Internal alkenes hydroformylation

Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

Scheme 2 Position-selective hydroformylation of internal alkenes with a rhodium(I)/-phosphabarrelene catalyst...

Cobalt carbonyls are the oldest catalysts for hydroformylation and they have been used in industry for many years. They are used either as unmodified carbonyls, or modified with alkylphosphines (Shell process). For propene hydroformylation, they have been replaced by rhodium (Union Carbide, Mitsubishi, Ruhrchemie-Rhone Poulenc). For higher alkenes, cobalt is still the catalyst of choice. Internal alkenes can be used as the substrate as cobalt has a propensity for causing isomerization under a pressure of CO and high preference for the formation of linear aldehydes. Recently a new process was introduced for the hydroformylation of ethene oxide using a cobalt catalyst modified with a diphosphine. In the following we will focus on relevant complexes that have been identified and recently reported reactions of interest. 

Figure 7.3 gives an overview of the reactions involved in the hydroformylation of internal alkenes to linear products. It has been suggested that cobalt, once attached to an alkene, runs along the chain until an irreversible insertion of CO occurs. Thus, the alkene does not dissociate from the cobalt hydride during the isomerisation process. There is no experimental support for a clear-cut proof for this mechanism. In alkene polymerisation reactions this type of chain running has been actually observed. 

One should be aware that the rate data are especially prone to variation. We have already seen that 1-alkenes are hydroformylated at a much higher rate, but at the same time they are rapidly isomerised to the much less reactive internal alkenes. This together with the highly exothermic reaction may result in low reproducibility. The results will thus strongly depend on the experimental procedures and how carefully they were executed. 

The fourth generation process for large-scale application still has to be selected from the potential processes that have been nominated . In the chapters to follow several of these candidates will be discussed. The fourth generation will concern higher alkenes only, since for propene hydroformylation there are hardly wishes left [13], Many new phosphite-based catalysts have been reported that will convert internal alkenes to terminal products [6,7,14] and recently also new diphosphines have been reported that will do this [15,16,17],... 

Internal alkenes. Dibenzophosphole- and phenoxaphosphino substituted xantphos ligands 31 and 32 [62] (Figure 8.14) show a high activity and selectivity in the rhodium catalysed linear hydroformylation of 1-octene (l b = > 60). More importantly, ligands 31 and 32 exhibit an unprecedented high activity and selectivity in the hydroformylation of trans 2- and 4-octene to linear nonanal. 

They constitute the first rhodium phosphine modified catalysts for such a selective linear hydroformylation of internal alkenes. The extraordinary high activity of 32 even places it among the most active diphosphines known. Since large steric differences in the catalyst complexes of these two ligands are not anticipated, the higher activity of 32 compared to 31 might be ascribed to very subtle bite angle effects or electronic characteristics of the phosphorus heterocycles. 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

Since hydroformylation of internal alkenes is also relatively fast with this catalyst the overall linearity obtained may become rather low (20-30%). This explains the low linearity of a non-optimised reaction using a similar bulky-phosphite R = 2,6-Me2C6H30 quoted in Table 8.5. 

Thermal cracking of wax. From thermal cracking a thermodynamic mixture might have been expected, but the wax-cracker product contains a high proportion of 1-alkenes, the kinetically controlled product. Still, the mixture contains some internal alkenes as well. For several applications this mixture is not suitable. In polymerisation reactions only the 1-alkenes react and in most cases the internal alkenes are inert and remain unreacted. For the cobalt catalysed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatisations the isomer composition is pivotal to the quality of the product. 

Kuil, MSoltner, T., van Leeuwen, P.W.N.M. and Reek, J.N.H. (2006) High-precision catalysts Regioselective hydroformylation of internal alkenes hy encapsulated rhodium complexes. J. Am. Chem. Soc., 128, 11344-11345. 

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... 

Of the isomeric aldehydes indicated in Eq. (7.1), the linear aldehyde corresponding to anti-Markovnikov addition is always the main product. The isomeric branched aldehyde may arise from an alternative alkene insertion step to produce the [RCH(Me)Co(CO)3] or [RCH(Me)Rh(CO)(PPh3)2] complexes, which are isomeric to 2 and 8, respectively. Alternatively, hydroformylation of isomerized internal alkenes also give branched aldehydes. The ratio of the linear and branched aldehydes, called linearity, may be affected by reaction conditions, and it strongly depends on the catalyst used. Unmodified cobalt and rhodium carbonyls yield about 3-5 1 mixtures of the normal and iso products. 

A particularly interesting problem is to develop catalysts that exhibit satisfactory activity in the hydroformylation of internal alkenes and produce linear aldehydes preferentially. The transformation of a technical mixture of octenes containing all isomers into octanal is of practical importance ... 

This becomes especially apparent in hydroformylation reactions of internal alkenes, since not only does (E)/(Z)-isomerization take place, but -aldehydes are obtained. Thus, in the hydroformylation of ( )-4-octene by Co2(CO)g, n-nonanal (78%), 2-methyloctanal (10%), 2-ethylheptanal (6%) and 2-pro-pylhexanal (6%) are obtained. This isomerization is supressed with the phosphine-modified catalysts, in the presence of excess phosphine and at high CO pressures. Both carbon monoxide and phosphine can react with a 16-electron complex to provide an 18-electron complex (e.g. 4 — 5 Scheme 2), the reverse (3-hydride elimination is prevented, a requirement for this elimination being the presence of a vacant co-... 

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. 

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