Cyclopropenylium ion

In contrast to the cyclopropenylium ion, which has been known for a long time, its phosphorus analogue was first synthesized just recently. The phosphirenyHum cation, 32, can be obtained by addition of the strong Lewis acid B(OTf)3 to a solution of the IH-phosphirene, 31, in liquid SO2 [77] (Scheme 19). 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

Bordwell, 1988), contributed to the successful isolation of the first hydrocarbon salt [l 2 ], when it was combined with Agranat s cation, tris(3-guaiazulenyl)cyclopropenylium ion [1" ] (Agranat and Aharon-Shalom, 1976). 

Similarly, three dissociative hydrocarbons [25-2], [99-2] and [100-2], whose structures were determined from their nmr spectra, have been prepared from substituted cyclopropenylium ions phenyl- [25 ], m-methylphenyl- [99 ] and m,m -dimethylphenyl- [100 ] dicyclopropylcyelo-propenylium ions (Seheme 2). These hydrocarbons reversibly dissociate (24) into their ionic moieties in DMSO (Takeuchi et al., 1993). 

In view of the ease of dissociation of the a bond into resonance-stabilized hydrocarbon ions, it is expected that an authentic hydrocarbon salt could be isolated from an extraordinarily stable cation and anion. The first synthesis of a salt composed solely of carbon and hydrogen was achieved on the combination of tris(3-guaiazulenyl)cyclopropenylium ion [1" ] and tris(7//-dibenzo[c,g]fluorenylidenemethyl)methanide ion [2 ] (Okamoto et al., 1985). 

The interesting zwitterionic compound (39) with the cationic component a butadien-2-yl cation was obtained by reaction of l,4-di(t-butyl)butadiyne with 2 mol of di(r-butyl)aluminium hydride, with the structure being established by X-ray analysis.80 The reactions of the l,2-diferrocenyl-3-(methylthio)cyclopropenylium ion with carbanions derived from active methylene compounds were investigated.81 Products were derived by ring opening of the cyclopropene ring after the initial carbanion addition. The bis(ferrocenylethynyl)phenylmethylium cation (Fc-C=C-)2C+Ph (Fc = ferrocenyl) was prepared 82 This cation proved to be much less stable than its bis-ethenyl analogue (Fc-CH=CH-)2C+Ph. 

The spectrum of furan18 shows, besides the molecular ion peak (M+), a very pronounced peak of the cyclopropenylium ion [1] at mass 39. This is in accordance with the rather low aromatic character of furan in comparison with other heterocyclics, facilitating its... 

The trithiodeltate dianion (185) in its phosphonium salt, (Ph3MeP)2 (c-C3S3) 3H20, is very close to symmetry and is aromatic (see 185a) The mean C—C bond length of 1.405 A is longer than in cyclopropenylium ions the C—S bond length, 1.676 A, indicates partial double-bond character" ... 

Derivatives of the cyclopropenylium ion have also been studied intensively. A few examples may serve as illustrations. For the open shell system C4H4, which is one of the major primary fragment ions produced in the unimolecular dissociation of many excited precursors, it is now well established that there exist at least two stable species, the most stable one may have the methylenecyclopropene structure (213) and the less stable one may be an acyclic species, perhaps vinylacetylene (214). 

The methyl-substituted cyclopropenylium ion (215) was suggested to be generated from various 4118 precursors, dissociating via consecutive losses of H and... 

For the next higher homologue, 41150, in addition to the species 154-156 (Figure 4), three further stable structures (220-222) could be identified by means of their characteristic unimolecular and collision-induced dissociations quite surprising is the finding that the methoxy-substituted cyclopropenylium ion (222) is by 9 kcal moP less stable than the methoxypropargyl cation (220), which is a reversal of stabilities observed for the C3H3 system (i.e. 204 and 207). 

The base peaks in the 70 eV electron impact mass spectra of 3,3-dimethylcyclopro-pene (223) , 3-substituted 1,2-diphenylcyclopropenes (224), 3-thiocyanato-1,2,3-triphenylcyclopropene (227) and tri-t-butylcyclopropenyl azide (229) just to mention a few examples, are often found to correspond to the loss of the (3)-substituent, thereby generating the substituted cyclopropenylium ions 215, 225, 226, 228 and 230, respectively (Scheme 33). It should be mentioned, however, that there exists at least one report describing the complete absence of cyclopropenyhum-type ions in 70 eV mass spectra of 1,2-dialkyl-substituted cyclopropenes. The reported spectra indicate that allylic cleavage seems to be operative, although no further experimental results are known which would lend support to such a suggestion. 

