Decarbonylation

Decarbonylation of aldehydes is catalysed by Vaska s compound, Ir(CO)Cl(PPh3)2 according to scheme (12.351). 

In the decarbonylation of / am-substituted benzoyl chlorides and phenylacetyl chlorides intermediate acyl complexes, [RhClzCCORXPPha), can be isolated. The kinetics of the rearrangement of these acyl complexes to alkyl complexes and alkyl complexes to rran -tRhClCCOXPPha) have been measured and the results are summarized in Table 3. The entropies of activation for these reactions are small, as 

The rhodium(i) complex RhCl(PPh3)3 catalyses decarbonylation of substituted cyclopropylaldehydes and of allylic alcohols. For the former 

Above ca. 210 °C the rhodium catalyst becomes deactivated rapidly and the optimum temperature ran for the decarbonylation is 200— 210 °C. The activation energy for the process is ca. 30 kcal mol . 

Both reactions deserve attention for the rationalization of unexpected side products observed in hydroformylation reactions, but they also have a large synthetic potential in fine chemistry and for the lab-scale synthesis of complex natural compounds. A special case concerns the decarbonylation of formaldehyde to CO and H2, which is treated separately in Section 3.2. 

The reactions may proceed via a cationic intermediate (87). The reactions provide another source of cyclobutadienes. Heterogeneous Group VIII transition metals deformylate benzilic acid and its derivatives to the corresponding ketones. Solutions of Na2PtCl4 in acetic acid have similarly been shown to deformylate benzilic acid, possibly via a r-complex (88), to benzo-phenone. Benzhydrol does not deformylate under these conditions, but does undergo hydrogen-deuterium exchange.  

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Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

Substituted aroyl- and heteroaroyltrimethylsilanes (acylsilanes) are prepared by the coupling of an aroyl chloride with (Me3Si)2 without decarbonylation, and this chemistry is treated in Section 1.2[629], Under certain conditions, aroyl chlorides react with disilanes after decarbonylation. Thus the reaction of aroyl chlorides with disilane via decarbonylation is a good preparative method for aromatic silicon compounds. As an interesting application, trimel-litic anhydride chloride (764) reacts with dichlorotetramethyidisilane to afford 4-chlorodimethylsilylphthalic anhydride (765), which is converted into 766 and used for polymerization[630]. When the reaction is carried out in a non-polar solvent, biphthalic anhydride (767) is formed[631]. Benzylchlorodimethylsilane (768) is obtained by the coupling of benzyl chloride with dichlorotetramethyl-disilane[632,633]. 

Acyi halides are reactive compounds and react with nucleophiles without a catalyst, but they are activated further by forming the acylpalladium intermediates, which undergo insertion and further transformations. The decarbonyla-tive reaction of acyl chlorides as pseudo-halides to form the aryipalladium is treated in Section 1,1.1.1. The reaction without decarbonylation is treated in this section. 

Various organotin reagents react with acyl and aroyl halides under mild conditions without decarbonylation to give carbonyl compounds[390,39l]. Alkyl- or alkenyltin reagents react with acyl and aroyl chlorides to give ketones[548.733,734]. One example is the preparation of the a,/3-dnsaturated 7-keto esters 860 and 861, carried out under a CO atmosphere[735]. The reaction has been applied intramolecularly to the synthesis of the macrocyclic keto... 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

The decarbonylation-dehydration of the fatty acid 887 catalyzed by PdCl2(Ph3P)2 fO.Ol mol%) was carried out by heating its mixture with acetic-anhydride at 250 C to afford the terminal alkene 888 with high selectivity and high catalyst turnover number (12 370). The reaction may proceed by the oxidative addition of Pd to the mixed anhydride[755]. 

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

From these facts, a mechanism of the Rosenmund reduction has been proposed, in which the formation of the acylpalladium species 893 is the first step of the aldehyde formation and also the decarbonylation, although the Rosenmund reduction proceeds under heterogeneous conditions[744]. 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

Arylsilylation of conjugated dienes to give 88 takes place at 80 °C by the reaction of a diene, disilane, and benzoyl chloride, which undergoes facile decarbonylation at 80°C[83]. 

Decarbonylation of aromatic aldehydes proceeds smoothly[71], Terephthalic acid (86), commercially produced by the oxidation of p-.xylene (85), contains p-formylbenzoic acid (87) as an impurity, which is removed as benzoic acid (88) by Pd-catalyzed decarbonylation at a high temperature. The benzoic acid produced by the decarbonylation can be separated from terephthalic acid (86) based on the solubility difference in water[72]. 

The cyano ketone 89 is converted into the nitrile 90 by heating at 140 C with Pd(Ph3P)4[73,74]. The a-lcetophosphonate 91 is decarbonylated with PdMe2(PMePh2)2 complex to give the phosphonate 92[75]. 

