Ethene hydroformylation

Gas Recycle technology has been licensed worldwide by Union Carbide-Davy for the hydroformylation of propene.[9] It has also been operated by Union Carbide for ethene hydroformylation. Its use with butene is feasible, but at the margin of operability. Liquid Recycle, described below, is a better option for butene. 

Scheme 8.4 The preparation step for Rh2/Si02 and the structural change during ethene hydroformylation.

Table 8.7 Catalytic Performances of Ethene Hydroformylation on the Rh Dimer Catalysts at 413 K...

Other technologies, already commercially applied or under development, are summarized in Figure 2.63b. Alternative routes of synthesis include (i) ethene hydroformylation to propionaldehyde, which then forms methacrolein by condensation with formaldehyde methacrolein is then oxidized to methacrylic acid (BASF process) (ii) isobuthyraldehyde conversion into isobutyric acid and then oxidative dehydrogenation to methacrylic add (Mitsubishi Kasei/Asahi process) and (iii) oxidation of terf-butyl alcohol to methacrolein followed by oxidation to methacrylic acid and esterification. 

Formation of diethyl ketone has been observed in the cobalt-catalyzed ethene hydroformylation. From other olefins, only trace amovmts of ketones were observed. For a review of the ketone formation vmder hydroformylation conditions, see Refs. (28) and (30). 

Scheme III Proposed cycle for photocatalysis of ethene hydroformylation by Ru(CO)4PPh3...

The disubstituted complex is converted by photolysis under the reaction conditions to Ru(CO)4PPh3 and this appears to be the precursor for the photocatalytic process. The unsubstituted cluster Ru3(CO)i2 proved be ineffective as a photocatalyst for ethene hydroformylation, although under the photolysis conditions the cluster should fragment to the unsubstituted mononuclear species Ru(CO)5 [17]. 

Another high-level MO study on the ethene hydroformylation by HRh(PH3)2(CO) looked at the effect of the geometry of the complexes having the phosphines in trans- or f -positions, stemming respectively from the bisequatorial (ee) and equtorial-apical (ea) trigonal-bipyramidal precursor.They concluded that the /ra r-isomer was the most active one. For this favored pathway, the insertion of CO was found to be the slowest one, which is not... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

As early as 1972 Parshall described the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate melts [1]. [NEt4][SnCl3], the ionic liquid used for these investigations, has a melting point of 78 °C. Recently, platinum-catalyzed hydroformylation in the room-temperature chlorostannate ionic liquid [BMIM]Cl/SnCl2 was studied in the author s group. The hydroformylation of 1-octene was carried out with remarkable n/iso selectivities (Scheme 5.2-13) [66]. 

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. 

The asymmetric hydroformylation of aryl ethenes such as substituted styrene or naphthylethene is of industrial interest because the hydroformylation products of these substrates are precursors to important nonsteroidal antiinflammatory drugs such as (S )-ibuprofen and (S )-naproxen. Strong efforts have been made to improve the branched/linear ratio, as well as the enantioselectivity of the product. 

The asymmetric hydroformylation of aryl ethenes such as substituted styrene and substituted -naphthyl ethene will lead to the intermediates for important pharmaceuticals. Much concerted effort has been applied to achieve high enantioselectivity as well as high regioselectivity toward the branched aldehydes. The research work in this area is of great industrial interest, and it continues to be a dynamic field of study. 

A typical feature of hydroformylation is the fact that both sides of the double bond are in principle reactive, so only ethene yields propanal as a single product. From propene, two isomers are formed linear or normal butanal and 2-methylpropanal (branched or iso product). With longer chain 1-alkenes, the isomerization of the double bond to the thermodynamically more favored internal positions is possible, yielding the respective branched aldehydes (Fig. 1). Frequently, terminal hydroformylation is targeted because of the better biodegradability of the products. Thus, not only stability, activity, and chemoselectivity of the catalysts are important. A key parameter is also the regioselectivity, expressed by the n/i ratio or the linearity n/(n+i). 

Scheme 2 General mechanism for rhodium-catalyzed hydroformylation of ethene...

Figure 8.3. Simplified mechanism for hydroformylation of ethene (L = PPh3)...

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