Ketenes, preparation from dimers

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. 

For thermally induced [2 + 2] cycloadditions, the concerted mechanism is operative only in particular cases, such as in the reactions between an alkene or alkyne and a ketene. The ketene can be generated directly in the reaction mixture from the appropriate acid chloride with triethylamine. The cycloaddition reaction is stereospecific and occurs exclusively in a cis fashion. Although the intermolecular cycloaddition with ketene itself proceeds in poor yields due to the propensity of the unsubstituted ketene to undergo dimerization, it is quite an efficient reaction with ketenes containing electron-withdrawing substituents. Usually, a-chloro ketenes are employed as reagents formed in situ from the corresponding a-chloro acid chlorides. Typical examples are represented in the preparation of cycloadducts such as 378 and 379 (Scheme 2.127). The latter cycloadduct, prepared in modest yield (ca. 20%),... 

Because many ketenes have a great tendency to dimerize, fission of the dimers is an important method for their preparation. The dimers are cleaved when heated at 500-600°, giving the monomers free from by-products, so that formation of and regeneration from the dimers can be used for purification of the monomers.176... 

Dimers of other ketenes also can be prepared from the monomers, but frequently it is more convenient to prepare the dimers directly by extending the reaction time or increasing the reaction temperature. The most convenient preparation of higher aldoketene dimers is that of dehydro-halogenation of acyl halides with tertiary aliphatic amines." ... 

If the ketene is not isolated from the solvent but converted into the anilide by addition of aniline, an amount of anilide corresponding to a 34% yield of the ketene is isolated. However, if the ketene is isolated from the solvent by vacuum distillation, the yield of monomer is much lower (not given). If pure monomeric ethylcarbethoxyketene is desired, it is preferable to prepare the dimer and depolymerize it thermally (p. 137). 

The known tetrakis(trifluoromethyl)allene has been observed as a byproduct in the preparation of bis(trifluoromethyl)thioketen via pyrolysis of its dimer" at atmospheric pressure and l,l-dimethyI-3,3-bis(trifluoromethyl)-allene, apparently a new compound, has been synthesised from bis(tri-fluoromethyl)keten (see Scheme 15)." The new allenes CFa CFa-CClrCiCMea and CF2CbCF2-CCl C CMe2 have been obtained by dehydrochlorination of adducts prepared from isobutene (see Table 3, p. 12). 

In context with the formation of peraminosubstituted 1,4,5,8-tetraazaful-valenes of type 85 it must be mentioned that the bis-vinylogous compounds 94 can be easily prepared by reaction of acetamidine with bisimidoylchlo-rides derived from oxalic acid (96S1302). In the course of a complex reaction a cyclic ketene aminal was produced it immediately underwent an oxidative dimerization to yield deeply colored TAFs. Tlieir high chemical stability can be compared with that of indigoid dyes and manifests itself, for example, by the fact that they are soluble in hot concentrated sulfuric acid without decomposition. Tire same type of fulvalene is also available by cy-... 

Ketenes, which are even more reactive than isocyanates, afford ketenimines at or below room temperature [62CRV247 84JOC2688 89JCS(P1)2140]. At elevated temperatures, dimerization or polymerization occurs (21HCA887). Although N-aryl- and A-vinyliminophosphoranes react smoothly with ketenes, strong acceptor substituents on the nitrogen hamper the reaction thus V-acyliminophosphoranes do not react with ketenes. Vinylketenimines such as 2-aza-l,3-dienes prepared in this way from... 

A different reaction pathway and a remarkably stable ketene-containing l,3-dioxin-4-one has been prepared by cross dimerization of ketenes 213 and 214, generated in situ by FVP of appropriate precursors. The product, l,3-dioxin-4-one 215, was obtained in 40% yield after recrystallization from hexane <2002J(P1)599> (Equation 74). 

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. 

Thioketenes can be prepared in several ways, from carboxylic acid chlorides by thionation with phosphorus pentasulfide [1314-80-3], P2S5, from ketene dithioacetals by p-elimination, from 1,2,3-thiadiazoles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimerization of thioketenes to 2,4-bis(alkylidene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolytically (63). For a review see Reference 18. 

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

A simple synthesis of symetrically functionalized dithiosalicylides 77a-d from the 377-1,2-benzodithiol-3-ones 75 and triphenylphosphine was described (Scheme 6) <1997JOC9361>. As indicated, the preparation of precursors 75 was based on a known transformation of the substituted anthranilic acid derivatives 73 to the thiosalicylic acid derivatives <1943OSC580>. Upon treatment with thioacetic acid in sulfuric acid, cyclization <1990JOC4693> afforded products 75 in 38-64% yield. Dimerization of a benzothietan-2-one 78a or ketene 78b <1987JOC3838>, arising presumably from initially formed intermediate 76, was proposed to account for the product formation. As revealed... 

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... 

Keteniminium salts arc an attractive alternative to ketenes for cycloaddition with alkenes to give cyclobutanones31-32. Keteniminium salts do not dimerize and are more electrophilic than ketenes. They can be easily prepared by treatment of a dialkyl amide with 2,4,6-trimethylpyri-dine (collidine) and triflic anhydride in an inert solvent or from an a-haloenamine and a Lewis acid. The cycloadditions of keteniminium salts with alkenes are stepwise, not concerted, so that products from syn addition to the alkene are not always obtained stereoselectively. Of greater concern is the fact that the major product from alkenes which can form a tertiary carbocation is the Kriedel-Crafts product. The cycloaddition procedure is therefore limited to mono- and 1,2-disubstituted alkenes. 

Ketenes are usually prepared in situ by elimination from acid chlorides with a tertiary amine. Thus (6) gives dimethyl ketene. If no other reagent is present, dimer (5) is formed. If a nucleophile is present, product (3) is formed, whilst thermal 2 + 2 cycloadditions take place with alkenes to give cyclobutanones (7). ... 

In situ prepared ketene by dehydrochlorination of the corresponding acyl chloride with Hiinig s base, afforded similar results [262c]. A further modification was the dimerization of ketenes 182 in situ formed from different alkanoyl chlorides to afford 3,4-dialkyl-ci5-a-Iactone derivatives [262d]. 

Monoalkylketenes are also prone to dimerization, but dialkylketenes have longer lifetimes. The remarkably crowded and unreactive di-teri-butylketene 5 bears strong steric protection and was first prepared in 1960 from the acyl chloride using a strong base (Eqn (4.4)), and identified by the characteristic ketenyl IR absorption. The dehydrochlorination reaction has also been carried out with triethylamine as the base using ultrasound in 86% yield or by reaction with neat tri- -butylamine at 80 °C, also in 86% yield. The use of the aldehyde i-Bu2CClCH=0 as an alternative precursor to 5 by an elimination reaction has also recendy been reported. This ketene is stable indefinitely as a neat liquid and reacts slowly with and there is no... 

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. 

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