第六章氢甲酰化反应Outline:A.Introduction and Cobalt catalystB.Cobalt Phosphine-Modified CatalystsC.Rhodium Phosphine Catalysts

D. Aqueous-Phase Rh HydroformylationE. Asymmetric Hydroformylation

Hydroformylation was discovered by Otto Roelen in 1938.

Roelen's observation that ethylene, H2and CO

were converted into propanal, and at higher pressures, diethyl ketone, marked the beginning of hydroformylation.

A. Introduction and Cobalt catalyst

Three commercial processes1. Cobalt salt2. Cobalt salt/phosphine ligand (Shell)3. Rhodium salt/phosphine ligand(Ruhrchemie/Rhone-Poulenc, Mitsubishi-Kasei, Union Carbide, and Celanese)

Both linear and branched products can be produced, although the more desirable linear aldehydesare usually the major products when using aliphatic olefins.

Hydroformylation: Thermodynamics

Thermodynamics at standard conditions:At higher temperatures the entropy loss becomes more important: ΔG will be less negative.

The TD favored product is the alkane. The obtained product is the aldehyde because of kinetic control.Mechanism of the Co-catalyzed hydroformylationAll steps are reversible, except product formation,which is often rate-determining.

Preferential formation of thelinear product is due to stericinteractions.Trapping of the alkyl by CO determines the regioselectivity.β-hydride elimination is suppressed due to the high CO pressure.

Hydrogenolysis is the rate-determining step for simple terminal olefins.

HCo(CO)4Catalyst

※The first step is a dissociative substitution of alkene for CO. ※The second step:Migratory insertion can result in either a primary or secondary metal alkyl. Although this is the step that sets the regiochemistry of the products, it is rapidly reversible. This rapid reversibility results in alkene isomerization and H/D exchange. ★Since ß-H elimination requires an open coordination site, isomerization and isotope exchange are inhibited by increased CO pressures.

※The next step ( third)is a second migratory insertion to form the coordinatively unsaturated acyl that can coordinate another CO to give the 18 electron acyl complex. Under standard catalytic conditions with 1-octene, this is the only species observed by IR.

The mechanism of the hydrogenolysis step is less clear.

An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow.

The inverse dependence on CO pressure is consistent with the mechanistic requirement for CO dissociation from the various saturated 18e species to open up a coordination site for alkene or H2binding.

Hydroformylation: Kinetic studiesWhen using a 1:1 ratio of H2/CO, the reaction rate is essentially independent of pressure due to the opposing orders of H2and CO..Increasing the H2/CO ratio is of limited use for increasing the overall reaction rate because HCo(CO)4is only stable under certain minimum CO

partial pressures at a given temperature

Increasing the CO partial pressure decreases the

hydroformylation reaction rate and the amount of alkene isomerization side reactions, while increasing the aldehyde linear to branched product ratio.

Why? ?

Pino proposed that the apparent marked differencebetween HCo(CO)4 catalyzed hydroformylation at low and high CO partial pressures was due to the existence of two active catalyst species, HCo(CO)4and HCo(CO)3, formed

from the CO association/dissociation equilibrium:

16e-HCo(CO)318e-saturated HCo(CO)

4

Regioselectivity:

Under lower CO partial pressures an unsaturated 16e-RCo(CO)3will have a long enough lifetime to allow reverse β-hydride eliminationand increase the possibility for alkene reinsertionto the branched alkyl species, which is slightly more favored thermodynamically.The regioselectivity of HCo(CO)4(or HCo(CO)3) for producing the more valuable linear aldehydes varieswith reaction conditions and alkene substrates used.

◆High CO partial pressure slows the rate of catalysis, but increases the linear to branched aldehyde product ratio. ◆Higher CO partial pressures also lower alkene isomerization side reactions. ◆Higher temperatures increase the reaction rate, but lower the linear aldehyde product regioselectivity and increase various undesirable side reactions.

Side reactions of the product aldehydes to form heavier productsgenerally occur, particularly at higher reaction temperatures, and usually account for ~ 9% of the product distribution.

The regioselectivity can be modifiedby adding phosphineto the Co-catalyzed hydroformylation system.This advance was discovered at Shell chemical. Using HCo(CO)3PBu3as catalyst.