Trapping of cyclopropenylium ions by anions represents a very common, and often remarkably regioselective, route to cyclopropenes 1. Further examples can be found in the literature given in Table 3. 

Cyclopropenylium ions were alkylated by reaction with a 1,3-diketone in the presence of triethylamine to give cyclopropenes 2, " ... 

Trisubstituted cyclopropenylium ions reacted with alcohols and thiols under controlled conditions to give mainly one cyclopropene 5. Under more vigorous conditions indenes were isolated the cyclopropcnes 5 rearrange to indenes on heating (see Section 2.B.3.2.2.). 

The cyclopropenylium ions were also trapped by Grignard reagents giving cyclopropencs 7 and... 

The reaction is thought to involve formation of a dihydrofurylcarbene 4, cyclization to a 2-oxabicyclo[3.1.0]hex-5-ene and then formation of 3 via the cyclopropenylium ion 5, which is quenched by water. 

Reaction of 3,3 -bicyclopropenyls 4 with bromine leads either to fragmentation to produce a cyclopropenylium ion or to ring opening to give trienes or cyclobutenes. ... 

The corresponding analog of cyclopropene 17 with methyl in place of hydrogen at C3 and 1,2-di- cr/-butyl-3-methyl-3-phenylcyclopropene do not react under these conditions, and a mechanism involving the formation and trapping of a cyclopropenylium ion by direct oxidation of the hydrocarbon, followed by fragmentation has been proposed. ... 

Oxidation of l,2,3-tri-tf>/-Cbutylcyclopropene (2) with 3-chloroperoxybenzoic acid in dichlo-romethane at room temperature gave di-/err-butylacetylene in a reaction shown to involve hydrogen abstraction from C3 and trapping of the cyclopropenylium ion so produced by per-oxybenzoate ion to give 3. ... 

Addition of cyclopropenylium ions to cyclopropenes gives benzenes, sometimes in very good yield, e.g. formation of l. ... 

Stereochemical and kinetic studies appear to be consistent with the first step being a simple cycloaddition, rather than involving a two-step process, initiated by loss of halide ion to give a cyclopropenylium ion pair. ... 

The reaction of cyclopropenylium ions with active methylene compounds in the presence of base provides a general method of preparing stable triafulvenes. Thus, the reactions of 1-alkoxy-2,3-diphenylcyclopropenylium ions (e.g., 10), obtainable from 2,3-diphenylcyclopropenone, with active methylene compounds in the presence of tertiary amines afford a variety of 1,2-diphenyl-3-(disubstituted methylenc)cyclopropcnes 11 in good yields. ... 

Bis(dialkylamino)methylcyclopropenyliumions (e.g., 18), prepared from the bis(dialkylamino)-cyclopropenylium ions by treatment with an alkyllithium reagent followed by iodomethane, are versatile synthons for a variety of diaminotriafulvene derivatives, including a diamino-triafulvene free of substituents at the exo-methylene carbon and triafulvenylium ions. ... 

Some other triafulvenylium ions have also been synthesized by using,or by the intermediacy of, cyclopropenylium ions. 

Carbenes have been fairly extensively used for the syntheses of cyclopropenes, and some of the cyclopropenes thus obtained have been transformed to triafulvene and related eompounds via the cyclopropenylium ions. Among many kinds of carbenes, isobutylidene and 2,3-diphenylcycloprop-2-enylidene " were direct precursors to triafulvenes. In the former case, simple tetraalkyltriafulvenes 31 were generated as labile substances. 

The reaction of l-chloro-2,3-bis(diisopropylamino)cyclopropenylium ion 24 with cyclopen-tadienide 23 (R =R = H) and indenide 23 (R R = CH = CH —CH = CH) gave rise to the 2 1 reaction products 25, probably via highly dipolar diaminocalicene intermediates. ... 

Two methods have been employed for the synthesis of quinocyclopropenes (a) short-step syntheses by the reaction of cyclopropenylium ions with electron-rich aromatics, followed by deprotonation with a base, and (b) multistep syntheses via cyclopropene derivatives, transformation to cyclopropenylium ions by ionization or hydride abstraction, and treatment with base. 

By Reactions of Cyclopropenylium Ions with Electron-Rich Arenes... 

Another heptatriafulvenequinone and a calicenequinone were similarly prepared as stable substances by the use of l-chloro-2,3-bis(diisopropylamino)cyclopropenylium ion 19. ... 

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