Rhodium hydroformylation catalysts unmodified

Structure 4 is an intermediate for manufaeturing vitamin A (Scheme 2). The annual demand for vitamin A is about 3000 tons. Major producers are BASF, Hoffmann-La Roche and Rhone-Poulenc Animal Nutrition [55]. At an early stage in the synthesis BASF and Hoffmann-La Roche are using a hydroformylation step to synthesize 4 starting from l,2-diacetoxy-3-butene (5) and 1,4-di-aeetoxy-2-butene (6), respectively [56, 57]. The selectivity toward the branched product in the BASF process is achieved by using an unmodified rhodium carbonyl catalyst at a high reaction temperature. The symmetry of 6 in La Roche s process does not lead to regioselectivity problems. Elimination of acetic acid and isomerization of the exo double bond (La Roche) yields the final product 4 in both processes. 

W Leitner, D Koch. Hydroformylation with unmodified rhodium catalysts in supercritical carbon dioxide. PCT Patent No. WO 99/03810, 1999. 

Hydroformylation with Unmodified Rhodium Catalysts in scCC>2... 

Other Rhodium Processes. Unmodified rhodium catalysts, eg, 1 14(00)22 [19584-30-6] have high hydroformylation activity but low selectivity to normal aldehydes. 

Cobalt carbonyls are the oldest catalysts for hydroformylation and they have been used in industry for many years. They are used either as unmodified carbonyls, or modified with alkylphosphines (Shell process). For propene hydroformylation, they have been replaced by rhodium (Union Carbide, Mitsubishi, Ruhrchemie-Rhone Poulenc). For higher alkenes, cobalt is still the catalyst of choice. Internal alkenes can be used as the substrate as cobalt has a propensity for causing isomerization under a pressure of CO and high preference for the formation of linear aldehydes. Recently a new process was introduced for the hydroformylation of ethene oxide using a cobalt catalyst modified with a diphosphine. In the following we will focus on relevant complexes that have been identified and recently reported reactions of interest. 

Of the isomeric aldehydes indicated in Eq. (7.1), the linear aldehyde corresponding to anti-Markovnikov addition is always the main product. The isomeric branched aldehyde may arise from an alternative alkene insertion step to produce the [RCH(Me)Co(CO)3] or [RCH(Me)Rh(CO)(PPh3)2] complexes, which are isomeric to 2 and 8, respectively. Alternatively, hydroformylation of isomerized internal alkenes also give branched aldehydes. The ratio of the linear and branched aldehydes, called linearity, may be affected by reaction conditions, and it strongly depends on the catalyst used. Unmodified cobalt and rhodium carbonyls yield about 3-5 1 mixtures of the normal and iso products. 

Although the overall reaction mechanisms (catalytic cycles) written for hydroformylation reactions with an unmodified cobalt catalyst (Scheme 1) and the rhodium catalyst (Scheme 2) serve as working models for the reaction, the details of many of the steps are missing and there are many aspects of the reaction that are not well understood. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Both modified and unmodified rhodium catalysts have shown good activity and selectivity for the hydroformylation of 1-octene in SCCO2. With respect to the unmodified catalyst, higher reaction rates can be achieved compared to organic solvents or liquid CO2 under similar conditions. These modified systems exhibit higher regioselectivities compared to conventional solvent systems. 

Modem hydroformylation research is almost exclusively focused on four transition metals cobalt, rhodium, platinum and to considerable extent mthenium [13], The generally accepted order of hydroformylation activity for unmodified monometallic catalysts clarifies this picture [14] ... 

In the early 1960s Heck and Breslow formulated the generally accepted hydroformylation cycle depicted in Scheme 3 [89]. Originally formulated for cobalt catalysts, the mechanism is valid for unmodified rhodium complexes as well. The elemental steps (Scheme 3) are ... 

Table 2 shows some examples of parameters determined for triphenylphos-phine-modified and unmodified rhodium and cobalt catalysts in the hydroformylation of terminal olefins. The general reaction rate equation (eq. (6)) is used ... 

The most detailed and generally accepted kinetic study on triphenylphosphine-modified rhodium catalysts was published in 1980 [109]. It was concluded from the coefficients obtained (Table 2) that the fast alkene insertion is followed by the rate-determining step involving CO or TPP [110]. The apparent activation energy for propene hydroformylation was found to be 84 kJ/mol, very similar to the value obtained for unmodified cobalt catalysts. 

Highly active unmodified rhodium catalysts for the hydroformylation of various olefins in SCCO2 are formed under mild conditions from [(cod)Rh(hfa-cac)] (8 cod = cis,cis-l,5-cyclooctadiene) and a number of other simple rhodium precursors [24]. Especially for internal olefins, the rate of hydroformylation is considerably higher than using the same catalysts in conventional liquid solvents under otherwise identical conditions. A detailed study of the hydroformylation of 1-octene (Scheme 6) using the online GC setup shown in Fig. 3 revealed a network of competing isomerization and hydroformylation when 8 was used without additional modifiers. As a result, the regioselectivity for the desired linear n-aldehyde varied considerably with conversion. At 60% conversion, the product aldehydes contained almost 80% of nonanal, whereas only 58 % linear aldehyde were present in the final product mixture. 

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