Cyclopropenones double bonds

The first synthesis of a cyclopropenone was reported in 1959 by Breslowls who achieved the preparation of diphenyl cyclopropenone (11) by reacting phenyl ketene dimethylacetal with benzal chloride/K-tert.-butoxide. The phenyl chloro carbene primarily generated adds to the electron-rich ketene acetal double bond to form the chlorocyclopropanone ketal 9, which undergoes 0-elimination of HC1 to diphenyl cyclopropenone ketal 10. Final hydrolysis yields 11 as a well-defined compound which is stable up to the melting point (120—121 °C). 

As Table 10 shows, vinylic three-ring protons in cyclopropenone itself and in monosubstituted cyclopropenones appear at remarkably low field in the region of 0.9-1.6 r. Comparison with protons at the double bond of covalent cyclopropenes (e.g. 200 3.34 t174 ) and cyclopropenium cations (210 —0.35 r146 202 ... 

In general, cyclopropenones do not exhibit dienophilic qualities despite their highly strained C1—C2 double bond. An exception is made by unsubstituted cyclopro-... 

At low temperatures cyclopropenones and enamines or ketene acetals were shown to yield 2-azonia-bicyclo(3,l, 0)hex-3-enolates-3 (371, X=0), which can be isomerized thermally to penta-2,4-diene amides(372, X=0). At elevated temperatures the amides were found to be the principal products arising from C-N-insertion 237) (insertion of the cyclopropenone three-carbon unit into the C-N bond of the enamine). These were accompanied in some cases by 3-aminoenones 373 arising from C-C-insertion 237) (insertion of the cyclopropenone into the C-C double bond of the enamine) and a-amino cyclopentenones 375 formed by Stevens rearrangement of the ylide 369 and cyclopentenones 374 ( condensation 237)). 

In contrast, methyl cyclopropenone is reported283) to react with the Pt-olefin complex 455 at low temperature with replacement of the olefin ligand. In the resulting complex 456 the cyclopropenone interacts with the central atom via the C /C2 double bond according to spectroscopic evidence284). At elevated temperatures a metal insertion to the C1<2)/C3 bond occurs giving rise to 457. Pt complexes of a similiar type were obtained from dimethyl and diphenyl cyclopropenone on reaction with 455 and their structures were established by X-ray analysis285). 

The rather complex reactivity exhibited by cyclopropenones on interaction with enamines (see p. 74) is not re-found in the reactions of triafulvenes with enamines and ketene acetals. Instead of a (3 + 3) cycloaddition of enamine C=C—N sequence to the CI(2)/C3 bond of triafulvene (as represented by ylide 51 J) the addition of the enamine double bond to triafulvene C /C2 bond (operating with cyclopropenones only as a minor side-reaction) predominates in all reactions hitherto investigated. 

The high reactivity of the strained double bond in cyclopropenes has led to several other more recent developments especially involving cyclopropenone ketals as substrates97. 

Cyclopropenone undergoes many interesting reactions — one example is Diels-Alder addition, the product of which in methanol solution is a hemiketal. That the hemiketal is favored for the adduct, but not for cyclopropenone, indicates that the double bond of cyclopropenone has a considerable effect on the reactivity of the carbonyl group. 

In the course of mechanistic studies it was established that aniline does not react with the cyclopropenones (153 and 154) even under reflux conditions. It was therefore assumed that the formation of (158) involves initial nucleophilic attack by the aminopyridine ring nitrogen on the electrophilic cyclopropenone ring. In this way 155 is formed, which is then transformed via the reactive intermediates (156, 157, and/or 161) to the prodticts. Kascheres et al. noted that the formation of 157 is formally a stereospecific trans addition of the 2-aminopyridines to the double bond of the cyclopropenone (153). Such sterospecificity has been observed in kinetically controlled Michael additions. 

Nucleophilic additions to the cyclopropene double bond are particularly well documented for strained and/or highly reactive derivatives " Thus bicyclo-[4.1.0]hept-l(7)-ene (139) can be trapped by oxygen, sulphur and carbon nucleophiles (equation 52) and the rapid reactions effect a syn-addition. Cyclopropenone dimethyl... 

Cyclopropenyl imminium salts can be prepared by the reaction of cyclopropenones with substituted ammonium saltsor by the reaction of ethoxycyclopropenyl salts with amines The ease of rotation about the carbon-nitrogen double bond shows that the positive charge is not located solely on the nitrogen as in 67 but is shared with the cyclopropene ring as in 67a . 

The infrared spectra of cyclopropenones have been assigned using C-labeled compounds A strong band at 1850 cm" arises from coupling of the two double bonds. Another characteristic band at 880 cm " Ms a symmetrical stretching interaction of the ring carbon-carbon bonds. It is interesting that no hydrate can be detected by infrared spectroscopy in aqueous solution ". ... 

In contrast, when allowed to react with Rh(CO)(PPh3)2(OTf) at 60-65°C, cyclopropenone forms cationic complex 17, in which cyclopropenone is bound to rhodium through the oxygen atom instead of the C-C double bond. Further heating at 60-65°C leads to the formation of the metal carbonyl insertion product 18 [34]. In refluxing benzene, 18 decomposes to diphenylacetylene and Rh(CO)(PPh3)2(OTf). 

