Hydroformylation acrylic esters

An unusual enhancement of catalytic activity in a two-phase system has been reported by Fremy et al. (1998) for the hydroformylation of acrylic esters using Rh complex of TPTS as catalyst. Even though acrylic esters have reasonable solubility in water, rate enhancements in two-phase systems by a factor of 2 to 14 have been reported. It seems that water is not an inert solvent but also acts as a reactant or a co-ordinating solvent which can modify elementary steps of the catalytic cycle (Cornilis, 1997). 

The first work on a-functionalized olefins was focused on the hydroformylation of acrylic esters [Eq. (5)] [16-19]. 

Hydroformylation of acrylate esters is a potentially very important reaction since the branched aldehyde product gives an entry point to production of important pharmaceuticals and methacrylate ester monomers. Hydroformylation of methyl acrylate (Scheme 20) proceeded in toluene with [Rh(acac)(CO)2] + PlCeHsla at 50°C and 5 MPa CO/H2 with a TOF of 225 h , aldehyde yield 95.4% and a branched to normal aldehyde ratio of more than 200 (165). In an aqueous organic biphasic system (40 mL toluene -I-10 mL water), the initial TOF increased to 545 h, retaining the high regioselectivity (b/n = 128). This effect is not unprecedented— polar solvents may facilitate reactions with polar intermediates or products which are strongly solvated. However, when the reaction was catalyzed with the same Rh-catalyst in a supported aqueous phase on silica gel. 

In 1982, Okano and Kiji [13] were the first to use paraformaldehyde in excess for the hydroformylation of styrene in the presence of a Rh(hydrogencarbonate) catalyst (Scheme 3.3), aliphatic olefins, and acrylic esters. Aldehydes were formed as major products. 

Isobutyraldehyde is commonly available as a by-product of propylene/Oxo hydroformylation. Methyl isoamyl ketone is used as a solvent for cellulose esters, acrylics, and vinyl polymers. It is available in the United States from Eastman (Kingsport, Tennessee) (47) and Union Carbide (South Charleston, West Virginia) and was priced at 1.42/kg in October 1994. 

A considerable amount of work has been done on the hydroformylation of alkyl acrylates. The formation of y-oxobutyrates has previously been reported (15). Iwanaga (70) studied the effect of solvent variation and found that the rate of hydroformylation was in the order alcohols > acetone > toluene. Pyridine and some of its homologs also increased the rate of reaction. At higher temperatures and pressures, lactones were formed (32, 35, 135), presumably by reduction of the Oxo ester to the hydroxy ester followed by ring closure with elimination of alcohol. 

Aliphatic a,/3-unsaturated esters undergo the hydroformylation reaction. Ethyl acrylate, ethyl crotonate, and diethyl fumarate react as shown in Equations 1, 2, and 3, respectively. 

As reported above, a literature survey shows that the hydroformylation of this class of alkene has been scarcely investigated. Indeed, these studies have been devoted exclusively to the hydroformylation of arylic esters such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethoxyethyl acrylate, and 2-ethylhexyl acrylate (Eq. 4) [18-21], Most attention has been focused on the hydroformylation of methyl acrylate to 2-formylpropanoate ester since the latter is used extensively for the synthesis of pharmaceuticals and may also be considered as a potential source of methyl methacrylate [18]. 

In contrast with the first class of functionalized alkenes, immobilization of the catalyst in aqueous phase results in an enhancement of the catalytic activity [19]. Indeed, it has been observed that the hydroformylation rates of arylic esters having high solubility in water were much higher in biphasic systems than those observed under comparable homogeneous conditions. Except for 2-ethylhexyl acrylate, the initial rate was increased by a factor of 2.4, 12, 2.8, and 14 for methyl, ethyl, butyl, and 2-ethoxyethyl acrylate, respectively (see Figure 1) [20]. One of the most intriguing features is that the hydroformylation rates for ethyl and butyl acrylates in biphasic medium were respectively higher than and comparable with those observed with methyl acrylate. Actually, the water-solubilities of ethyl and butyl acrylates (18.3 and 2.0 g L-1 at 20 °C, respectively) are lower than that of methyl acrylate (59.3 g L-1 at 20°C). 

Epoxidation of oleic and linoleic acid was readily achieved by treatment with the acetonitrile complex of hypofluorous acid (55). Phase-transfer-catalyzed biphasic epoxidation of unsaturated triglycerides was accomplished with ethylmethyldioxirane in 2-butanone (56). The enantioselective formation of an a,P-epoxy alcohol by reaction of methyl 13()S)-hydroperoxy-18 2(9Z,llfi) with titanium isopropoxide has been reported (57). An immobilized form of Candida antartica on acrylic resin (Novozyme 435) was used to catalyze the perhydrolysis and the interesterification of esters. Unsaturated alcohols were converted with an ester in the presence of hydrogen peroxide to esters of epoxidized alcohols (e.g., epoxystearylbutyrate) directly (58). Homoallyl ethers were obtained from olefinic fatty esters by the ethylaluminium-in-duced reactions with dimethyl acetals of formaldehyde, acetaldehyde, isobutyralde-hyde, and pivaldehyde (59). Reaction of 18 2(9Z, 12Z) with 50% BF3-methanol gave monomethoxy and dimethoxy derivatives (60). A bulky phosphite-modified rhodium catalyst was developed for the hydroformylation of methyl 18 1 (9Z)and 18 1(9 ), which furnished mixtures of formylstearate and diformylstearate (61). 

Various a-formyl esters can be generated from the hydroformylation of a,P-unsaturated esters and diesters. Ethyl 2-formylpropanoate 33 can be obtained from the hydroformylation of ethyl acrylate 32. The reaction was catalyzed by the Rh2Cl2(CO)2/phosphine/NEt3 system at 20-40 °C in the presence of H2/CO (1/1, 20 atm), and yielded the product with excellent regioselectivity (98-100%). Particularly effective ligands for this reaction include l,4-bis(diphenylphosphino)butane (DPPB) 34, phosphole (o-TDPP) 35, and phosphanorbomadiene (DMTPPN) 36. 

Os-2-alkenes react a few times faster than tra 5-2-alkenes. Steric influences, both from the complex part and the alkene, play a dominant role in determining the fate of hydroformylation of alkenes not containing functional groups. For alkenes such as styrene, 3,3,3-trifluoropropene, acrylates, and vinyl esters, the electronic properties of the alkene determine to a large extent the regioselectivity. 

Scheme 4.30 Hydroformylation of acrylic add esters and subsequent transformations.

Most unsaturated esters are hydroformylated in good yield (table 18). The conjugation in a,p-unsaturated esters is weaker than in unsaturated aldehydes (the resonance energy of crotonaldehyde, for example, is 2.4 Kcal/mole higher than for ethyl crotonate [7]). Thus, conjugated unsaturated esters such as acrylates and crotonates, in contrast to acrolein and crotonaldehyde, can be converted to aldehyde products with synthesis gas. 

---