Inchemistry,carbonylation refers toreactions that introducecarbon monoxide (CO) intoorganic andinorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry.[1] The term carbonylation also refers tooxidation of proteinside chains.
Several industrially useful organic chemicals are prepared by carbonylations, which can be highly selective reactions. Carbonylations produce organiccarbonyls, i.e., compounds that contain theC=Ofunctional group such asaldehydes (R−CH=O),carboxylic acids (R−C(=O)OH) andesters (R−C(=O)O−R').[2][3] Carbonylations are the basis of many types of reactions, including hydroformylation and Reppe reactions. These reactions require metal catalysts, which bind and activate the CO.[4] These processes involvetransition metal acyl complexes as intermediates. Much of this theme was developed byWalter Reppe.
Hydroformylation entails the addition of both carbon monoxide andhydrogen to unsaturated organic compounds, usuallyalkenes. The usual products are aldehydes:
The reaction requires metalcatalysts that bind CO, forming intermediatemetal carbonyls. Many of the commodity carboxylic acids, i.e. propionic, butyric, valeric, etc, as well as many of the commodity alcohols, i.e. propanol, butanol, amyl alcohol, are derived from aldehydes produced by hydroformylation. In this way, hydroformylation is a gateway from alkenes to oxygenates.
Few organic carbonyls undergo spontaneousdecarbonylation, but many can be induced to do so with appropriate catalysts. A common transformation involves the conversion of aldehydes to alkanes, usually catalyzed by metal complexes:[5]
Few catalysts are highly active or exhibit broad scope.[6]
Large-scale applications of carbonylation are theMonsanto acetic acid process andCativa process, which convertmethanol toacetic acid. In another major industrial process,acetic anhydride is prepared by a related carbonylation ofmethyl acetate.[7]
Dimethyl carbonate anddimethyl oxalate are produced industrially using carbon monoxide and an oxidant, in effect as a source ofCO2+:[2]
Theoxidative carbonylation of methanol is catalyzed by copper(I) salts, which form transient carbonyl complexes. For the oxidative carbonylation of alkenes, palladium complexes are used.
Inhydrocarboxylation, alkenes andalkynes are the substrates. This method is used to producepropionic acid from ethylene usingnickel carbonyl as the catalyst:[2]
The above reaction is also referred to ashydroxycarbonylation, in which case hydrocarboxylation refers to the same net converstion but usingcarbon dioxide in place of CO andH2 in place of water:[8]
Acrylic acid was once mainly prepared by the hydrocarboxylation of acetylene.[9]
Hydroesterification is like hydrocarboxylation, but it uses alcohols in place of water.[10] For example, the carbomethoxylation ofethylene to givemethyl propionate, a precursor tomethyl methacrylate:[11]
The process is catalyzed byHerrmann's catalyst,Pd[C6H4(CH2PBu-t)2]2. Under similar conditions, other Pd-diphosphines catalyze formation ofpolyketones.
TheKoch reaction is a special case of hydrocarboxylation reaction that does not rely on metal catalysts. Instead, the process is catalyzed by strong acids such assulfuric acid or the combination ofphosphoric acid andboron trifluoride. The reaction is less applicable to simple alkene. The industrial synthesis ofglycolic acid is achieved in this way:[12]
The conversion ofisobutene topivalic acid is also illustrative:
Alkyl, benzyl, vinyl, aryl, and allyl halides can also be carbonylated in the presence carbon monoxide and suitable catalysts such asmanganese,iron, ornickel powders.[13]
In the industrial synthesis ofibuprofen, abenzylic alcohol is converted to the correspondingarylacetic acid via a Pd-catalyzed carbonylation:[2]
Metal carbonyls, compounds with the formulaM(CO)xLy (M = metal; L = otherligands) are prepared by carbonylation of transition metals. Iron and nickel powder react directly with CO to giveFe(CO)5 andNi(CO)4, respectively. Most other metals form carbonyls less directly, such as from their oxides or halides. Metal carbonyls are widely employed as catalysts in the hydroformylation and Reppe processes discussed above.[14] Inorganic compounds that contain CO ligands can also undergo decarbonylation, often via aphotochemical reaction.
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