Aldehydes catalytic hydroformylation production

Due to the wide availability of aliphatic aldehydes from hydroformylation (cf. Section 2.1.1), the principal method for the production of C3-C]o carboxylic acids is the catalytic oxidation of the corresponding aldehyde (eq. 1). 

The hydroformylation, or 0x0, process was introduced in 1938 and is the oldest homogeneous catalytic process in commercial use. It is used to convert terminal alkenes into aldehydes and other organic products, especially those having their carbon chain increased by one. Approximately 10 million tons of hydroformylation products are produced annually. The conversion of an alkene of formula R2C=CH2 into an aldehyde R2CH—CH2—CHO is outlined in Figure 14.18. ... 

It catalyses the aminolysis of epoxides in an extraordinarily efficient manner in aprotic solvents (e.g. toluene, CH2CI2) with complete trans stereoselectivity and high regioselectivity [Chini et al. Tetrahedron Lett 35 433 1994], It also catalyses the trans addition of indole (at position 3) to epoxides (e.g. to phenoxymetltyloxirane) in >50% yields at 60° (42 hours) under pressure (10 Kbar) and was successfully applied for an enantioselective synthesis of (+)-diolmycin A2 [Kotsuki Tetrahedron Lett 37 3727 799(5]. Of the ten lanthanide triflates, Yb(OTf)3 gave the highest yields (> 90%, see above) of condensation products by catalytically activating formaldehyde, and a variety of aldehydes, in hydroformylations and aldol reactions, respectively, with trimethylsilyl enol-ethers in THF at room temperature. All the lanthanide triflates can be recovered from these reactions for re-use. [Kobayashi Hachiya J Org Chem 59 3590 1994.]... 

Due to its highly metal functionalized Si-0 framework 2 can be seen as a model compound for Si-O-supported transition metal catalysts. In first experiments we have studied the catalytic activity of 2 in the hydroformylation of 1-hexene. The experiments were performed in toluene at a temperature of 120°C and a reaction time of 18 h. The initial CO/H2 pressure at room temperature was 70-80 bar. The use of a catalyst formulation of 2 and triphenylphosphane in a 1 8 stoichiometry led to complete conversion of 1-hexene to the corresponding aldehydes. NMR and GC analyses of the hydroformylation products showed a 3 1 mixture of 1-heptanal and 2-methylhexanal had been formed. Filtration of the reaction mixture led to the isolation of a brownish solid, which still showed catalytic activity. According to IR spectroscopic results it is supposed that the catalytically active species formed in situ is a substitution product of 2 and triphenylphosphine. However, the mechanistic pathway of this catalysis is not yet understood. Experiments leading to a further understanding are under investigation. 

In order to make sure under catalytic hydroformylation conditions that the H in HRe (CO)5 was incorporated into the product aldehyde, deuteroformylations were conducted with D2 and then HRe(CO)5 was injected into the system. These experiments showed that the H label in HRe(CO)s was exclusively incorporated in the formyl group of the organic product [75]. 

The desired reaction in catalytic hydroformylation is the addition of carbon monoxide and dihydrogen to the olefin substrate usually to obtain aldehyde. To some extent, however, concurrent reactions of the olefin (substrate) such as hydrogenation, isomerization, and special carbonylations, and consecutive reactions of the aldehyde product such as hydrogenation to alcohol, aldol reaction, trimer-ization, and formate formation take place under the reaction conditions of hydroformylation, which affect both yield and selectivity of the aldehyde products. For an example of product composition obtained using an unmodified cobalt catalyst in the BASF process, see Table 3. 

In the presence of strong bases, the hydroformylation product can undergo in situ P-elimination to give a,P-unsaturated aldehydes (Scheme 4.109) [20]. Noteworthy, in this protocol no additional phosphite ligand had to be added. With amine bases, stoichiometric amounts were required to achieve full elimination, whereas catalytic amounts of K2CO3 (0.1 equiv) were sufficient. 

As the second selectivity issue, the catalyst should usually favor w-aldehyde over iso-aldehyde formation. This task is mainly addressed by the right choice of ligand. The ligand influences both the electronics and sterics of the catalyst in the step of the catalytic cycle that determines regioselectivity (see -II versus iso-11 in Scheme 6.14.4). Note that the transition state leading to the linear hydroformylation product involves a linear alkyl chain attached to the metal center that requires less space compared to the branched counterpart. Moreover, the electronic properties of the ligand influence the hydride transfer from the metal complex to Cl versus C2 during formation of the metal-carbon bond. 

In the Co-based hydroformylation reaction, the product aldehydes are generated from 5.46 and 5.47 by reaction with dihydrogen as well as by reaction with 4.49. In the latter case, the organometallic product is Co CCO). The latter on reaction with and loss of CO regenerates 5.38. However, under the catalytic conditions product formation by this pathway, indicated by arrows of lighter shade, is insignificant. 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

Figure 2 shows the generally accepted dissociative mechanism for rhodium hydroformylation as proposed by Wilkinson [2], a modification of Heck and Breslow s reaction mechanism for the cobalt-catalyzed reaction [3]. With this mechanism, the selectivity for the linear or branched product is determined in the alkene-insertion step, provided that this is irreversible. Therefore, the alkene complex can lead either to linear or to branched Rh-alkyl complexes, which, in the subsequent catalytic steps, generate linear and branched aldehydes, respectively. 

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