金属有机化学第九章

H Ph3P Ph3P Rh CO CO Ph3P OC
H Rh PPh3 CO
H Ph3P Ph3P OC Rh CO CO Ph3P
H Rh CO
PPh3
O Ph3P
- CO H
R O
H H2
R
Rh PPh3 CO
CO R
Ph3P Rh PPh3 CO
O R H R
Ph3P Ph3P
Rh CO + CO
(Ph3P)2Rh(CO)H H2, CO molten PPh3 100 C, 50 atm O H
The Rh-system is sufficiently more reactive than the Co-systems to offset the dramatic price difference ($Rh:$Co = ca. 300:1).
Ph3P H Rh CO
R
R Ph3P Rh PPh3 CO
R Ph3P Rh PPh3 CO
Ph3P
The key Rh complex is a 5-coordinate 18 electron species. Due to the high trans-effect ligands (CO, H), the large phosphines occupy cis coordination sites. Both phosphines can be equatorial, or one can occupy an axial position.
• Steric: Substituting PBu3 for CO increases the steric bulk of the metal center, which would be expected to favor formation of a primary Co-alkyl. • Electronic: Substituting PBu3 for CO significantly decreases the acidity of the cobalt-hydride. Therefore, there is a lower preference for Markovnikov-type addition of H-Co to the alkene.
R
+ H2 + CO
Co2(CO)8 R
CHO +
CHO R
Catalytic cycle: Although the exact nature of some steps have not been conclusively determined, the catalytic cycle is as follows.
Dissociative substitution of alkene for CO proceeds through a 4-coordinate, square planar intermediate. These catalyst have very high linear:branched ratios. Alkene migratory insertion largely determines the regioisomeric ratio, because the square planar Rh-alkyl is rapidly trapped by CO in an irreversible reaction under the reaction conditions. For a primary Rh-alkyl this is followed by CO migratory insertion and then hydrogen cleavage. Secondary Rh-alkyls undergo CO insertion slowly, so alkene isomerization products are also observed.
The generally accepted mechanism for the Rh systems is similar to that for the Co catalysts. It was first proposed by Sir Geoffrey Wilkinson in 1968. (J. Chem. Soc. (A) 1968, 3133-3142)
The Shell phosphine-modified catalyst is slower than HCo(CO)4, so higher temperatures are required for acceptable rates. The phosphinemodified catalyst is more stable though, so it can run at lower CO pressures. 2. Rhodium-based hydroformylation catalysts Simple rhodium carbonyls are inactive for the hydroformylation reaction due to the formation of stable, catalytically inactive Rh-carbonyl clusters occurs. HRh(CO)4 can be formed under high H2 and CO pressures to give a very active catalyst, but it also isomerizes alkenes and gives low linear:branched ratios.
Improved linear to branched ratios are obtained using chelating phosphines with large bite angles. Texas-Eastman has commercialized hydroformylation processes using ligands such as BISBI that give linear:branched ratios of 96:4 under mild conditions and low L:Rh ratios. (U.S. patents 4,694,109 (1987) and 4,879,416 (1989)) BISBI (ß = N 113°) gives a much more regioselective catalyst than dppe (ß = 86 °). N
O R Co(CO)4 HCo(CO)4 - CO R O R H O Co(CO)3
both mechanisms has been observed. HCo(CO)4 reacts 20-30 X faster with the Co-acyl than H2 does at room temperature (Organometallics, 1986, 5, 209-215). Under the catalytic conditions, the H2 concentration is about 2 orders of magnitude larger than that of HCo(CO)4.
Chapter 9
Hydroformylation and Related Alkene Carbonylations
1. Cobalt-Based Hydroformylation Catalysts
Hydroformylation or the oxo process was originally developed by Roelen in the late 1930’s. The original catalyst was Co2(CO)8, which is still widely used. In the hydroformylation reaction, the elements of formaldehyde (H and CHO) are added across a double bond to give an aldehyde. Both linear and branched products can be produced, although the more desirable linear aldehydes are usually the major products when using aliphatic olefins. Depending on the catalyst and conditions, the aldehydes can be directly reduced to alcohols during the reaction.
Addition of phosphines to these rhodium systems dramatically improves both the chemo- and regioselectivity, while maintaining high reaction rates of the initial Rh-carbonyls. This system has been commercialized by Union Carbide.
Regioselectivity: Generally the linear isomer is more valuable industrially than the branched isomer. Since the catalytic cycle is irreversible, the observed regioselectivity represents the kinetic product ratio rather than the thermodynamic preference.