Ketene dimers preparation

Ketenes can be prepared by treatment of acyl halides with tertiary amines. The scope is broad, and most acyl halides possessing an a hydrogen give the reaction, but if at least one R is hydrogen, only the ketene dimer, not the ketene, is isolated. However, if it is desired to use a reactive ketene in a reaction with a given compound, the ketene can be generated in situ in the presence of the given compound. ... 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

Acetoacetanilide has been prepared by the reaction of aniline with ethyl acetoacetate 2 3 and by the reaction of ketene dimer with aniline.4... 

W. E. Hanford, J. C. Sauer, Preparation of Ketenes and Ketene Dimers, Org. React. 1946, 3, 108-140. G. M. Coppola, H. F. Scuster, a-Hydroxy Acids in Enantioselective Synthesis, Wiley-VCH, Weinheim, New York, 1997. 

Only a few ketenes can be isolated, and diphenylketene is one of those. The majority of the other ketenes dimerize quickly, as exemplified by the parent ketene H2C=C=0. Cycloadditions with reactive ketenes therefore can be observed only when they are prepared in situ and in the presence of the alkene to which they shall be added. Dichloroketene generated in situ is the best reagent for intermolecular [2+2]-cycloadditions. Dichloroketene is poorer in electrons than the parent ketene and therefore more reactive toward the relatively electron-rich standard alkenes. The reason is that the dominating frontier orbital interaction between these reactants involves the LUMO of the ketene, not its HOMO (see Section 12.2.4). 

A critical discussion of methods for the preparation of ketenes and ketene dimers including many experimental procedures has been elegantly presented, For the most part, the methods are modifications of those used for the synthesis of olefins. Ketenes are very reactive substances and are prepared for immediate consumption. The simplest member, ketene, reacts with various groups which contain hydrogen to form acetyl derivatives. Even t-butyl alcohol reacts readily to form t-butyl acetate when a small amount of sulfuric acid is present as a catalyst. ... 

Ketene, H,C = C = 0, has been obtained by the pyrolysis of many compounds containing the CHjCO—group. However, its preparation from acetone has been the most successful from the standpoint of the laboratory and is carried out by passing the vapors through a combustion furnace at 650° (30%) or over a hot Chromel A wire filament at 700-750° (90%). The product is contaminated with ethylene, carbon monoxide, and methane. It may be purified by dimerization followed by depolymerization (cf. method 246). More often than not, since ketene dimerizes readily, it is passed directly from the generator into a reaction vessel for immediate consumption. 

Ketene acetals, preparation, 267, 269 Ketene dimers, depolymerization, 407 reaction with alcohols, 483... 

The method is apparently limited to the preparation of certain aryl and high-molecular-weight ketoketenes, which are relatively resistant to dimerization. Thus, the dehydrohalogenation of a low-molecular-weight acyl chloride such as isobutyryl chloride gives dimethyl ketene dimer (60%). It is quite possible that the tertiary amine salt catalyzes the dimerization. ... 

An important method for the preparation of /3-keto esters is by the action of alcohols on ketene dimers in the presence of acid catalysts. Diketene and alcohols give acetoacetic esters in 60-80% yields. Dimers of higher ketenes are made by dehydrohalogenation of acyl halides and are converted to /S-keto esters in one operation (cf. method 245). 

The requisite a-diazo thiol esters are conveniently prepared by using the "detrifluoroacetylative" diazo transfer strategy previously developed in our laboratory. Cycloadditions are best carried out by using as little as 0.006 equiv of rhodium(II) acetate to promote the thia-Wolff rearrangement. Reactions involving the more nucleophilic ketenophiles proceed smoothly in refluxing dichloromethane (40°C), while cycloadditions with less reactive partners are best accomplished in 1,2-dichloroethane (83°C). As is standard for ketene cycloadditions, the optimal protocol involves slowly adding a solution of the diazo thiol ester to a solution of the ketenophile and catalyst in order to minimize competitive ketene dimerization. 

Ketenes and derived products. Triethylamine dehydrohalogenates an acid chloride having an a-hydrogen atom to give a ketene isolable as the ketene dimer, which can be converted into a j8-ketoacid or a symmetrical ketone (Sauer An example is the preparation of laurone from lauroyl chloride. An ethereal solution of the acid... 

Preparation of Ketenes and Ketene Dimers W. E. Hanford and John C. Sauer... 

The reaction with ketene dimers effects the formation of bridges that are narrowing the pores of the cotton fiber so that the passage of water across the fiber is greatly delayed. The main stages of preparation are (28) ... 

Preparing an aqueous alkaline emulsion of alkyl ketene dimer with cationic starch and a retention agent... 

The ease of preparation of ketenes and their use in the Staudinger reaction depends on their reactivity. Most ketenes dimerize readily, are hydrolyzed easily, and are sensitive to oxidation by oxygen. However, some ketenes, such as trimethylsilylketene and diphenylketene, may be isolated as pure compounds. ... 

NBE dicarboximides and subsequent cyclopolymerization of Meldrum s acid-containing 1,6-hexadiynes (72) were reported by Choi et al. [46] (Scheme 9.11b). The prepared polymers assembled in core-shell supramolecules (nanospheres), with the inner core being the insoluble polyacetylene block functionalized with Meldrum s acid. Thermal treatment at 240 °C for 10 min led to cross-linking of the inner core (74) by ketene dimerization (Scheme 9.11c). 

The dimerization of ketens has been used to prepare the spirocyclic cyclobutane-1,3-diones (273), (274), and (275). The dimethylene and trimethylene ketens were generated by thermolysis of the malonate esters (276) and (277). In the cyclohexane case, the necessary keten was prepared by elimination of hydrogen chloride from the acid chloride. Thermal dimerization of benzylphenylketen gives mainly the cis-dimer (279). An acyclic dimer is also formed. 

Table IV. Preparation of Ketenes by Pyrolysis of Ketene Dimers. . . 120...

The ketenes are prepared by modifications of the general methods for the synthesis of olefins. The ketenes are much more reactive than simple olefins, however, and are more likely to enter into combination with the reagents from which they are prepared or with the solvents used or into self-condensation to yield dimers or polymers. Many of the aldoketenes, which generally are more reactive than ketoketenes, have not been isolated as the pure monomers. The dimers of aldoketenes (p. 127) have some of the properties of ketenes, and certain of them are useful reagents. 

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