Aldehydes hydrogenation, homogeneous catalysis

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. 

The reaction between alkenes and synthesis gas (syngas), an equimolar mixture of carbon monoxide and hydrogen, to form aldehydes was discovered in 1938 by Otto Roelen [1,2]. Originally called oxo-reaction , hydroformyla-tion is the term used today. This reflects the formal addition of formaldehyde to the olefinic double bond. Commercially, homogeneous metal complexes based on cobalt and rhodium are used as catalysts. With more than 10 million metric tons of oxo products per year, this reaction represents the most important use of homogeneous catalysis in the chemical industry. 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

An important modern example of homogeneous catalysis is provided by the Monsanto process in which the rhodium compound 1.4 catalyses a reaction, resulting in the addition of carbon monoxide to methanol to form ethanoic acid (acetic acid). Another well-known process is hydro-formylation, in which the reaction of carbon monoxide and hydrogen with an alkene, RCH=CH2, forms an aldehyde, RCH2CH2CHO. Certain cobalt or rhodium compounds are effective catalysts for this reaction. In addition to catalytic applications, non-catalytic stoichiometric reactions of transition elements now play a major role in the production of fine organic chemicals and pharmaceuticals. 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

Hydroformylation is an industrially important carbonylation process that is used in the synthesis of aldehydes from olefins, CO and molecular hydrogen by homogeneous catalysis. Efficient catalyst recovery is crucial for the process to be economically viable. Pioneering work in fluorous chemistry was published in 1994 by Horvdth and Rabai, who studied the fluorous hydroformylation of olefins catalyzed by an Rh complex with a fluorous phosphine as a ligand. ... 

Oxo synthesis, or more formally hydroformylation, is an olefin/CO coupling reaction which in the presence of hydrogen leads to the next higher aldehyde. The process was discovered in 1938 by Otto Roelen at Ruhrchemie, where it was first commercialized [4]. This reaction is the most important industrial homogeneous catalysis in terms of both scale and value. The most important olefin starting material is propene, which is mainly converted to 1-butanol and 2-ethylhexanol via the initial product butyraldehyde (Eq. 3-1). 

The shift from coal to oil, of course, had major consequences for the chemical industry. Alkenes, instead of synthesis gas and acetylene, became the basic building blocks. Homogeneous catalysis became increasingly important for the production of chemicals. We already mentioned the Reppe chemistry, based on acetylene as the main feedstock. In 1938, the same year that Reppe started his work, another German scientist, Otto Roelen, discovered the reaction of olefins with carbon monoxide and hydrogen over a cobalt carbonyl catalyst to form aldehydes, known... 

The prototypical C-C bond forming hydrogenation, hydroformylation combines basic feedstocks (a-olefins, carbon monoxide, and hydrogen) with perfect atom economy and accounts for the production of over 10 million metric tons of aldehyde annually, making it the largest volume application of homogeneous metal catalysis. 

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