Regioselective hydroformylation stabilization

The regioselective hydroformylation of functionalized and nonfunctionalized olefins can also be performed by platinum compounds [26] in chlorostannate ionic liquids as solvents for homogeneous catalysis (entries 20-22, Table 6.1). Dissolved in chlorostannate ionic liquids, the Pt catalyst shows enhanced stability and selectivity in the hydroformylation of methyl-3-pentenoate compared to the identical reaction in conventional organic solvents. The moderate Lewis acidity of these ionic liquids allows the activation of the Pt catalyst combined with tolerance of the functional groups in the substrate. In the case of 1-octene hydroformylation, a biphasic reaction system could be performed using the chlorostannate ionic liquid. 

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). 

The production of 10-methylcarbapenem (184), which has antibacterial activities and enhanced chemical and metabolic stability, has been reported by asymmetric hydroformylation of 4-vinyl-0-lactams 185 catalyzed by Rh-BINAPHOS complexes (Scheme 12.75). Under optimized conditions, the observed regioselectivity was 55/45 (b/1), enantioselectivity was 93/7 (1860 18600 at 95% conversion, and S/C = 1000.233... 

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

Hydroformylation (Equation (14)) is one of the very largest homogeneous catalytic reactions carried out by industry making over 15 billion pounds of aldehyde products each year. These are subsequently hydrogenated to alcohols or oxidized to carboxylic acids. There are several recent excellent reviews on hydroformylation catalysis (cf. Refs 7,7a-7c). Industry is generally more interested in the linear aldehyde product, and much of hydroformylation catalyst development work has been directed at increasing the linear to branched regioselectivity (L B, also referred to as normal to iso), reaction rates, and catalyst stability (lifetime). There are three main hydroformylation catalysis... 

This branched regioselectivity in these cases results from the greater stability of the branched alkyl complex when the alkyl group bears an electron-withdrawing substituent on the a-carbon. This regioselectivity was discussed in Chapter 10 on insertion processes. The high selectivity for formation of branched products from the hydroformylation of vinylarenes results from the formation of an iri -benzyl intermediate, - - as discussed in Chapter 10. The formation of these chiral, branched products has been a particular focal point for the development of enantioselective hydroformylation. 

As has been seen above, ligands have a considerable influence on the control of regioselectivity because they stabilize a preferred M-alkyl intermediate or accelerate or avoid the (i-elimination process. An example of this is the hydroformylation of 2,3- and 2,5-dihydrofuran [27]. Thus, the hydrofonnylation of both dihydrofurans 22 and 23, using P(0-o- BuCsH4)3 gives practically the same ratio of products 24 25 (Figure 9) (entries 2 and 4 Table 2). 

Stabilizing the catalyst against acids was needed for the Rh/BINAS-catalyzed aqueous hydroformylation of internal olefins (see Table 5.1) [8]. Reaction rates were low (averaged TOF = 62h for 2-pentene), but very high regioselectivities of 99% toward the terminal aldehydes were obtained for the hydroformylation of 2-pentene and 2-octene, respectively, under optimized reaction conditions. Controlling pH was found to be essential to increase both the selectivity and the aldehyde yield. Best results were obtained in solutions buffered at pH 8-9, or with additional triethanolamine or TMEDA employed to trap formic acid suggested to be formed in a side reaction. 

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