Methyl ketene decomposition

According to Blake and Hole , methyl ketene decomposes into CO2 and pentadiene-2,3 as well as into CO and butene-2 subsequent polymerization and decomposition processes produce other products, especially at higher temperatures. The results indicate that the overall orders of both CO2 and CO formation are 1.5 and each reaction path is inhibited by isobutene. Blake and Hole suggested tentative chain mechanisms to account for the observed product formation and for the kinetics of the decomposition. Initiation and termination was assumed to occur at the surface of the vessel. 

The analogous reaction paths and the similar overall kinetics of the pyrolyses of ketene and methyl ketene seem to suggest more similarities in the mechanisms than is assumed at present. A detailed examination of this point could be profitable. 

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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.). 

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... 

Benzocvclobutenone was obtained from decomposition of 0-alkyl substituted benzoyl chlorides189. Thus, with 2-propylbenzoyl chloride at 600 °C, the 0-quinonoid ketene intermediate is stabilized by 1,5-hydrogen shift to give the aldehyde (equation 101). In this case, the 0-alkyl contains a /1-hydrogen. However, if the 0-substituent lacks a /1-hydrogen, i.e. it is a methyl group, then the cyclobutenone is obtained (equation 102). 

Using 2,2-dimethylcyclobutanone [26a], as a specific example, initial excitation to produce an excited state species followed by a-cleavage would produce the acyl alkyl biradical [30]. Subsequent decomposition of [30] would then afford ester [27a] (via ketene), cyclopropane [28a], and acetal [29a], the observed photoproducts. The intermediacy of biradical [30] was supported by (a) the nearly exclusive formation of methyl acetate (as opposed to methyl isobutyrate), (b) the exclusive formation of the 5,5-dimethyl substituted acetal [29a] (as opposed to its 3,3-dimethyl substituted isomer), (c) its role as a common intermediate for all products, and (d) analogy to the photochemistry of cyclopentanones and cyclohexanones. Recently, Wasacz and Joullie have reported that photolysis of oxacyclohexanone [32] affords a 3% yield of acetal [29a] (18). It is conceivable that the formation of [29a]... 

The anhydrides can be prepared by the action of acetic anhydride on the corresponding malonic acid in the presence of a small amount of sulfuric acid, followed by neutralization of the mineral acid with powdered barium carbonate and evaporation to dryness in a high vacuum. The residual malonic anhydride is then heated to the decomposition point at a low pressure, and the ketene is collected in a cold receiver. This procedure has been applied to the synthesis of low-molecular-weight dialkylketenes (R is methyl, ethyl, f2-propyl, or isopropyl) in 50-80% yields. ... 

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. 

Both types of reactivity of ynolate anion have been reported in the literature. The O-attack is typical for the reactions of lithium ynolates with trialkylchlorosilanes b24,25 dialkyIchlorophosphates. Lithium ynolates, generated as shown in equations 5-10, react with sterically hindered trialkylchlorosilanes in THF affording silyl ynol ethers as primary products (equation 12). However, in some cases the silyl ynol ethers are unstable at room temperature and isomerization to the more stable ketenes, or decomposition, occurs The ketene rearrangement usually occurs in reactions of lithium alkynolates with methyl substituted silyl chlorides a typical example of such a rearrangement is represented by reaction 13 ". ... 

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... 

The formation of the low molecular weight compounds CO, CO2, H2, propene, ketene, methanol and methyl hydroperoxide can be explained by the decomposition of biradicals as shown in Chapter 2, Fig. 13. In view of the atmospheric relevance of the ozonolysis of alkenes, especially the influence of water vapour on the reaction mechanism and the product formation needs careful investigation. Under such conditions heterogeneous effects cannot be neglected and large-scale reaction chambers would be useful to account for such effects. 

Scheme 9.121. A representation of the formation and subsequent decomposition of methyl 3-ketopropanoate. It is formed by the ring opening of ketene dimer on reaction with methanol. Decomposition in the presence of aqueous acid and aqueous base, separately, produces ketone in the former and fragmentation in the latter.

The diazo compound from methyl benzoylacetate gives phenylcarbo-methoxyketene in 70% jdeld when the final decomposition is carried out in refluxing xylene, but it is converted largely to the ester of phenyl-malonic acid by heating in the absence of a solvent. The ethyl ester of ethylmalonic acid is the principal product from the decomposition (in the absence of a solvent) of the diazo compoimd obtained from ethyl pro-pionylacetate. A small amoimt of the ketene evidently was formed also. The diazo compound obtained from ethyl ethoxalylacetate on decomposition in warm xylene gives 54% of dicarbethoxyketene. ... 

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