Product selectivity, hydroformylation

NAPS can be run with a very wide variety of both polar and non-polar ligands that are enantioselective. One could consider the synthetic possibilities and solvents for the separation to select potential process/separation combinations. The mild separation conditions of NAPS are well suited to maintain enantiomeric excess. Water extraction of the hydroformylation product of methoxyvinylnaphthalene, (2-(6-methoxy)-naphthylpropanal) is not feasible. 

However, the balance between sterically demanding ligands and their ability to remain coordinated so that the product selectivity could be influenced is a fine one. This aspect is discussed in more detail in Section 5.2.4. Although not directly related to hydroformylation, it is appropriate to note here that Markovnikov additions accompanied by /3-hydride elimination is a general pathway for alkene isomerization. This is shown in Fig. 5.2 for the isomerization of both terminal and internal alkenes. 

Bimetal carbonyl clusters of RhFe were synthesized inside supercages of NaY by reaction of Rh4(CO)i2 with [HFe3(CO)n] /NaY in vacuo at 100°C (262). The pale red-brown RhFe carbonyl clusters in NaY catalyze the hy-droformylation of ethylene and propylene with high selectivity to alcohols, whereas this selectivity is lower for supported pale grey Rh carbonyls and pink-purple Fe carbonyls. With Pd phosphine carbonyl clusters in NaY the hydroformylation of propene displays high selectivity for C7 ketones, which are formed by reaction of propene with the primary hydroformylation product, C4 aldehyde (263). 

Here, we describe the simulated effect of a second catalytic function (hy-droformylation or cracking) on product selectivity. Hydroformylation in-... 

Atmospheric pressure hydroformylation of ethylene and propene was conducted at 373-453 K on reduced [Rh5]-NaY and RhFe-NaY. The results show that acetaldehyde is catalytically obtained as the hydroformylation product on [Rhg]-NaY (142). In contrast, it is of interest to find that the bimetallic RhFe-NaY catalyst gives much higher activities and selectivities for the normal alcohols, as compared to those on [Rhg]-NaY. In particular. 

Addition of hydrogen predominantly from the opposite face to a 3-methoxycarbonyl substituent has also been achieved with HMn(CO)5, the 1,2-diphenyl compound giving a 7 1 facial selectivity with HCo(CO)4 a single hydroformylated product is isolated, together with three isomeric reduced compounds. ... 

The use of ionic liquids in combination with CO2 has the potential to produce cleaner processes with improved selectivity. The negligible miscibility of the ionic liquid in CO2 compared with appreciable amounts of CO2 that can be found in the liquid phase make the use of CO2 as a green solvent attractive for continuous reaction processes. Sellin, Webb, and Cole-Hamilton conducted a hydroformylation of hex-l-ene and 1-octene catalyzed by rhodium based catalyst in l-butyl-3-methylimidazolium hexafluoro-phosphate (BMIMHF) in contact with CO2. Improved n iso product selectivity was obtained, compared with that using toluene with similar selectivity, but substantially lower yield (40% compared to >99%). Using 1-octene as a substrate and [Rh2(OAc)4]/[1-propyl-3-methylimidazolium] [PhP(C6H4S03)2] as catalyst, over 20 hr of continuous operation was achieved with minimal catalyst leaching at 373 K. 

Cobalt, rhodium and platinum complexes modified with numerous chiral phosphanes have been used in asymmetric hydroformylation of styrene. The results are compiled in Table 4. Iso-product selectivities of >95% and stereoselectivities of >90% ce are reported, in many cases, however, only with low conversion rates and yields. Early results based on optical rotation measurements had to be reevaluated due to wrongly adopted rotation values for hydra tropaldehyde4-. ... 

This reaction is observed as a side reaction of hydroformylation. With ethylene the formation of diethyl ketone can be optimized to give high product selectivities, however, with higher, especially unsymmetrically substituted alkenes, only low ketone product selectivities and re-gioselectivities are observed. Thus, the reaction is only rarely applied to open chain ketone synthesis and little information is available about the stereochemistry of this reaction type1 -3. 

Hydroformylation is usually carried out under catalytic conditions. The alkene, catalyzed by metal complexes under carbon monoxide and hydrogen in hydrocarbon, alkyl halide or ether solvent, generates the hydroformylation product. Rhodium catalysts are preferred for laboratory syntheses because of their higher activity and selectivity. Improvements in regioselectivity and yields have been found when the reaction is carried out in the presence of added donor ligands such as trialkylphosphines, or under UV irradiation. Catalysts supported on polymers have been used for easy separation of product and reuse of catalysts. 

Hydroformylation, the addition of H and CHO to an olefinic double bond, is the oldest and largest industrial process that involves homogeneous transition-metal catalysts. The original catalyst was derived from Co2(CO)g. Later, phosphine modification of the system was introduced to afford better product selectivity and lower operating pressures. Still later, related Rh systems were introduced. ... 

Figure 11 Proposed cobalt-phosphine hydroformylation and aldehyde hydrogenation cycles showing the dominant linear product selectivity.

Figure 12 Proposed Rh-PPhs hydroformylation cycles showing linear product selectivity.

Structure (ATO) gives a product distribution that is dominated by adipic acid. This is thought to result because the narrower channels inhibit the release of cyclohexanol and cyclohexanone and the reaction proceeds further to the more mobile linear products, such as adipic acid. Selectivity is also observed in the aerial oxidation of linear alkanes. If the reaction is performed over large-pore solids, w-alkanes are oxidised preferentially at carbon atoms at C2 and C3 positions in the chain, in accordance with the C-H bond strengths at these positions. If a small-pore structure such as CoAPO-18 is used, however, the product selectivity favours Cl oxyfunctionalised products. The synthesis of terminally oxidised alkanes would be of use for many applications, because linear terminal alcohols could be prepared from alkane feedstocks, rather than from a-olefins (via hydroformylation). 

The main problem related to SLPC is the loss of solvent because of evaporation in a continuously operated catalytic reactors. This problem can be overcome by using ionic liquids as solvent [17-20]. Ionic liquids are molten salts and their partial pressure is low under conditions commonly used for hydroformylation and hydrogenation reactions. As generally observed for SLPC, the catalytic activity and product selectivity depends on the liquid loading and the nature of the porous support [21]. A detailed discussion can be found in [22]. In order to diminish internal diffusion resistances within the supported liquids by using microstruc-tured supports with high porosity like foams or fibrous materials, are proposed for SLPC [23]. 

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