Cyclopropenones reactions

The reaction of the ethynylestradiol derivative (50) with difluorocarbene represents an equally fascinating series of reactions. The major product obtained in 48% yield is the difluorocyclopropene (51). The latter is hydrolyzed to the cyclopropenone (53), and is readily alkylated at C-21 to (54), which is also obtained by difluoromethylation of the propynyl derivative (57). The cyclopropenone (53) loses carbon monoxide at elevated temperature to... 

Care should be taken to prevent either the dibromoketone or the cyclopropenone from coming into contact with the skin, as allergic reactions have been observed in several cases. The use of gloves is recommended especially for the bromoketone. The latter product has a wide melting range because it is a mixture of the meso- and d,/-compounds. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

Independently Volpin17 synthesized diphenyl cyclopropenone from diphenyl-acetylene and dibromo carbene (CHBr3/K-tert.-butoxide). This reaction principle of (2 + 1) cycloaddition of dihalocarbenes or appropriate carbene sources ( caibenoids ) to acetylenic triple bonds followed by hydrolysis was developed to a general synthesis... 

The reactivity of dichloro carbene towards acetylenic bonds was systematically investigated by Dehmlow19, 20 with respect to substitution of the acetylene, especially those containing additional C-C multiple bonds. It was shown that with aiyl alkyl acetylenes, e.g. 1-phenyl-butyne-l, often the normal cyclopropenone formation occurs only to a minor extent (to yield, e.g. 14), whilst the main reaction consists of an insertion of a second carbene moiety into the original acetylene-alkyl bond (giving, e.g. 15) ... 

Trichloromethyl lithium (generated from BrCCl3 and CH3Li at —100 °C) adds to dialkyl acetylenes and to monoalkyl acetylenes23, thus monoalkyl cyclopropenones became accessible which could not be obtained from terminal acetylenes by reaction with the above carbene sources. The 3,3-dihaIogeno-A1,2-cycIopropenes formed as primary products in the dihalocarbene reactions are usually not isolated, but are hydrolyzed directly to cyclopropenones. 

This procedure is the method of choice for the preparative chemist, since it not only provides the advantage of generally good yields (40—60%), but also can be carried out on a large scale. The modified Favorski reaction of dibromoketones offers a remarkably wide scope of application and has made possible the preparation of a large number of dialkyl-, diaryl-, and monoaryl cyclopropenones (see Table 1). Interestingly, the bicyclic cyclopropenones 5943) and 4041 have become accessible by use of the dibromoketone method. 

One of the first syntheses of a triafulvene utilized the Wittig reaction, when diphenyl cyclopropenone was reacted with triphenyl carbomethoxymethylene phos-phorane giving 696 ) ... 

The Wittig reaction of diphenyl cyclopropenone with the phosphorane 68 failed to give the methylene cyclopropene instead a hydrocarbon of probable structure 67 was obtained71. ... 

As was shown for the mechanism of quinocyclopropene formation in acetic anhydride75 (see p. 20), acylation of the cyclopropenone is reasonable for the primary reaction step, then the O-acyl-cyclopropenium ion 74 forms methylene cyclopropene 73 through addition of the anion of the C-H acidic component and elimination of acetic acid. 

The reaction of diphenyl cyclopropenone with aryl malononitriles75 or aryl cyano acetone84 unexpectedly gave rise to 4-cyano-4-aryl triafulvenes 90, as well as the formation of quinocyclopropenes (see later) ... 

Diphenylcyclopropene thione (156) was prepared11S-12°) from 3,3-dichloro-1,2-diphenyl cyclopropene (154) by reaction with thioacetic acid, since transformation of the carbonyl function of diphenyl cyclopropenone with P4S10121 was complicated by ring expansion to the trithione 155122 In a useful recent thioketone synthesis123) 156 was obtained directly from diphenyl cyclopropenone in a quantitative yield by simultaneous treatment with HC1 and H2S. 

Application of the decarbonylation reaction to cyclohepteno cyclopropenone (J9)43) led to the intermediate formation of the highly strained cycloheptyne (246) as indicated by the formation of its cyclotrimerization product 247 (in analogy to... 

The 4-hydroxy aryl substituted cyclopropenone 251 was found by West193 to exhibit a remarkable cycle of decarbonylation and oxidation-reduction reactions ... 

Phenyl cyclopropenone is not capable of thermal dimerization. On treatment with Cu2+ ions, however, a well-defined tetramer is formed54, to which structure 265 of a polyene dilactone was assigned. Its generation can be rationalized via 264 with both the above dimerization types contributing in metal-catalyzed reactions. 

