Diphenylketene, cycloaddition with

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). 

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... 

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],... 

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). 

The carbonyl ir-bond has been found to add chemo- and regio-selectively across the alkenic ir-bond of ketenes. Thus diphenylketene readily reacts with benzoquinone to yield a stable [2 + 2] adduct (equation 1). With an excess of diphenylketene the bis-adduct is formed, which decomposes into tetraphe-nylquinodimethane and carbon dioxide (equation 2). With the less stable ketene, thermal [2 + 2] cycloadditions are observed with highly electrophilic carbonyl compounds (equation 3). With unactivated aldehydes and ketones, yields are much lower due to a faster oligomerization of the ketene reagent. However, in the presence of a Lewis acid catalyst, most aldehydes or ketones form P-lactones with ketene (equation 4). Cycloadditions with ketones usually require more active catalysts than with aldehydes. The catalyzed reaction of ketene with methyl vinyl ketone is chemoselective, yielding a 10 1 ratio of [2 + 2] versus [4 + 2] adducts (equation 5). In the absence of catalyst, methyl vinyl ketone reacts with ketene to give exclusively the [4 + 2] adduct. 

Diphenylcyclobutanones. Contrary to earlier reports, diphenylketene undergoes cycloaddition with simple alkenes to give 2,2-diphenylcyclobutanones.1 However, a long reaction time is needed for satisfactory yields. cw-Olefins react much faster than trans-olefins. Thus the reaction with cis-butene-2 requires 3 days, but the reaction with frans-butene-2 requires 3 months. With low-boiling alkenes, the reaction is carried out in a sealed tube or in an autoclave. 

Site selectivity in ketene cycloadditions is also explained by the frontier orbitals. Diphenylketene reacts with isoprene 6.397 mostly at the more substituted double bond to give the cyclobutanone 6.398 as the major product.890 In contrast, it reacts with cw-piperylene 6.399891 and with cw-butadiene-l-nitrile 6.400890 at the less substituted double bond. In all three cases the site of attack is the double bond having the largest coefficient in the HOMO. 

The 4-benzoyl-2,3-thiophenedione (260) undergoes thermal or photochemical cycloaddition with diphenylketene to give the spirothiete (261) and the thiophen-2-one (262) <91T3045>. The probable mechanism is shown in Scheme 49. With ketenimines, however, the reaction takes a different course, the initial step being a [4 - - 2] cycloaddition (Scheme 49) <87H(26)625>. The product is either (263) or (264) depending on the substituent R. 

Imines, generally can be subjected to [4 + 2] cycloaddition with acylke-tenes. However the reaction of diphenylketene with phenylbenzoazete 258... 

Vinyl azides undergo slow cycloaddition with diphenylketen involving loss of nitrogen to give five-membered-ring enamino-ketones (84). The non-concerted mechanism suggested involves nucleophilic attack of the )5-carbon... 

Diphenylketene reacts with the l-arsa-3-germaallene 163 by a [3 + 2] cycloaddition forming 165, fully characterized by NMR, by a process interpreted as involving the transient heterocycHc arsa(germa)carbene 164... 

Across N=0 bonds The [2+2] cycloaddition reaction of ketenes with nitroso compounds affords the four-membered ring cycloadducts. For example, diphenylketene reacts with nitrosobenzene to give the cycloadduct 312. On heating of the cycloadduct, dissociation... 

Since vinyl azides like 34 are electron-rich olefins, [2 + 2] cycloaddition with electron-deficient alkenes such as diphenylketene could lead to azidocyclobutanes. " The stability of the cycloadducts 211, prepared from 34 or 52 and tetracyanoethene (TCNE), allowed characterization in solution but not isolation of these products because rapid ring-expansion regioselectively afforded the dihydropyrroles 212 already at room temperature (Scheme 5.25). "" A similar mechanism via [2 + 2] cycloaddition and quick ring-enlargement may perhaps explain the formation of 213 from 52 and 4-phenyl-l,2,4-triazole-3,5-dione (PTAD). In this " " and other " " cases, however, different interpretations were offered. The 2-azidobuta-l,3-dienes 92a,b underwent [4 + 2] cycloaddition in the... 

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). 

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. 

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]. 

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). 

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). 

---