Catalysis homogeneous hydrogenation with rhodium

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. 

Heterogeneous catalysis (Pd) in tritiation of l,4-dien-3-ones leads mainly to l) -tritiation (ca. 76%). Tritium distribution studies indicate that the reaction is probably a 1,4-addition of tritium onto the more exposed p-face of the dienone, which is tilted downwards with respect to ring b (211). Homogeneous catalysis with tris(triphenylphosphine)rhodium chloride, however, is known to favour la-tritiation. Similarly, homogeneous hydrogenation of the less reactive... 

An important milestone in the history of asymmetric homogeneous catalysis was the development of asymmetric homogeneous hydrogenation catalysts based on rhodium complexes with chiral phosphine ligands. Scientists at Monsanto, over a period of years, developed catalysts for the production on an industrial scale of L-Dopa [31-33]. The key step is the asymmetric hydrogenation of an enamide (Scheme 7.6). 

Tris-m-sulfonatophenylphosphine (tppts) plays an important role in the history of homogeneous catalysis [39], mainly due to its use in the Ruhrchemie/Rhone-Poulenc hydroformylation process [40], now operated by Celanese (see 1.2 and Chapter 7). It is also used in a number of fine chemical processes, such as selective hydrogenation with ruthenium [41], carbon-carbon bond formation with rhodium [42], and the Heck reaction [43]. Monosidfonated triphenylphosphine (tppms) is used for the preparation ofnonadienol [44] (see Figure 5). 

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. 

Elimination. Two kinds of elimination reaction are important in homogeneous catalysis. A catalytic cycle which has involved an oxidative addition generally ends with the reverse process of reductive elimination (equation 6). Rhodium-catalyzed hydrogenations end with this step. 

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