Rhodium aldehyde hydrogenation

Optically active aldehydes are important precursors for biologically active compounds, and much effort has been applied to their asymmetric synthesis. Asymmetric hydroformylation has attracted much attention as a potential route to enantiomerically pure aldehyde because this method starts from inexpensive olefins and synthesis gas (CO/H2). Although rhodium-catalyzed hydrogenation has been one of the most important applications of homogeneous catalysis in industry, rhodium-mediated hydroformylation has also been extensively studied as a route to aldehydes. 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

Data are presented to identify some of the important factors in aldehyde hydrogenation and to characterize rhodium carbonyl chemistry under hydroformylation conditions. Comparison is made of the effects of monomeric and of polymeric amines, and a possible reaction mechanism is examined in the light of the data. 

The effect of DMBA and polyDMBA on the hydroformylation of 1-hexene is shown in Table I. Product distributions are reported at 80% total olefin conversion for various rhodium sources and olefin concentrations. As shown, olefin isomerization is complete in all cases. In the presence of 2M DMBA, significant aldehyde hydrogenation occurred producing nearly 30% alcohol. With the exception of RhCl3 3H20 which is inactive, no differences were found between various rhodium sources. 

In our experiments, no CO inhibition was observed, instead the rhodium-catalyzed hydrogenation of aldehydes in the presence of amines showed a first-order dependence on both CO and H2 (Table V). The absence of any significant inhibition by CO is attributed to lower operating temperatures and lower H2 pressures feasible in the presence of amine, relative to the earlier reports (16). Maxima in the rates are reported only at CO partial pressures exceeding 1000 psig (11, 16) and were neither expected nor observed in this investigation. 

The kinetics of aldehyde hydrogenation over cobalt and rhodium carbonyl have been examined by Heil and Marko (16, 19) who proposed Mechanism 2. 

In the hydroformylation of olefins over rhodium carbonyl catalysts, aldehyde hydrogenation was noted under mild conditions in the absence of amine. Rates of hydrogenation passed through a maximum with increasing amine basicity and concentration. 

Unlike cobalt, the rhodium catalyst exhibits little or no aldehyde hydrogenation activity, but it is 10 -10 limes more active than the cobalt analog. The rhodium system is also a highly active isomerization catalysts but gives rise to a low n iso ratio (about I compared to 4 of Co2(CO) ). 

Palladium is usually the prefeired metal of choice for aromatic aldehyde hydrogenation in neutral non-polar solvents such as hexane, DMF, or ethyl acetate (5-100 °C and 1-10 bar) although ruthenium, which is less active, can be considered and run in aqueous alcohol at similar temperatures and pressures. If higher pressures are accessible ruthenium may be preferable because of its lower (historical) cost. Its use has recently been reviewed [4]. Although platinum and rhodium could... 

A recent paper describing the use of these anchored complexes for aldehyde hydrogenation stated that cationic complexes were required for the formation of the anchored species and that this anchored catalyst was, essentially, an ion pair comprised of a rhodium cationic species and an oxygen... 

Scheme 2 Rhodium catalyzed hydrogenation of aldehydes using a pyrrolo-acylguanidinium containing a phosphine ligand.

According to Heil and Marko [328], aldehyde hydrogenation begins to occur at CO partial pressures over 100 atm with rhodium chloride. Rhodium chloride normally requires much more severe conditions to form an active hydroformylation catalyst than rhodium oxide or rhodium carbonyl does [329]. 

Jung, C.-K., Krische, M. J. (2006). Asymmetric induction in hydrogen-mediated reductive aldol additions to a-amino aldehydes catalyzed by rhodium Iniramoleeular hydrogen-bonding. Jonmal of the American Chemical Society, 128,17051-17056. 

Process Technology. In a typical oxo process, primary alcohols are produced from monoolefins in two steps. In the first stage, the olefin, hydrogen, and carbon monoxide [630-08-0] react in the presence of a cobalt or rhodium catalyst to form aldehydes, which are hydrogenated in the second step to the alcohols. 

Often the aldehyde is hydrogenated to the corresponding alcohol. In general, addition of carbon monoxide to a substrate is referred to as carbonylation, but when the substrate is an olefin it is also known as hydroformylation. The eady work on the 0x0 synthesis was done with cobalt hydrocarbonyl complexes, but in 1976 a low pressure rhodium-cataly2ed process was commerciali2ed that gave greater selectivity to linear aldehydes and fewer coproducts. 

The hydroformylation reaction is carried out in the Hquid phase using a metal carbonyl catalyst such as HCo(CO)4 (36), HCo(CO)2[P( -C4H2)] (37), or HRh(CO)2[P(CgH3)2]2 (38,39). The phosphine-substituted rhodium compound is the catalyst of choice for new commercial plants that can operate at 353—383 K and 0.7—2 MPa (7—20 atm) (39). The differences among the catalysts are found in their intrinsic activity, their selectivity to straight-chain product, their abiHty to isomerize the olefin feedstock and hydrogenate the product aldehyde to alcohol, and the ease with which they are separated from the reaction medium (36). 

Dialdehydes 8 have been converted to y-lactones 9 in the presence of a rhodium phosphine complex as catalyst. The example shown below demonstrates that this reaction works also with aldehydes that contain a-hydrogen atoms. 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

---