Ketenes diphenylketene

Staudinger H (1907) Zur Kenntnis der Ketene. Diphenylketen. Liebigs Ann Chem 356 51-123... 

The phosphorylated ketenes obviously represent the most stable group among ketenes. In reactions with nucleophils having hydrogen atoms they are more reactive than ordinary organic ketenes (diphenylketene). 

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]. 

Physical Properties. Ketenes range ia their properties from colorless gases such as keteae and methylketene [6004-44-0] to deep colored hquids such as diphenylketene [525-06-4] and carbon subsulftde [627-34-9]. Table 1 lists the physical state mp, and bp for certain ketenes, thioketenes, and ketenimines. 

StericaHy hindered or very electrophilic substituted ketenes, such as diphenylketene, di-Z rZ-butylketene [19824-34-17, and bis(trifluoromethyl)ketene, are quite stable as monomers. Ketenimines tend to polymerize. The dimerization of thioketenes results in 1,3-dithiacyclobutanones (6) (45), a type of dimer not observed with ketenes. 

The lithium chloride-catalyzed addition of 2-phenylthiirane to diphenylketene may involve attack on carbon by chloride ion followed by addition of the anion of 2-chloro-l-phenylethanethiol to the ketene, but no data about the mechanism were given (69TL259). 

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the Diels-Alder reaction. The 2+2 cycloadditions were classified into the symmetry-allowed 2+2 cycloaddition reactions [6, 7], It was quite momentous when Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme 11) [53, 54], The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products. 

When a ketene acetal is used instead of a ketene, e.g. diphenylketene glycol acetal 22... 

Ketenes and isocyanates also undergo facile [6 + 2]-photocycloaddition with metal complexed cyclic polyenes. Irradiation of 232 in the presence of diphenylketene gave 256 in good yield (Scheme 58)120. This should be contrasted with the normal behavior of ketenes toward alkenes, which typically involves [2 + 2]-cycloaddition. Isocyanates such as 257 work as well. The adducts are produced in high yields and have considerable potential in synthesis. 

A novel heterocyclic system has been achieved from methyl 3-aminopyra-zine-2-carboxylate and several aroyl chlorides, leading to 3-aroylamino derivatives the latter are cyclized with dibromotriphenylphosphorane to 2-arylpyrazino[2,3-rf][3,l]oxazin-4-one (94S405). Furthermore, vinylimino-phosphoranes and diphenylketene react (Scheme 87) to give nonisolable vinylketenimines (233) which afford, with a second equivalent of ketene in a [4 + 2]-cycloaddition, 1,3-oxazinones (234) [89JCS(P1)2140]. 

The three-component reaction with ketene as the heterocumulene component and 6-aminouracil (Scheme 136) leads to both zwitterionic het-eropolycyclic uracils and their monohydro products. For example, 377 is obtained by treatment of diphenylketene and pyridine in a 42% yield. However, diphenylketene and quinoline are transformed into the zwitterionic 378 in a satisfactory yield of 54% (94UP2). 

Many cycloaddition reactions have been carried out with ketenes and thioketones. The products are thiolactones (52). Hexafluorothioacetone and diphenylketene, however, do not undergo cycloaddition even after prolonged heating at 100°C. Good results can be obtained when the more stable dimer of this fluorinated thioketone (53) is used. Anionic monomer 54 could be released by the action of potassium fluoride in an aprotic solvent. Two-step cycloaddition to diphenylketene yields ketone 55. 

Cycloaddition may be varied by the use of fluorinated thioketenes in place of ketenes. Olefins and Schiff bases serve as the other component. Thus thioketene 56 cycloadds to a SchifT base to give product 57 in a 79% yield. In a further variation, reaction of dithiobenzoate esters with diphenylketene yields 61% of product 58. ... 

A 2 -H 2 cycloaddition of diphenylketenes with tosylated sulfurdiitnides 162 produces the l,2-thiazetidin-3-one as well as the isomer 1,2-thiadiazolin-3-one (Eq. 32) Reaction of a ketene with N-sulfinylaniline in acetone at... 

The structure of ketene imine 188 was elucidated by means of a single-crystal X-ray diffraction analysis. Surprisingly, bond lengths of 133 and 120 pm for the cumulated system (C=C=N) hardly deviate from those found in a linear analogue (i.e., diphenylketene p-toluylimine) (193). Apparently, the cumulated bond system of 188 is not linear but rather is bent to an angle of 163.8°. 

A variety of concave pyridines 3 (Table 1) and open-chain analogues have been tested in the addition of ethanol to diphenylketene (59a). Pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 °C by recording the disappearance of the ketene absorption [47]. In comparison to the uncatalyzed addition of ethanol to the ketene 59a, accelerations of 3 to 25(X) were found under the reaction conditions chosen. Two factors determine the effectiveness of a catalyst basicity and sterical shielding. Using a Bronsted plot, these two influences could be separated from one another. Figure 4 shows a Bronsted plot for some selected concave pyridines 3 and pyridine itself (50). 

The selectivity studies show that concave pyridines 3 (Table 1) not only catalyze the addition of alcohols to ketenes, but they may also differentiate between different OH groups in inter- and intramolecular competitions. They are not only reactive but also selective. First experiments with a chiral concave 1,10-phenanthroline show that enantioselectivity is also possible [20]. Structure 9 shows the concave 1,10-phenanthroline 21 g which catalyzes the addition of R-1-phenylethanol (R-68) to diphenylketene (59a) 20% faster than the addition of the S-enantiomer S-68. 

Aldehydes, ketones, and quinones react with ketenes to give p-lactones, diphenylketene being used most often. The reaction is catalyzed by Lewis acids, and without them most ketenes do not give adducts because the adducts decompose at the high temperatures necessary when no catalyst is used. When ketene was added to chloral Cl3CCHO in the presence of the chiral catalyst (+ )-quinidine, one enantiomer of the p-lactone was produced in 98% enantiomeric excess.777 Other di- and trihalo aldehydes and ketones also give the reaction enantioselectively, with somewhat lower ee values.778 Ketene adds to another molecule of itself ... 

Azido-l-methylbenzimidazole with diphenylketene gave the imidazobenzimidazole (232 R = Ph), but decomposition of the azide to give a nitrene, followed by cycloaddition to the ketene, was ruled out as a mechanism. Instead, the authors propose that 232 is formed by attack of the endocyclic ring nitrogen on the C=C bond of the ketene followed by nucleophilic displacement of N2 from the azide.168 Preparation of an analog (232, R = H) would constitute a possible route to an aromatic azapentalene 233. 

Additional supporting evidence for the mechanism includes the formation of pyrylium salts when arylacetyl chlorides, which yield ketenes by loss of HC1, react with the phosphorane (63CR(257)926). It is also known that diphenylketene reacts in a similar manner to give a pyran, whilst aryl isocyanates yield pyran-4-one derivatives. 

The known disuhstituted ketencs include dialkyl-ketcnes, diary lkctcncs. and ihe ester analogs. Rimcthylkelene may be made from u-bromoisobuiy-ryl bromide by reaction with zinc in boiling ether, Diphenylketene may be made similarly, but the usual way lo prepare it is to oxidize benzil hydrazone with yellow mercuric oxide to benzoylphenyldiuzomethnne which, on healing in benzene solution, decomposes inio Ihe ketene. 

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