion stage) and electronic (to donate electron density to metal in order to stabilize CO ligands) properties. Studies indicate that the rate-determining step is likely to be hydrogenation of the acylrhodium intermediate (as with unmodified Co hydroformylation), but the mechanism of this apparent OA-RE step is not completely understood.34 DFT-level theoretical studies have suggested that the selection for linear versus branched aldehydes... 

The use of CO + HjO as an in situ source of H2 via the shift reaction or as a source of reducing electrons with concomitant oxidation of CO to CO2 has recently been explored with homogeneous catalyst solutions. Equations (a) and (b) are modifications (Reppe) of hydroformylation (see 14.6.3) and olefin hydrogenation, respectively. The most effective catalysts for equation (a) are Ru3(CO),2, Rh fCOlig, Ir4(CO)i2 in alkaline THE or MeOH. The first of these shows great selectivity in the formation of linear vs. branched aldehyde . With Rhg(CO),5, the aldehyde is reduced further to the alcohol. A different catalyst based on Co2(CO)g/diphos in polar ether solvents has also been used to catalyze equation (a) with propylene as substrate. ... 

Extensive mechanistic studies have been performed on reactions catalyzed by rhodium and platinum complexes containing enantiopure C2-symmetric diphosphine ligands.As discussed above, (1) the formation of the Tr-olefin-Rh(H) complex 19, (2) stereospecific cis addition of the hydridorhodium to the coordinated olefin to form the alkyl-Rh complex 20 (and then 2, and (3) the migratory insertion of a carbonyl ligand giving the acyl-Rh complex 17 with retention of configuration, have been established in the hydroformylation of 1-alkenes or substituted ethenes. Thus, it is reasonable to assume that the enantioselectivity of the reaction giving a branched aldehyde is determined at the diastereomeric (1) TT-olefin-Rh complex 19 formation step, (2) alkyl-Rh complex 20 formation step, or (3) acyl-Rh complex 17 formation step. 

The substrate selectivity observed during the hydrogenation of a 1 1 mixture of olefins and the increase in the linear to branched aldehyde ratio during the hydroformylation of 1-octene were attributed to the formation of transient adducts between the substrate and a water-soluble rhodium complexes bearing /3-CD-modified diphosphines (Figure 18). ... 

Our system selection for studies on the stability of the W/CO2 microemulsions in the presence of organometallic catalysts is based on the hydroformylation of higher olefins. This reaction involves the formation of branched or linear aldehydes by the addition of H2 and CO to a double bond according to Scheme 1. The linear aldehydes are the preferred products and the selectivity in such reactions is usually expressed in terms of n iso ratio, which is the ratio of the linear aldehyde to the branched aldehyde. When conducted in the aqueous phase, the reaction is catalyzed with complexes formed in-situ from Rh(CO)2acac and 3,3 ,3 -Phosphinidynetris (benzenesuifonic acid), trisodium salt (TPPTS) in the presence of synthesis gas. 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

The hydroformylation reaction was discovered in 1938 by Otto Roelen in the Ruhrchemie laboratories at Oberhausen (Germany) [1, 2], Since that day, hydroformylation has become a widely studied and interesting reaction for both academic and industrial researchers. This reaction consists formally in the transformation of olefins under carbon monoxide and hydrogen pressure leading to linear and branched aldehydes as primary products (see Section 2.4.1.1). The interest of such a reaction resides in the formation from an olefin of a new carbon-carbon bond with the introduction of a carbonyl function which can be easily transformed into different products of industrial interest like detergents, plasticizers, and pharmaceutical products. The overall production capacity of oxo products was estimated to be aroimd 10 million tons per year in 2001 and this production is still increasing. 

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