Thermal reactions of N-aryl cyclopropenone imines 268 are differentiated by the nature of the N-aryl substituent. Imines 268 (Ar = phenyl, p-nitro-phenyl) undergo isomerization to N-aryl-2-phenyl-indenone imines 271 when heated in aprotic solvents202. Since in protic solvents, e.g. ethanol, only the iminoester 272 is isolated, evidence seems to be given for the intermediacy of 269 implying carbene and ketene imine functionality, which may either cause electrophilic ring closure with a phenyl group to form 271 or may add to the hydroxylic solvent (272). 

Oxidation of diphenyl or di-tert. butyl cyclopropenone with wi-chloro peroxy-benzoic acid207 proceeds via intermediates corresponding to a Bayer-Villiger-type mechanism 277/278) to unrearranged products (1,2-diketones) or rearranged products (ketones) depending on the reaction conditions. 

The reaction with hydroxide ion is frequently used as proof for the chemical structure of cyclopropenones and has been examined in some detail with respect to the factors governing ring-cleavage. Thus, methyl cyclopropenone23 and aqueous NaOH react to yield a mixture of methacrylic and crotonic acids in a ratio of 3 1 as expected from the relative stabilities of the two possible intermediate carbanions (type 317) ... 

A rather complex reactivity towards the cyclopropenone system is exhibited by N-nucleophiles. Thus, ammonia reacts with diphenyl cyclopropenone to yield either the enamino aldehyde 323222> or a mixture of the cis and trans isomeric diphenyl azetidinones 522223 depending on the reaction conditions these products result from primary addition of the nucleophile at C,(2 ... 

This is further accentuated by the surprising results of the reaction between aziridine and diphenyl cyclopropenone which was elucidated by Dehmlow224. In aprotic media two molecules of aziridine react with a cyclopropenone moiety eliminating ethylene and forming enamino amide 527, whereas in protic media one molecule of aziridine reacts with the exclusive formation of the aziridide 326 ... 

Similiarly, cyclopropenone reacts with Grignard s compounds via conjugate addition, which is followed by an ene -reaction of intermediate 341 with a second cyclopropenone moiety (342) leading to 2-substituted resorcinols20... 

Finally, a reaction should be mentioned in which a nucleophile gives support to another reacting species without appearing in the final product. Diphenyl cyclopropenone interacts with 2,6-dimethyl phenyl isocyanide only in the presence of tri-phenylphosphine with expansion of the three-ring to the imine 344 of cyclobutene-dione-1,2229,230 Addition of the isocyanide is preceded by formation of the ketene phosphorane 343, which can be isolated in pure formss 231 it is decomposed by methanol to triphenyl phosphine and the ester 52. 

In the reaction scheme (formulated abo e for enamines) the primary formation of an acyl ylide 369 (the formal product of addition of the enamine sequence C=C-N to the C /C3 bond of cyclopropenone) was first suggested by Dreiding237. ... 

The bicyclic enamine 383 deviates from the above reaction scheme when interacting with diphenyl cyclopropenone the betaine 384 formed initially does not iso-merize to the amide 385, but to the a-amino cyclopentenone 386, possibly favored by steric reasons248. ... 

N-Heteroaromatic compounds like pyridine, pyridazine, pyrazine, isoquinoline, and their derivatives42,250 react with diphenyl cyclopropenone in a formal (3+2) cycloaddition mode to the C=N bond of the heterocycle. As expected from the results discussed earlier (p. 67), the reaction is initiated by attack of nitrogen at the cyclopropenone C3 position and followed by stabilization of the intermediate betaine 390 through nucleophilic interaction of the Cl/C3 bond with the activated a-site of the heterocycle, giving rise to derivatives of 2-hydroxy pyrrocoline 391—394). In some cases, e.g. diphenyl cyclopropenone and pyridine42, further interaction with a second cyclopropenone molecule is possible under the basic conditions leading to esters of type 392. 

Arylidene alkylamines and diphenyl cyclopropenone gave rise to products 397-399, whose formation can be interpreted by means of oxidative secondary reactions of the 5 H-A2-pyrrolin-4-one 396 (R2 = H) initially generated252. ... 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

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. 

In further agreement with cyclopropenones, primary adducts 548 may use a second pattern of stabilization, as observed in the reactions of diazoalkanes with... 

This unexpected expansion of the triafulvene skeleton to a four-membered ring system presents further evidence in support of the reaction scheme of triafulvenes toward ylides 427 suggested for cyclopropenones (p. 81). 

Berson and colleagues119 re-examined the Diels-Alder reaction between 1,3-diphenyli-sobenzofuran and cyclopropenone. They selectively obtained the exo adduct, as was confirmed by X-ray analysis. Ab initio calculations indicated a kinetic preference for the exo isomer due to stabilizing interactions between the ether oxygen and the carbonyl carbon in the exo transition state120. 

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