THERMAL DECOMPOSITION OF KETENE

Little attention has been paid to the thermal decomposition of ketene. It seems, however, necessary to deal in some detail with this reaction since it shows some fundamental differences compared to the thermal decomposition of the ketones. 

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The thermal decomposition of ketene into carbon monoxide and ethylene is prevented, as far as possible, by the rapid removal of ketene from the hot tube, which is accomplished by the undccomposed acetone vapor. About half the acetone originally used should be collected unchanged as distillate by the vertical condenser. The yield of ketene will fall considerably if less distillate is formed. 

Singlet state carbenes produced in photo- or thermal decomposition of ketene or diazomethane... 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Thermal decomposition of l,2-dihydro-477-pyrrolo[2,l-r-][l,4]benzoxazine-l,2,4-triones 425 yielded 7-(2,4,6-trimethylbenzoyl)-8-[(2,4,6-trimethylphenoxycarbonyl)-6,10-dihydropyrido[2,l-r-][l,4]benzoxazine-6,10-dione by [4+2] cycloaddition of the initially formed ketene 426 and the subsequent rearrangement of 6,8,9,10-tetrahydropyrido[2,l-c][l,4]benzoxazine-6,8,10-trione 427 (Scheme 36) <1999RCB2131>. 

In practice ketene is prepared by the thermal decomposition of acetone (Schmidlin) ... 

The thermal decomposition of ketene1>60 93 142 146 apparently does not occur by the simple unimolecular decomposition CH2CO CH2 + CO. Young146 suggests that CH2 actually results from breakdown of allene, which is an intermediate product. Ketene is known to dimerize readily, even in the gas phase on Pyrex surfaces to form the d-methylene cyclic lactone. 

Thermal decomposition of 2-azidotropone (8) gives a ketene (9) as primary product which cyclizes to o-salicylonitrile or reacts with protonic solvents to give the corresponding carboxylic add derivatives.1 167 The reaction occurs at a quite moderate temperature (60-80°) and is therefore best explained by a concerted mechanism. 

SCHMIDLIN KETENE SYNTHESIS. Formation of kelene by thermal decomposition of acetone over electrically heated wire at 500-750 degrees by a reaction involving radical formation with generation of methane and carbon monoxide. 

Grassie, N., and I. C. McNeill Thermal degradation of polymethacrylo-nitrile. IV. Formation and decomposition of ketene-imine structures. J. Polymer Sci. 33, 171-182 (1958). 

The self condensation of a-carbonyl ketenes also constitutes a type (i) synthesis of 2//-pyran-2-oncs. Cyclophanes 627 are synthesized by the thermal decomposition of bis(4,6-dioxo-l,3-dioxanes) 628 and dimerization of the resulting bis(a-carbonyl ketenes) 629 (Scheme 141) <1999JA8270>. Similarly, thermal decomposition of 4,5-diarylfuran-2,3-diones 630 forms the intermediate a-carbonyl ketenes, which dimerize to afford 3,4,5-aryl-2//-pyran-2-ones (Scheme 142) <2003RJ0103>. 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

Through a stirred suspension of HgO (21.7 g, 100 mmol) and Hg(OAc)2 (31.8 g, 100 mmol) in dry MeOH (217 mL) at rt was gently bubbled ketene (generated by the thermal decomposition of acetone). The reddish suspension turned into a white suspension. After stirring for 12 h, a gray clear soln was obtained... 

The main reaction is followed by a slow pressure increase , which is due to the thermal decomposition of the ketene formed. It was proved that ketene is an intermediate . 

Though radicals react with acetone, chains are not propagated below about 450 °C. On the other hand, at higher temperatures where the thermal decomposition of acetone has been generally studied, the acetonyl radical is unstable and decomposes into ketene and methyl radical. Thus, under such conditions, the reaction is a chain process. 

Allene and its decomposition products increase the rate of ketene consumption and that of CO formation, while they hardly influence the rate of CO2 formation. A similar effect was caused by addition of methylene cyclobutane, which decomposes into allene and ethylene. The decomposition of ketene is not inhibited significantly by the usual chain inhibitors, nor is it initiated by biacetyl. Thus, chains, if any, are very short in the thermal decomposition. 

Derivation By spontaneous polymerization of ketene obtained by thermal decomposition of acetone or from bromoacetylbromide and zinc. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

Formation of 5-methyltricyclo[3.3.0.0 ]oct-6-en-3-one (1) by thermal decomposition of l-diazo-3-(l-methylcyclopenta-2,4-dienyl)propan-2-one succeeded remarkably well. It is worth noting that the Wolff rearrangement leading to [(l-methylcyclopenta 2,4-dienyl)methyl]ketene is of no importance, in contrast to the direct, unsensitized irradiation of the diazo compound (see Section 1.2.1.2.4.2.6.2.). 

The thermal decomposition of the sodium salt of the tosylhydrazone of. -dialkyldisulfanyl-carbonic acid generates bis(organosulfanyl)carbene which is trapped by an enol ether, ketene acetal or enamine (it does not react with cyclohexene),e.g. formation of 2. ... 

In principle, the thermal decomposition of -butoxyacetylenes can provide an efficient route to ketenes by a fragmentation according to equation 48 (Section III.B). However, only silylketenes possess sufficient stability to be isolated as individual compounds. A number of silylketenes 88 have been prepared by Pericas and coauthors by a gentle... 

Additional preparative method. Thermal decomposition of di-acetyltartaric anhydride (Klemenc et al.). The yield is better, but the C3O3 contains ketene, which cannot be completely separated. 

Reactions of the Side-chain of Benzothiophens.—The C n.m.r. chemical shifts in 2-benzo[ j]thenyl carbenium ions have been investigated.The thermal decomposition of 2-azidophenyl 2-(3-methylbenzo[ >]thienyl) sulphide and of 2-azidophenyl 3-(2-methylbenzo[ ]thienyl) sulphide proceeded efficiently through spiro-benzothiazolines to give (161) and (162). 5-Hydroxy-3-methyl-benzo[ ]thiophen-2-carboxylic acid is most conveniently decarboxylated by refluxing with 48% hydrobromic acid. The decomposition of benzo[/)]thiophen-2(3//)-one and of 3-diazobenzo[/ ]thiophen-2-one at high temperatures provided convenient syntheses of benzothiet and the transient benzothiet keten. The decomposition reactions were carried out in the reactor of a photoelectron spectrometer.Heterotriptycenes have been obtained from... 

A unique preparation of ketenes is by thermal decomposition of malonic anhydrides, which form ketenes and carbon dioxide (Scheme 7.47). Ketene, methylketene, and dimethylketene are formed by this methodology. Surprisingly, the monomethyl malonic anhydride is the most reactive and the dimethyl is the slowest. 

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