Diphenylketene. cycloaddition

Analogously to ynamines and o, /3-acetylenic ketones, 4-aminobut-3-yn-2-ones react with 1,3-dipoles (68HCA443 73HCA2427 92KGS867). The reaction of 4-dimethylaminobut-3-yn-2-one with diphenylketene follows a route of [2-1-21-cycloaddition (30°C, THF, 1 h) to give 2-acetyl-3-dimethylamino-4,4-diphenyl-cyclobut-2-en-l-one (377) in 15% yield. With ethyl azidoformate (30°C, THF, 3 h), the tiiazole 378 is formed in 82% yield, whereas with phenyl isocyanate, the quinoline 379 is the product (by a [2- -4] scheme) in 70% yield (68HCA443). 

Cycloaddition of diphenylketene to the benzothiazepinones 1 gives the fused /J-lactams 2 re-giospecifically.47... 

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. 

Several unusual cycloaddition reactions of 9 with unsaturated ketones should be mentioned in conclusion the heterocumulene generated photolytically from 7 undergoes [8 + 2]-cycloaddition with tropone to form 33 (40%) the structure of the product has been unequivocally established by X-ray structure analysis 22,23). Once again, the affinity of phosphorus for oxygen is manifested an entirely analogous cycloaddition reaction is known for diphenylketene 26). 

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. 

Diphenylketene undergoes 2 + 2- or 2 -E 4-cycloaddition reactions with various 1,3-diazabuta-1,3-dienes. The 2 - - 4-, 4 -E 2-, 6 -E 4- and 8 -E 2-cycloaddition reactions of heptafulvenes have been reviewed. ... 

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 N—S—N grouping in the [3 -1- 2] (type A) thiadiazole syntheses is functionally equivalent to either an N,N - or S,.S -disubstituted sulfur diimide, diaminosulfane, sulfamide or tetrasulfur tetranitride. In the cycloaddition of diphenylketene to A,fV -diphenylsulfur diimide the 1 1 adduct... 

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... 

A new and completely different methodology involving a cycloaddition reaction has been described. The reaction between diphenylketene, ferf-butylcyanoketene or dimethylketene with 2,4,6-trimethylbenzonitrile A-oxide gave the corresponding 5(4//)-oxazolones 107 in moderate yields (Scheme 7.30). 

Diphenylketene and more reactive haloketenes will undergo cycloaddition with allenes such as 1,1-dimethylallene giving regioisomeric mixtures of cyclobutanones. Yields of these reactions are only modest5-7 and distributions of isomers are solvent dependent, which is indicative of a nonconcerted process involving ionic intermediates.8... 

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. 

The cycloaddition of phenylarsine or a trimethylsilylphenylarsine to a pentadiyn-3-one (Scheme 6) leads to a 1 -phenyl-2-arsolen-4-one of type (24). This reacts with diphenylketene to give the diphenylmethylene analogues (25) which are intermediates in the synthesis of the benz[/]arsindoles (26) (76T2131). 

Boedeker and Courault215 reacted Schiff bases (151) derived from 2-aminopyridine and aromatic aldehydes with diphenylketene. In benzene at room temperature 4-oxopyrido[l,2-a]pyrimidines (152) were obtained in a reversible [4 + 2] cycloaddition reaction, whereas upon boiling mesitylene, irreversible [2 + 2] cycloaddition yielded azetidinones (150). Previously Sakamoto et al.216 prepared the azetidinones (150) in boiling xylene. 

Since stepwise reactions are not subject to the rules of pericyclic reactions, they are often invoked to explain how reactions in which the rules have been subverted take place. However, there is a small group of thermal [2+2] cycloadditions that seem to be disobeying the rules, and yet may well be pericyclic. One is the reaction of ketenes with electron-rich alkenes, illustrated by the reaction of diphenylketene 2.167 with ethyl vinyl ether 2.166 giving the cyclobutanone 2.168. Another is a group of electrophile-... 

Treatment of diphenylketene (407) with a catalytic amount of Co2(CO)8 produces tetraphenylethylene (410,) involving a carbene complex as an intermediate. In this reaction carbene complex 408 is formed from 407 and Co2(CO)8, the cobaltacyclo-butanone 409 is generated by cycloaddition of 407 and 408, and cleaved to give 410 [130],... 

The reaction of diphenylketene with 1,3,2-diazaboroles 234 in hexane at — 20 °C leads to the formation of 1,3,2-oxazaborolidines 238 in 64-70% yield. The reaction proceeds via the coordination of the ketene oxygen to boron 236 followed by a [2+3] cycloaddition yielding the bicyclic intermediate 237. Fission of B-N bond leads to the product formation as yellow to colorless crystals (Scheme 40) <20000M5791>. 

Others are not so obvious. Cyclobutanone 22 might come from diphenylketene and the diene 21 (disconnection 22a) or from ketene itself and the diene 23 (disconnection 22b). No doubt both dienes could be made but the regioselectivity of both cycloadditions looks in doubt. 

Another example of cycloaddition selectivity was observed for the reaction of cyclopentadiene with diphenylketene (Equation (37)). Originally only products of [2+2] cycloaddition were observed and these reactions were considered as highly periselective. 

The metastable byproduct of [4+2] cycloaddition was detected when the reaction of cyclopentadiene with diphenylketene was (Equation (38)) was examined by low-temperature NMR experiment at — 30°C.67 The [4+2] cycloadduct was not observed at elevated temperature because easily isomerizes to [2+2] cycloaddition product via [3,3] sigmatropic (Claisen) rearrangement. This observation was supported by kinetic measurements NMR) and isolation of [4+2] cycloadduct.68 Mechanism proposed by Machiguchi and Yamabe was re-examined by Singleton.69... 

Competition between C=0 and C=C was observed in diphenylketene from which a lactone and a ketene acetal were obtained with the ylide of dimedone [23]. A different type of cycloaddition occurred with PhI=C(C02Me)2 this reacted catalytically with several alkenes (1-hexene, styrene, methyl methacrylate, etc.) with formation of lactones [24] ... 

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). 

Cycloaddition of diphenylketene to some Schiff bases (192) derived from 2-amino-5-phenyl-l,3,4-selenadiazole (185 R = Ph) affords 7-(4-substituted phenyl)-2,6,6-triphenyl- 6,7-dihydro-5if- l,3,4-selenadiazolo[3,2-a]pyrimidin-5-one (193 Scheme 84) (76CPB2532). The selenadiazolopyrimidinium perchlorates of type (194) are prepared by the reaction of (185 R = Ph) and dicarbonyl compounds (74KGS1435). 

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