Cyclohexenes hydroformylation

Example 11.1. Hydroformylation of cyclohexene with phosphine-substituted cobalt hydrocarbonyl catalyst. The most probable network of cyclohexene hydroformylation catalyzed by a phosphine-substituted cobalt hydrocarbonyl is shown on the facing page. HCo(CO)3Ph (cat) is in equilibrium with the CO-deficient HCo(CO)2Ph (cat ) and CO. For greater generality, quasi-equilibrium of these species with the 7r-complex, X, is not assumed. Actual hydroformylation olefin — aldehyde proceeds via a Heck-Breslow pathway (cycle 6.9 that includes the trihydride, X2) but without... 

Industrial practice often confronts the development engineer with networks that are considerably more complicated than that of cyclohexene hydroformylation in the example above. Additional simplifications may then be desirable or necessary in order to arrive at a model that remains manageable in the highly iterative applications called for in reactor design and optimization and possibly on-line process control. A useful and usually successful way of achieving such streamlining is to place all network nodes at end members or non-trace intermediates, ignoring the fact that some of them may be at trace-level intermediates [10]. 

Scheme 1. Catalytic cycle of the cobalt-catalyzed cyclohexene hydroformylation reaction.

Figure 1. Enhancement of the reaction rate of the cobalt-catalyzed cyclohexene hydroformylation by addition of Ru3(CO),2 (data from [20]).

For Rh4(CO)12 as hydroformylation catalyst, several mechanistic studies including isotope labeling and kinetics have been undertaken. Thus, the deuteroformylation of 2,3,3-trimethylbut-l-ene and of styrene in the presence of Rh4(CO),2 have been studied In the first case, the 3,4,4-trimethyl-pentanal formed was found to be deuterated exclusively in the formyl position (238). In the latter case, the aldehydes formed were deuterated in the formyl and in the corresponding a-position ( > and (Zy PhCH=CHD and PhCH=CD2 were also observed (239). The mechanistic implications of these findings are not entirely clear. However, a kinetic study of the Rh4(CO)12-catalyzed low-pressure cyclohexene hydroformylation provides some evidence in favor of intact cluster catalysis A mechanism proposed on the basis of these findings includes fission of one Rh-Rh bond, whereby the tetranuclear cluster framework remains intact (Scheme 12) (240). 

From 1974 onwards the scope of different reactions using biphasic catalyst systems, preferably with precious metals, was tested in laboratory-scale experiments. Among these were butadiene hydrodimerization, hydrogenation of acrylonitrile or cyclohexene, hydroformylation of propene, and some other conversions to fine... 

According to these calculations, the rate of acylcobalt reduction at 80°C and 9.5 MPa (C0 H2 = 1 1) pressure by dihydrogen was found to be 37 times and 8 times faster in 1-octene and in cyclohexene hydroformylation, respectively, than by HCo(CO)4. 

The initial rate of Co2(CO)g-catalyzed cyclohexene hydroformylation, triethyl orthoformate carbonylation, and CoH(CO)4 formation from Co2(CO)g and H2 is reduced by the addition of dinitrogen, argon, or xenon. It is assumed that the additional gas competes with one or more reactants for a coordinatively unsaturated site responsible for their activation, thus affecting the reaction rate [1]. 

In addition to rhodium(III) oxide, cobalt(II) acetylacetonate or dicobalt octacarbonyl has been used by the submitters as catalyst precursors for the hydroformylation of cyclohexene. The results are given in Table I. 

A recent example where Co2(CO)8 serves as a precatalyst is in the preparation of linear and branched aldehydes via propylene hydroformylation in supercritical C02 (93-186 bar 66-108 °C). Cyclohexane carbaldehyde is produced from cyclohexene using Co2(CO)8 and an acid RCOOH, or else is successful with another established Co catalyst, Co(OOCR)2, assumed to form in situ in the former case. Oligomerization of aldehydes such as n-butanal is achieved with Co2(CO)6L2 as catalyst (L = CO, PR3).1364... 

The formation of multinuclear clusters is much more favorable for rhodium than for cobalt. Additional evidence was obtained in comparative hydroformylation rate studies of 1-heptene and of cyclohexene at 75°C and 150 atm 1/1 H2/CO (19). For the acyclic olefin the kinetics followed the kinetic expression (except at low olefin) ... 

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). 

The experiments in Tables I and II have dealt with the reactions of 1-hexene only. It is known that hydroformylation rates for various olefins are in the order, 1-hexene > 2-hexene > cyclohexene (15). Little dependence on structure would be expected in the aldehyde hydrogenation step, however. 

The hydroformylation of alkenes generally has been considered to be an industrial reaction unavailable to a laboratory scale process. Usually bench chemists are neither willing nor able to carry out such a reaction, particularly at the high pressures (200 bar) necessary for the hydrocarbonylation reactions utilizing a cobalt catalyst. (Most of the previous literature reports pressures in atmospheres or pounds per square inch. All pressures in this chapter are reported in bars (SI) the relationship is 14.696 p.s.i. = 1 atm = 101 325 Pa = 1.013 25 bar.) However, hydroformylation reactions with rhodium require much lower pressures and related carbonylation reactions can be carried out at 1-10 bar. Furthermore, pressure equipment is available from a variety of suppliers and costs less than a routine IR instrument. Provided a suitable pressure room is available, even the high pressure reactions can be carried out safely and easily. The hydroformylation of cyclohexene to cyclohexanecarbaldehyde using a rhodium catalyst is an Organic Syntheses preparation (see Section 4.5.2.5). 

Solvent effects on the rate of hydroformylation have been found to be small, however. A small increase in the rate of hydroformylation of cyclohexene has been found in the series methanol > benzene > heptane but the overall increase is only by a factor of 1 5 (158). Alcohols have been reported to increase the yield of hydrogenated products (66, 88). 

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). 

Among the hydroformylated l-methyl-4-[alpha-alkoxy-isopropyl]-l-cyclohexenes, i.e. 8-methoxy-p-menthan-2-carboxyaldehyde, the methyl ether 36 (Eq. 15.3.4) smells mangofruit-like and might be used as a perfumery material. The ethyl derivative has a green citrous fragrance, the propyl one has a fresh grass flavour and the butyl one smells woody. 

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