Dimers diphenylketene

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. 

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. 

Only a few ketenes can be isolated, and diphenylketene is one of those. The majority of the other ketenes dimerize quickly, as exemplified hy the parent ketene H2C=C=0. The fewer or smaller the substituents that are hound at the sp2-hybridized carbon atom the quicker these dimerizations proceed ... 

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). 

Irradiation of 2-phenylbenzoxazole forms a head-to-tail dimer, 1,3-diazetidine 436, whereas benzoxazole itself, in the presence of oxygen, gives the dimeric product, 437. Isothiazoles yield lactams 438 on reaction with diphenylketene. Benzotriazole and its 2-alkyl derivatives undergo photochemical ( > 290 nm) [2 + 2] cycloaddition to maleinimides to give adducts 439 <20020L1487>. 

The (4+2)-cycloaddition reactions are theoretically and experimentally well founded for electron-rich dienes. However, those reactions with electron-rich dienophiles are less investigated. An early application of the inverse type of (4+2)-cycloaddition to pyridazine synthesis, i.e., from electron-deficient azoalkenes and alkenes, was reported by Sommer in 1977, [Eq. (4)]. A mixture of regioisomeric products 22 and 23 was obtained.107 108 However, in the presence of diphenylketene, products of either (2 + 2)- and/or (4+2)-cycloaddition (i.e., pyridazines) are formed arylazoalkenes thus behave similarly to a,/ -unsaturated carbonyl compounds in cycloadditions.109 2-Phenylazo-l-alkenes dimerize (a (4+2)-cycloaddition) in the absence of solvent to give pyridazines, or they may react with dienophiles.110-115 Some other cycloadditions of this type are reported.116-120... 

Mit Diphenylketen 620 und 673 bildet sich das Tetraphenylpenta-tetraen 622 das unter den Reaktionsbedingungen in sein Dimeres 192) iibei eht 90), Mit Methylisocyanat 621 entsteht aus 615 das kumu-lierte Ketenimin 623, das bei der Aufarbeitung mit Wasser das AUen-derivat 624 Kefert 9). 

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. ... 

Stereoselective formation of unsymmetrical ketene dimers is conveniently achieved by a process where a more reactive ketene is generated in the presence of a less reactive ketene in the presence of a chiral catalyst. The catalyst adds to the more reactive ketene and the resulting reactive eno-late reacts with the less reactive ketene to form the unsymmetrical dimer. By this process methylketene formed by dehydrochlorination in methylene chloride in the presence of the chiral catalyst TMS-quinine reacts with diphenylketene forming the mixed dimer 44 (Eqn (4.24)). 

Diphenylketene (1) has been reported to form several dimers and tri-mers, with initial findings by Staudinger with GoUer that heating 1 with quinohne forms tetraphenyl-l,3-cyclobutanedione 45 (Eqn (4.25)), as... 

Erickson and Kitchens found heating of neat diphenylketene yielded 7% of 46, as well as 54% of a trimer of diphenylketene, while reaction of diphenylketene with sodium methoxide at 160 °C was reported to give a faster reaction forming the same two products. The trimer was then shown by Das and Kooyman to have the structure 47a and was proposed to form by enolization and acylation of the dimer 47 (Eqn (4.28)). ... 

A further trimer of diphenylketene 48 was formed by reaction of diphenylketene with hexamethylphosphoramide, together with the dimer 46 (Eqn (4.29)). =... 

Cp2Ti(CO)2], a precursor of titanocene, undergoes an unusual reaction with diphenylketene (Scheme 3). The product dimer, characterized by crystal structure determination, contains diphenylketene j -bonded to the titanium through the CO moiety. Previously such jr-interaction of a ketonic C=0 group was only known for complexes of hexafluoroacetone with transition metals. An A -ray crystal structure has been determined for the dimer, further reaction of which affords a metallocycle (Scheme 3). 

Disubstituted ketenes usually form the symmetric dimers. However, Staudinger and Klever also obtained a small amount of the j8-lactone dimers. This dimer is the only reaction product if the dimerization of dimethylketene is conducted in the presence of aluminum chloride as the catalyst Diphenylketene produces the -lactone dimer on addition of a catalytic amount of sodium methoxide . 

The cyclodimer 22 of diphenylketene, formed by a [4-1-2] cycloaddition sequence, is also obtained. This dimer reacts with diphenylketene at 100 °C, in the presence of a catalytic amount of quinoline to give the trimer 23 . 

The symmetrical trimers of disubstituted ketenes are obtained by heating the corresponding dimers with a catalytic amount of base. For example, heating of the symmetrical dimer of dimethylketene 77 with sodium methoxide at 100 °C affords a 97 % yield of the trimer 78 . In a similar manner, 78 is obtained from the unsymmetrical dimethylketene dimer 79 in high yield. The symmetrical trimer of diphenylketene was similarly obtained... 

The [2+2] cycloaddition reaction of ketenes with olefins was already observed by Staudinger and his coworkers, who postulated the four-membered ring structure for the cycloadducts. The rate of reaction of ketenes with olefins is as follows diphenylketene > dimethylketene > butylethylketene > ketene. Because of its slow rate of dimerization butylethylketene is an especially useful reagent and its cycloaddition to slow reacting olefins can be forced by using elevated temperatures. 

Hydroxy-2,4-diphenyl-3-butenoic lactone (neutral diphenylketene dimer) heated briefly with NaOH in water-dioxane -> - 2,4-diphenylcyclobutenol-3-one (acidic diphenylketene dimer). Y 98%. J. E. Baldwin and J.D. Roberts, Am. Soc. 85, 2444 (1963). 

Oxathietans, Oxathiets, and Dioxathietans.—Oxathietan intermediates have been considered in the reaction of thiocarbonyl ylides with diphenylketen and in the vapour-phase thermolysis of S-methoxymethyl thioacetates. The behaviour of solutions of monothiobenzil may indicate isomerization to an oxathiet. The photo-oxidation of tetramethyl-3-thiocyclobutane-l,3-dione by oxygen may proceed via a l,2-dioxa-3-thietan. The l,3-dioxa-2-thietan 2,2-dioxide (210) was obtained by treatment of 3,3,3-trifluoro-2-tri-fluoromethylpropanoic acid with sulphur trioxide. It is cleaved by various nucleophiles to bis-trifluoromethylketen, which dimerizes, and its reactions with fluoroethylenes have been determined. Thermolysis of (210) yields anhydride (211). ... 

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