Furan is produced from furfural commercially by decarbonylation loss of carbon monoxide from furfural gives furan direcdy. Tetrahydrofuran (3) is the saturated analogue containing no double bonds. 

Tetrahydrofuran (3) is produced commercially from furfural by decarbonylation followed by hydrogenation it is also produced by several different methods from other raw materials. A complete discussion of tetrahydrofuran is found under Ethers. Polymers of tetrahydrofuran are covered under the general topic. Polyethers. Several other compounds containing the tetrahydrofuran ring, which are most readily produced from furfural, are discussed here. 

Furfural can be oxidized to 2-furoic acid [88-14-2] reduced to 2-furanmethanol [98-00-0] referred to herein as furfuryl alcohol, or converted to furan by decarbonylation over selected catalysts. With concentrated sodium hydroxide, furfural undergoes the Cannizzaro reaction yielding both 2-furfuryl alcohol and sodium 2-furoate [57273-36-6]. 

Uses. Furfural is primarily a chemical feedstock for a number of monomeric compounds and resins. One route produces furan by decarbonylation. Tetrahydrofuran is derived from furan by hydrogenation. Polytetramethylene ether glycol [25190-06-1] is manufactured from tetrahydrofuran by a ring opening polymeri2ation reaction. Another route (hydrogenation) produces furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-methylfuran, and 2-methyltetrahydrofuran. A variety of proprietary synthetic resins are manufactured from furfural and/or furfuryl alcohol. Other... 

As can be seen, most of the furfural produced in this country is consumed as an intermediate for other chemicals. Hydrogenation to furfuryl alcohol is the largest use. Some of the furfuryl alcohol is further hydrogenated to produce tetrahydrofurfuryl alcohol. The next major product is furan, produced by decarbonylation. Furan is a chemical intermediate, most of it is hydrogenated to tetrahydrofuran, which in turn is polymerized to produce polytetramethylene ether glycol (PTMEG). 

Manufacture. Furan is produced commercially by decarbonylation of furfural in the presence of a noble metal catalyst (97—100). Nickel or cobalt catalysts have also been reported (101—103) as weU as noncatalytic pyrolysis at high temperature. Furan can also be prepared by decarboxylation of 2-furoic acid this method is usually considered a laboratory procedure. 

Miscellaneous Methods. Exhaustive evaluation of the decarbonylation of ben2oyl duorides, ArCOF, by Wilkinson s catalyst [14694-95-2] Rh[(CgH )2P]3Cl, to give aryl duorides has estabUshed (81) that previous claims (82) caimot be reproduced. 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Reactions and Uses. The common reactions that a-hydroxy acids undergo such as self- or bimolecular esterification to oligomers or cycHc esters, hydrogenation, oxidation, etc, have been discussed in connection with lactic and hydroxyacetic acid. A reaction that is of value for the synthesis of higher aldehydes is decarbonylation under boiling sulfuric acid with loss of water. Since one carbon atom is lost in the process, the series of reactions may be used for stepwise degradation of a carbon chain. 

A significant use of dkect-process waste is realized by C H -Si bond formation via silylative decarbonylation (Fig. 2) (49,50). A novel route to CgHg—Si bond formation has also been described (eq. 2) (51). 

Other methods for preparing -chlorotoluene include cx-elimination from an organoteUurium(IV) halide (57), paHadium-cataly2ed decarbonylation of 4-methylben2oyl chloride (58), and desulfonylation of -toluenesulfonyl chloride cataly2ed by chlorine (59) or cblorotris(tripbenylpbospbine)rbodium (60). 

Although the thermolytic decarbonylation of 2-pyrone gives only a low yield of furan, an excellent yield of benzo[i]furan is obtained from coumarin 70JOC135). 

Imidazole-4-carbaldehyde, 5-mercapto-1 -phenyl-reactions, 5, 444 Imidazolecarbaldehydes oxidation, 5, 437 Imidazole-2-carbaldehydes condensation reactions, 5, 436 deacylation, 5, 93 decarbonylation, 5, 436 oximes, 5, 436 reactions, 5, 93... 

WILKINSON Cartxinylalion decarbonylation catalyst Rh catalyst lor cartxinylation, decarbonylation, oxygenation, benzyl cleavage... 

The product described here, 4-(4-chlorophenyl)butan-2-one, was previously prepared in the following ways a) by reduction of the corresponding benzalacetone, b) by catalyzed decarbonylation of 4-chlorophenylacetaldehyde by HFeiCO) in the presence of 2,4-pentanedione, - c) by reaction of 4-chlorobenzyl chloride with 2,4-pentanedione under basic catalysis (K2CO3 in EtOH), d) by reaction of 4-chlorobenzyl chloride with ethyl 3-oxobutanoate under basic catalysis (LiOH), - and e) by reaction of 3-(4-chlorophenyl )-propanoic acid with methyl lithium. - ... 

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