Regioselective isomeric 2-octenes

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

The ligands synthesized were also apphed to the isomerizing hydroformylation of more reactive 2-pentene. At 120 °C/ 20 bar quantitative conversion of olefin to aldehydes was achieved within 40 min. Trends similar to those described for internal octene hydroformylation were found. The regioselectivity obtained for the individual ligands tends to be 5% higher compared to that for the octenes. Thus, in the presence of 10 75% of n-hexanal were determined, compare Table 3. Obviously, 2-pentene is able to react more smoothly to the terminal isomer compared to olefins having the double bond in an more internal position. Illustrative for this effect are also literature results obtained for 2- and 4-octene.4,5... 

The hydroformylation of trflns-3-octene at room temperature using the (non-encapsulated) rhodium catalyst based on tris(weta-pyridyl)phosphine afforded 2-ethylheptana] and 2-propylhexanal in exactly a 1 1 ratio. The encapsulated catalyst provided an unprecedented selectivity for 2-propylhexanal of 75% (Scheme 8.3). Again the selectivity is largely retained at 40 °C whereas at 80 °C the isomerization side reaction prohibits the selective formation of aldehydes. Similar regioselectivities were obtained in the hydroformylation of frflns-2-hexene, trans-2-nonene and trans-3-nonene at 25 °C. 

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. 

A phosphite-modified calixarene with unsubstituted hydroxyl groups was used as a ligand in 1-hexene hydroformylation catalyzed by rhodium complexes [224], The reaction was carried out at a synthesis gas pressure of 6.0 MPa and 160 °C. Rh(acac)(CO)2 was a catalyst precursor. In 3 h, the conversion of the initial alkene virtually reached its theoretically predicted value the yield of aldehydes was 80-85%, and the normal-to-isomeric aldehyde ratio was approximately 1 1. Some similar phosphites 83 were also studied as components of catalytic systems for 1-octene hydroformylation [225]. It was shown that the nature and steric volume of substituent R have no essential effect on the main laws of the process. For example, the conversion was 80-90% at a selectivity with respect to nonanal of about 60% in all cases. The regioselectivity with respect to nonanal was considerably increased to 90-92% by using the chelate biphosphite 84 [220]. 

Thus, by using XANTPHOS analogues [14] and appropriate reaction conditions such as high temperature and low pressure, 2- and 4-octene are hydroformylated to give a remarkable 90% of the lineal aldehyde (Table 1) (Figure 4) (see chapter 4). Under these conditions, the isomerization takes place very quickly and all the possible alkenes are present in equilibrium in the reaction medium, and the terminal alkene is hydroformylated faster than the internal ones. Moreover, the catalytic system proved to be remarkably regioselective towards the linear aldehyde. 

In the hydroformylation of terminal olefins, superior results in comparison to BISBI at higher temperature were noted l/b = 54.2 in comparison to BISBI Hb = 2.4) [66]. Moreover, with these tetraphosphines more than 95% linear selectivity and up to 94% yield of total aldehydes starting from 2-alkenes (2-pentene, 2-hexene, 2-octene) were observed in isomerizing hydroformylation [67]. The M-regioselectivity increased in relation to the nature of Ar in the following order, indicating a remarkable electronic effect ... 

Isomeric pentenes, hexenes, and octenes have been used frequently in academic research to study the properties of new catalytic systems [38, 39]. Kragl and coworkers [40] have screened the hydroformylation of a series of terminal olefins (1-pentene until 1-dodecene) with a rhodium catalyst based on BIPHEPHOS and achieved excellent n-regioselectivities and TOFs (turnover frequencies) between 1895 and 8200 h (Scheme 4.7). 

Pyrrole-based tetraphosphoramidites were suggested by Zhang and coworkers [103] for isomerization-hydroformylation of 2-octene and 2-hexene (Figure 5.8). The ligand/metal ratio had a dramatic effect on regioselectivity. At a ratio of 1 1, low regioselectivities were observed. To achieve an l/b ratio of 41 1, a minimum ligand/metal ratio of 2 1 had to be applied and the reaction had to be run at a temperature above 100 °C. By substitution in 3,3, 5,5 -positions (R) at the biphenyl unit, selectivity could be further improved [104]. In the best case, a ratio of lib = 207 was stated. A clear conclusion about the contributions of steric or electronic effects could not be derived from these experiments. 

With methyl oleate, 53% yield of the terminal alcohol was observed. With unmodified internal olefins (2-decene, 2-tridecene, 4-octene), even higher regioselectivities in favor of the terminal alcohol could be achieved l/b up to 12 1). Proof was given that both rhodium and ruthenium complexes catalyze the isomerization-hydroformylation-hydrogenation reaction in a cooperative manner. 

Marko and co-workers reported the activity of monomeric, moisture and air-stable NHC-Pt such as 133 (Figure 13.15). When complex 133 was applied to the hydrosilylation of a variety of terminal alkenes, only the anti-Markovnikov adducts were obtained, with isomerization by-products representing less than 2% of the reaction products. This catalyst tolerated functional groups such as ethers, carbonyls and esters, but internal alkenes were inert under similar conditions. A related series of six- and seven-mem-bered ring-expanded NHC-Pt complexes 134 was tested in the hydrosilylation of 1-octene with bis(trimethylsiloxy)methylsilane excellent activity and regioselectivity were observed. ... 

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