Cyclopropenones formation

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) ... 

Two other methods of cyclopropenone formation have become known although interesting in principle they have not yet found general application. 

Cyclopropenone formation should involve the bisketene 46 and its decarbonylation to the monoketene 47 (a valence tautomer of cyclopropenone 44), since photolysis in protic media like ethanol produces diethoxy diethyl tartrate 45) (meso and d,l). This method was also successful in the case of l,2-diphenyl-3,3-dichloro-cyclo-butene dione (48) giving rise to diphenyl cyclopropenone49 (but as for 44 only a moderate yield was produced) ... 

The elimination of carbon monoxide, ie. the (formal) reversal of cyclopropenone formation from divalent carbon species and alkynes, takes place when cyclopropenones are heated to higher temperatures (130—250 °C) or when subjected to photolysis or electron impact191 ... 

Bisketenes (11) can decarboxylate and then ring close to give cyclopropenones (12) subsequent further decarboxylation yields alkynes (13). A theoretical study shows that the first reaction is favoured by electronegative substituents, whereas electropositive substituents favour the second. The calculations do not indicate conclusively whether cyclopropenone formation is concerted, or proceeds via a syn-ketenylcarbene (14). 

Dimethyleneketene acetals 546 are easily available by a thermolytic rearrangement of methylenecyclopropanone ketals 544, in turn prepared from the corresponding cyclopropenone ketals [146a]. The thermolysis is a two-step process, involving the reversible formation of a dipolar TMM intermediate 545 [146b], followed by the irreversible production of stable 546 at higher temperature (Scheme 75) [145],... 

Furthermore, the addition of dichlorocarbene to ene-ynes proved to be remarkably sensitive to substituent effects. Trans-1,4-diphenyl butenyne gave only the cyclopropenone 17 via hydrolysis of dichlorocyclopropene 16, however, 2-methyl-pentene-l-yne-3 favored the formation of the dichlorocyclopropane 18 with only traces of products resulting from addition to the triple bond ... 

Application of the usual hydrolysis procedures to tetrachlorocyclopropene does not lead to the formation of dichloro cyclopropenone (26). This unstable compound is obtained, however, by a special procedure from trichloro cyclopropenium tetra-chloroaluminate (24) via the AlCl3-adduct 2532. ... 

In 1963 Breslow published a synthesis of cyclopropenones, which approached formation of the three-ring without the use of divalent carbon species41-45, a, a -Dibromo ketones 36, in general readily available from ketones R-CH2—CO—CH2—R, are de-... 

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) ... 

This principle of formation proved to be general for quinocyclopropenes of type 61a ( phenylogous cyclopropenones )- The required p-hydroxy-phenyl cyclopropenium cations were available by electrophilic substitution of phenolic components (preferentially 2,6-disubstituted) and heterosubstituted cyclopropenium cations (75 and 109), as the representative examples 11099 11176 112and 113 show ... 

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... 

C42 or in boiling toluene194 causes dimerization to spirolactone 257 (R = C6H5)195> 196. The formation of 257 can again be understood as an addition of one molecule of cyclopropenone through the C -CO bond to the C=0 group of a second molecule ... 

Dimerization of cyclopropenones has also been found to occur under reductive conditions. Tetraphenyl resorcinol is formed in addition to a small amount of tetra-phenyl p-benzoquinone on treatment of diphenyl cyclopropenone with aluminum amalgam200 its formation can be rationalized via dimerization of the cyclopropenone ketyl 266 and subsequent aromatization, possibly according to a prismane mechanism. 

The thermal cycloreversion of imines 268, i.e. formation of isocyanide and alkyne, which would be expected by analogy with cyclopropenone decarbonylation and in reversal of cyclopropenone imine formation (see p. 25), was found to be only a minor side-reaction203). 

Hydrogenation of di-n-propyl cyclopropenone with Pd/C catalyst, however, gave rise to 2-propyl-2-hexenal (287) as a major product according to attack of H2 at the cyclopropenone CVC3 bond43. A cyclopropanone could not be detected spectroscopically in any case. The formation of diphenylcyclopropanol 283 reported for... 

Electrophilic attack on cyclopropenones takes place at carbonyl oxygen, as indicated by the formation of hydroxy cyclopropenium cations on protonation (see p. 28). Hydrogen-bonded complexation between the carbonyl oxygen of diphenyl cyclopropenone and the O-H hydrogen in water212 and substituted acetic acids213 is reported to give rise to well-defined 1 1-adducts (296). 

Diphenyl cyclopropenone is transformed to the dichloride 154 under very mild conditions on treatment with oxalyl chloride115 a reasonable mechanism implies primary formation of an O-acyl cation 297 suffering fragmentation by loss of carbon monoxide und dioxide ... 

Cyclopropenone itself reacts with trifluoroacetic anhydride197 to the acylal 299, whose formation is easily understood by initial acylation to 298, which subsequently adds trifluoroacetate. 

Phenyl aryl cyclopropenones16 were cleaved by methanolic KOH to a mixture of cis aryl cinnamic acids (318/319 R = phenyl, R = aryl), whose rates of formation gave rise to a linear Hammett-type correlation with a values in the range of -0.268 to +0.373 and p = 0.75. This also indicates that cleavage yielding the more stable carbanion is preferred. Interestingly, ortho-substituted aryl phenyl cyclopropenones gave only a-phenyl-0-aryl acrylic acids (319 R = phenyl, R = aryl), which was explained in terms of steric interactions. 

However, ethyl phenyl cyclopropenone (14) was found to give the 0-keto amide 324 with ammonia in the presence of oxygen209 and this demands primary attack of NH3 at C3. Apparently product formation is influenced — in a fashion not yet com-... 

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 ... 

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. ... 

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. ... 

An analogous diazoketone formation is observed (409) with cyclopropenone and diphenyl ketene28. ... 

Analogously, the mesoionic jV-methyl thiazol-5-ones and l,3-dithiol-4-ones afforded A-methyl-4-pyridones and thiapyran-4-ones when reacting with diphenyl cyclopropenone and its thione261. Benzonitrile oxide apparently gives a 1,3-dipolar cycloaddition to the C=0 group of diphenyl cyclopropenone rationalizing the formation of triphenyl-l,3-oxazin-6-one 41626i ... 

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 ... 

Y (as confirmed by results with triafulvenes (see p. 101)). With phosphonium ylides 2-pyrone formation competes with Wittig olefination of the cyclopropenone carbonyl group. 

Interestingly, cyclopropenone exhibited comparable reactivity towards sulfur ylide 437 and phosphorus ylide 439 giving rise to 6-phenyl-2-pyrone and a-naphthol, respectively197. Again, the intermediacy of ketenes 438f440 may reasonably explain the formation of these products. 

Allyl pyridinium betaines 441 isoelectronic with enol betaines 427 likewise reacted with diphenyl cyclopropenone by elimination of pyridine272,213 The product formation, different in aprotic and protic media (phenol 443 in aprotic solvent, A3,5-hexadienoic esters 445 in alcohol solvent), suggested that the diene... 

Diazoalkanes add to l,2-diphenyl-4,4-diacetyl triafulvene 180) by analogy with diphenyl cyclopropenone (p. 79) across the CVC2 bond, as the formation of 4-diacetylmethyl-3,5-diphenyl pyridazines 547 certifies302. The bicyclic azo compound... 

The formation of pyrazolenines demands, as in the cyclopropenone series, the intermediacy of conjugated diazo compounds, e.g. 549, arising from valence tautomeriza-tion of diaza bicyclo(3,l,0)hexanes, e.g. 548. 

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. 

---