4571-chapt16-hydroformylation

4571-chapt16-hydroformylation - Hydroformylation 1...

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Hydroformylation 1 Hydroformylation (Oxo) Catalysis + CO + H 2 H O + O H linear (normal) branched (iso) Rh or Co R R R Aldehydes R alkene isomerization alkene hydrogenation R side reactions * * Largest homogeneous catalytic process * > 15 billion pounds of aldehydes (alcohols) per year * Commercial catalysts are complexes of Co or Rh * Selectivity to linear (normal) or branched (iso) products is important Hydroformylation was discovered by Otto Roelen in 1938 during an investigation of the origin of oxygenated products occurring in cobalt catalyzed Fischer-Tropsch reactions. Roelen's observation that ethylene, H 2 and CO were converted into propanal, and at higher pressures, diethyl ketone, marked the beginning of hydroformylation. Cobalt catalysts completely dominated industrial hydroformylation until the early 1970's when rhodium catalysts were commercialized. In 2004, ~75% of all hydroformylation processes are based on rhodium triarylphosphine catalysts, which excel with C 8 or lower alkenes and where high regioselectivity to linear aldehydes is critical. Most aldehydes produced are hydrogenated to alcohols or oxidized to carboxylic acids. Esterfication of the alcohols with phthalic anhydride produces dialkyl phthalate plasticizers that are primarily used for polyvinyl chloride plastics -- the Otto Roelen (1897-1993)
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Hydroformylation 2 largest single end-use. Detergents and surfactants make up the next largest category, followed by solvents, lubricants and chemical intermediates. HCo(CO) 4 Catalyst. Roelen's original research into hydroformylation involved the use of cobalt salts that, under H 2 /CO pressure, produced HCo(CO) 4 as the active catalyst. In 1960 and 1961 Heck and Breslow 1,2 proposed what is now accepted as the general mechanism for hydroformylation: C Co C C H C R - CO + alkene C Co C C C R + CO O R + H 2 - CO Co CO CO C O O R H H H O R C Co C C C C Co C C C Monometallic Bimetallic O O O O Co C C H C O O O + CO O O O O C Co C C C O O O O C Co C C H C O O O O O O O O O O O O Rate Determining Step anti-Markovnikov hydride addition to C=C bond to give linear alkyl increasing the CO pressure keeps the back reactions from occuring - this limits alkene isomerization and the corresponding opportunity for making branched alkyl 3 atm CO = 1.6:1 L:B ratio 90 atm CO = 4.4:1 L:B ratio proposed bimetallic pathway - NOT important in normal catalysis An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow. The acyl intermediate could react with HCo(CO) 4 to do an inter molecular hydride transfer, followed by reductive elimination of aldehyde producing the Co-Co bonded dimer Co 2 (CO) 8 . A common starting material for HCo(CO) 4 catalyzed hydroformylation, Co 2 (CO) 8 is well known to react with H 2 under catalysis reaction conditions to form two equivalents of HCo(CO) 4 . The Richard Heck (b. 1931)
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Hydroformylation 3 bimetallic hydride transfer mechanism is operational for stoichiometric hydroformylation with HCo(CO) 4 and has been proposed to be a possibility for slower catalytic hydroformylation reactions with internal alkenes.
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4571-chapt16-hydroformylation - Hydroformylation 1...

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