Hydroformylation internal olefins

Rhodium-phosphine catalysts are unable to hydroformylate internal olefins, so much that in a mixture of butenes only the terminal isomer is transformed into valeraldehydes (see 4.1.1.2). This is a field still for using cobalt-based catalysts. Indeed, [Co2(CO)6(TPPTS)2] -i-lO TPPTS catalyzed the hydroformylation of 2-pentenes in a two-phase reaction with good yields (up to 70%, but typically between 10 and 20 %). The major products were 1-hexanal and 2-methylpentanal, and n/i selectivity up to 75/25 was observed (Scheme 4.12). The catalyst was recycled in four mns with an increase in activity (from 13 to 19 %), while the selectivity remained constant (n/i = 64/36). 

As has been explained above, isomerization is a competitive process that can be diminished, or suppressed, by choosing low reaction temperatures and an appropriate catalyst. However, isomerization can be advantageously used to hydroformylate internal olefins to give the linear aldehyde. 

Linear terminal olefins are the most reactive in conventional cobalt hydroformylation. Linear internal olefins react at less than one-third that rate. A single methyl branch at the olefinic carbon of a terminal olefin reduces its reaction rate by a factor of 10 (2). For rhodium hydroformylation, linear a-olefins are again the most reactive. For example, 1-butene is about 20—40 times as reactive as the 2-butenes (3) and about 100 times as reactive as isobutylene. 

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 most influential parameter in cobalt-catalyzed hydroformylation was found to be carbon monoxide partial pressure. Piacenti et al. (30) showed this to be influential for both a- and internal olefins. Results are detailed in Tables V and VI. The percent of n-aldehyde rose rapidly as the carbon monoxide partial pressure was increased up to 30-40 atm CO further increase had little effect. 1-Pentene clearly gave a higher percentage of straight-chain aldehyde than 2-pentene, but the difference was insignificant in the lower Pco experiments. 

The catalyst containing 2.0% Rh, insoluble in organic solvent, was used for hydroformylation of 1-hexene at 80°C and 43 atm of 1/1 H2/CO. The catalyst concentration was 1 mmole Rh per mole of olefin. After 4 hours a 41% yield of aldehyde was obtained, with a 2.5 1 isomer ratio. Some isomerization to internal olefins also occurred. A significant feature was the rhodium concentration of 2 ppm in the product. 

Fig. 11 Some recent developments for isomerizing hydroformylation of internal olefins...

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

Even 2,3-disubstituted indoles can be achieved if internal olefins are used. Regioselective hydroformylation of a styrene-type olefin and subsequent hy-drazone formation and Fischer indolization gives an intermediate indole with a quaternary center in 3-position. The regained aromaticity is the driving force for the rearrangement of one substituent into the 2-position of the indole core (Scheme 39). 

Raffinate-II typically consists of40 % 1-butene, 40 % 2-butene and 20 % butane isomers. [RhH(CO)(TPPTS)3] does not catalyze the hydroformylation of internal olefins, neither their isomerization to terminal alkenes. It follows, that in addition to the 20 % butane in the feed, the 2-butene content will not react either. Following separation of the aqueous catalyts phase and the organic phase of aldehydes, the latter is freed from dissolved 2-butene and butane with a counter flow of synthesis gas. The crude aldehyde mixture is fractionated to yield n-valeraldehyde (95 %) and isovaleraldehyde (5 %) which are then oxidized to valeric add. Esters of n-valeric acid are used as lubricants. Unreacted butenes (mostly 2-butene) are hydroformylated and hydrogenated in a high pressure cobalt-catalyzed process to a mixture of isomeric amyl alcohols, while the remaining unreactive components (mostly butane) are used for power generation. Production of valeraldehydes was 12.000 t in 1995 [8] and was expected to increase later. 

Internal olefins (2-butene, 2-hexene) were also successfully hydroformylated in water with complexes prepared in situ from [PtCbCCOD)] and the tetrasulfonated diphosphines 37 at 100 °C and 80 bar syngas [148,149]. The same catalysts were suitable for the hydroformylation of 2- and 3-pentenoic acids and trans-2-pentenenitiile, too [150]. The -... 

