Phosphine-Modified Cobalt Catalysts

Tucci (54), studying mainly terminal olefins, cited two reasons for the high selectivity for linear products in the phosphine-modified cobalt catalysts (a) stereoselective addition of the hydride species to the olefinic double bond, and (b) inhibition of olefin isomerization. However, the results obtained with internal olefins as substrate tended to discount the likelihood of the second reason, and it is generally accepted that selective anti-Markovnikov addition arising from steric hindrance is the principal cause for linear products from nonfunctional olefins. 

Phosphine modified cobalt catalysts the Shell process... 

Na5[Co+(CO)3(19)2]5 was used as catalyst for the hydroformylation of 1-hexene and 1-octene in a two phase system without leaching of cobalt into the organic phase.122 The products obtained were almost exclusively aldehydes (4-38%) and very little (0.4-3%) or no alcohol formation122 in contrast with cobalt/phosphine catalysed hydroformylation in organic solvents which give alcohols. The n/i ratios of the aldehydes were low (1.1-2.5),122 however, and never approached that expected for a phosphine modified cobalt catalyst in non-aqueous media324,325,393 (see Table 8). 

Since its discovery some 50 years ago by Roelen, a great deal of research has been carried out on the reaction and its industrial importance is great. The initial work used as catalyst precursor [Co2(CO)8] or simple cobalt salts which were carbonylated under the reaction conditions. Subsequently, phosphine-modified cobalt catalysts were introduced and, more recently, rhodium and platinum catalysts. Only the cobalt and rhodium catalysts have found industrial use to date. 

In the unmodified catalyst system (Scheme 1), the rate shows a first-order dependence on hydrogen pressure and an inverse first-order dependence on carbon monoxide pressure, so that the rate is nearly independent of total pressure. The reaction is first order in alkene and first order in cobalt at higher CO pressures. With phosphine-modified cobalt catalysts, the rate-determining step depends on the ligand and the alkene. 

According to the mechanism described above, phosphine-modified cobalt catalysts Co2(CO)6(L)2 behave in the same way. It is generally accepted that the selective antCMarkovnikov addition of the hydridocobalt carbonyl to the olefin forced by steric hindrance determines the n/i ratio. 

Compared with cobalt carbonyl, the phosphine-modified cobalt catalyst introduced by Shell in 1966 leads to an increase of selectivity toward linear products, to an increase in the thermal stability and hydrogenation activity, but also to a lower reactivity. In order to compensate for the lower activity, reaction temperatures have to be kept at about 180 °C. With higher temperatures the n/i selectivity drops [130] as less coordinated cobalt species are involved in the catalytic cycle. The reduced steric demand around the metal center leads to increased formation of branched aldehydes. With respect to formation of by-products, modified cobalt catalysts behave similarly to their unmodified derivatives. 

Phosphine-modified cobalt catalyst is applied commercially only in the Shell process to hydroformylate olefins of medium chain length (C7-C14). The resulting alcohols are sold under the brand name Dobanol . 

Unmodified cobalt catalyst Phosphine modified cobalt catalyst Phosphine modified rhodium catalyst... 

Phosphine modified cobalt catalysts permit the hydroformylation reaction to operate at lower pressure and produce a higher proportion of the normal isomer. Pressure is typically about 35 bars (500 psig) and the nor-mal/iso ratio is between 6 and 7. In the 1970s, Union Carbide in conjunction with Johnson Matthey and Davy McKee developed and improved oxo process based on a rhodium catalyst, modified with a triphenylphosphine (TPP) lipnd. 

In the 1960s, Shell commercialized a one-step process using a phosphine modified cobalt catalyst. This process operates at a much lower pressure, provides a high selectivity for the normal aldehyde isomer, and carries out the hydroformylation and hydrogenation sequentially without isolating the aldehyde. 

In the case of phosphine-modified cobalt catalysts, Ugand effects have been thoroughly investigated. The influence of ligand basicity can be represented by the following equihbrium reaction (Eq. 2-100). 

The Aldox (aldolization-Hoxo reaction) process of Esso and Shell describes a one-pot transformation (Scheme 5.127) running with a phosphine-modified cobalt catalyst in the presence of additives such as Zn decanoate [13]. High yields... 

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

It should be recognized that the stability of cobalt complexes under carbon monoxide can be enhanced by the addition of ligands, as is the case for phosphine-modified cobalt hydroformylation catalysts (57, 58). The stability will also probably depend on properties of the solvent employed. Nevertheless, the plot shown in Fig. 4 appears to be quite useful for assessing long-term cobalt stability under H2/CO in the absence of strongly coordinating solvents or ligands. 

In 1961 Heck and Breslow presented a multistep reaction pathway to interpret basic observations in the cobalt-catalyzed hydroformylation.28 Later modifications and refinements aimed at including alternative routes and interpreting side reactions.6 Although not all the fine details of hydroformylation are equally well understood, the Heck-Breslow mechanism is still the generally accepted basic mechanism of hydroformylation.6,17,19,29 Whereas differences in mechanisms using different metal catalysts do exist,30 all basic steps are essentially the same in the phosphine-modified cobalt- and rhodium-catalyzed transformations as well. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

As mentioned above, most published papers and patent applications deal with ligands and their influence on activity and selectivity in the oxo synthesis. When alkylphosphine-modified cobalt catalysts were introduced by Shell [135], high n/i selectivities were reported. However, coordination of phosphine ligands makes the metal-hydrogen bond more hydridic, therefore leading to substantial formation of hydrogenation products. Hydroformylation of 1-pentene yielded hex-anol with 91 % linearity. 

Table 9.8 gives economic data about Oxo synthesis on propylene for processes using cobalt tetracarbonyl hydride phosphine-modified cobalt and phosphine-modified rhodium catalysts. 

A relatively new approach involves the functionalization of phosphines or other ligands with weakly basic or acidic functionalities. An early study reports on a modified cobalt catalyst [12]. This cobalt carbonyl complex containing the P(CH2CH2NEt2)3 ligand was claimed to be extractable into dilute carbonic acid and could be re-extracted into an organic phase by simply reducing the C02 pressure. 

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