The carbocupration of cyclopropenes has been especially investigated with cyclopropenone ketals as reactants. The cuprio cyclopropanes formed can serve as a synthon for the cyclo-propanone enolate. " Achiral ketals with an unsubstituted cyclopropene double bond 15 undergo instantaneous reaction with lithium dimethylcuprate at — 78 C to give the methyl derivative 17 (R = Me) in 96% yield after quenching with methanol. Similar reactions, with a deuterium oxide or iodomethane quench, indicate that the carbocupration takes place in a cis fashion. 

Chlorofluorocarbene generated under phase-transfer catalysis conditions undergoes addition not only to the double bond of enynes (as dichlorocarbene usually does, see Section 1.2.1.4.2.1.4.) but also to the triple bond chlorofluorocyclopropenes thus formed hydrolyze to the corresponding cyclopropenones. If the double bond is sterically crowded, for example, by the presence of a tert-butyl group, chlorofluorocarbene exclusively attacks the triple bond (see Houben-Weyl, Vol. E19b, p 1495). Typical examples 3-6 are given. 

Hexakis(tert-butylisocyanide)triplatinum (Pt3[CN-/-Bu],5) reacted with 2,3-diphenylcycloprop-enone to give /t-2-oxo-l,3-diphenylpropanediylidenebis[bis(t< -Tt-butylisocyano)platinum] 4. Synthesis of the complex involves C-C double-bond cleavage in cyclopropenone the resulting C3 fragment bridges symmetrically the two platinum atoms. ... 

For the coordination of cyclopropenones to transition metals, two modes of coordination are possible coordination of the oxygen atom and coordination of the double bond. The first cyclopropenone complexes prepared were those of the first type, The ligand diphenylcyclo-propenone (1, dpcp) was reacted with a number of metal salts in ethanol, and physical data of a number of metal complexes were determined. The IR absorptions of the double-bond vibrations were somewhat more complex but had similar values to the corresponding ones of the uncomplexed ligand. The products were analyzed by their composition, yields were not given in many cases, and structures were not determined. 

The first cyclopropenone complex with coordination of the double bond was formed by the reaction of (ethene)bis(triphenylphosphane)platinum(0) (2) with methylcyclopropenone (3) which resulted in a ligand exchange giving complex 4 at — 65 "C. ... 

As well as formation of metallacyclobutenones, reactions of cyclopropenones with transition-metal compounds can lead to acetylene-derived products by decarbonylation, to quinones by a nickel-catalyzed cyclodimerization, and to cluster compounds involving a cleavage of the double bond of the cyclopropenone. ... 

Reduotion. Catalytic reduction of cyclopropenone commonly leads to reduction of both the double bond and the three-membered ring with formation of a dialkyl ketone ... 

Reaction of cyclopropenones with carbon-nitrogen double bonds... 

The vibrational spectrum and normal co-ordinate analysis of cyclopropenone and its dideuterio-derivative have revealed that the carbon-carbon double-bond stretch occurs at the remarkably low wavenumber of 1483 cm" (1403 cm" in the dideuterio-compound). The authors have concluded that the bands at 1650 and 1850 cmin disubstituted analogues are an in-phase and out-of-phase mixture, respectively, of approximately equal amounts of the C=0 and C=C components conflicting with this is the observation that only the band at 1650cm is shifted to lower wavenumber ( 1620cm" ) on complexation with phenol in carbon tetrachloride. These latter studies have established the order of basicities of a series of cyclic polyenones relative to cyclohexanone in a non-co-ordinating solvent. 

Coordination of C - C unsaturated bonds to a metal center generally induces elongation of the C-C bond. The coordination can be viewed as oxidative addition of the 3t-bond to the metal center. In some extreme cases, such interaction eventually leads to cleavage of the C—C a-bond. For example, the C—C double bond of a cyclopropenone was cleaved upon treatment with Pt3(CNt-Bu)g (Scheme 1.69)... 

The reactions of cyclopropenone with a wide variety of reagents have been reported by Breslow, Oda, and Pecoraro. " Bromine gives an acyclic product after initial electrophilic attack at the carbonyl oxygen rather than at the double bond. As expected, triethyloxonium tetrafluoroborate gives ethoxycyclo-propenium tetrafluoroborate, and trifluoroacetic acid opens the C-1—C-2 bond, but this could conceivably occur after initial addition to the C-2—C-3 double bond. A cyclopropenyl cation is also a proposed intermediate in the... 

The thiiren 1,1-dioxide system has been investigated theoretically and by u.v. photoelectron spectroscopy. Charge transfer from the carbon-carbon double bond to the SO.2 unit is about one-third that predicted for the cyclopropenone system. Aromaticity decreases in the order cyclopropenone > tropone >... 

For an unusual generation of the CC double bond in a cyclopropenone by double photoelimination of nitrogen from an a,a -his diazoketone, see Scheme 12 in Section 15.3. Examples of photoeliminations of CO from a cyclobutanedione (109) and a cyclobutenedione (112), to afford a cyclopropanone and a cyclopropenone, respectively, are given in Section 15.4. The latter reaction appeared to proceed by a two-step mechanism involving an initial ring opening. 

Diol (1) is used to prepare cyclopropenone acetal (2). Its highly nucleophilic double bond forms addition products with alcohols and amines and cycloaddition products with dienes to give norcarenes, ketones to give furanones and oxetanes, aldehydes to give butenolide, furan, and y-keto ester derivatives (eq 2), electrophilic alkenes to give cyclopropanes and functionalized cyclopentenones, and an a-pyrone to give a... 

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