The discovery and use of fluorophosphites and chlorophosphites as trivalent phosphorus ligands in the rhodium catalyzed, low-pressure hydroformylation reaction are described. The hydroformylation reaction with halophosphite ligands has been demonstrated with terminal and internal olefins. For the hydroformylation of propylene, the linear to branched ratio of the butyraldehyde product shows a strong dependency on the ligand to rhodium molar ratios, the reaction temperature, and the carbon monoxide partial pressure. 

Derivatives of the steroids androstene and pregnene have been transformed directly into A-acyl amino acids by an orthogonal catalysis procedure, utilizing [RhCl(nbd)]2 and Co2(CO)8 (Scheme 11). The rhodium phosphine catalyst (generated in situ in the presence of syn-gas and phosphine) affects hydroformylation of the internal olefin to generate aldehyde. In the presence of Co2(CO)8, A-acyl amino acids are obtained as the major products. An unstable amido alcohol intermediate, formed by reaction of the amide with aldehyde, is proposed to undergo cobalt-catalyzed GO insertion to yield the desired A-acyl amino acid. 

Table 9 summarizes further biphasic hydroformylation reactions of various mid range terminal and internal olefins such as 1-hexene, 1-octene and 2-hexene catalysed by different water soluble systems. 

Straight-chain terminal olefins react most readily in hydroformylation. Internal alkenes exhibit decreased reactivity, whereas branched olefins are the least reactive.6,11,43 Rhodium modified with phosphite ligands, however, was shown to... 

Optical yields up to 17% and 25%, respectively, have been reached in the styrene hydroformylation in the presence of cobalt or rhodium catalysts using N-alkylsalicylaldimine or phosphines as asymmetric ligands. Furthermore the hydroformylation of aliphatic and internal olefins have been achieved using rhodium catalysts in the presence of optically active phosphines. With the same catalysts, cis-butene surprisingly undergoes asymmetric hydroformulation with optical yields up to 27%. On the basis of the results obtained for cis-butene and the asymmetric induction phenomena in dichlor(olefin)(amine)platinum( 11) com-... 

The results of the hydroformylation of internal olefins are reported in Table 9. In the case of (Z)- and (E)-2-butene, the same fare of the unsaturated carbon atom is formylated with either a rhodium- or platinum (—)-DIOP-containing catalytic system. With the rhodium catalyst, when an acyclic olefin is used as the substrate, the same fare is always attacked, and it is only the notation but not the geometric requirement that is different for (E)-l-phenyl-1-propene. The only exception is represented by bicyclo[2,2,l]heptene. However, using (—)-CHIRAPHOS instead of (—)-DIOP, also bieyelo[2,2,l]heptene behaves like internal butenes. No regularity is observed for the cobalt or ruthenium (—)-DIOP catalytic systems. With the same system, only in 3 cases out of 15 the face of the prochiral atom preferentially formylated has different geometric requirements. 

Table 9. Face of the Unsaturated Carbon Atom Prevailingly Attacked by CO in Internal Olefins Hydroformylation... 

From the results of the hydroformylation of (Z)-2-butene it is possible to predict, when the same catalytic system is used, the prevailing enantiomer which would arise from other C2v olefins, such as bicyclo[2.2.2]oct-2-ene and 2,5-dihydrofurane and for other internal olefins like (Z)-2-hexene. 

As shown in Table 10,4 predictions of the chiralities of the prevailing enantiomers fit well, not only for (Z)- but also for (E)-internal olefins. In fact, due to the greater space availability in quadrants Q, and Q2 with respect to quadrants Q t and Q 2 (as shown by the isomeric ratios found in the hydroformylation of (Z)- and (E)-2-hexene), the difference in space availability between quadrants Q., and Q 2 are expected to influence mainly the energy of the activated complexes yielding one or the other antipode. 

In view of the many differences noted above between the hydroformylation of olefins and epoxides, it is not surprising to find that changes in structure result in a different order of reactivity in each case. Thus for epoxides the reactivity to cobalt hydrocarbonyl is cyclohexene oxide > propylene oxide, whereas with olefins the order is terminal olefins > internal olefins > cyclic olefins (145). 

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