Cyclopropenylium

Tlie smallest compounds among the azafulvalenes described to date are the azacalicenes, which are formed by combining a cyclopropenylidene moiety with a nitrogen-containing cyclopentadienylidene. Tlius, heating di-or triphenylpyrroles with diphenyl-methylsulfanyl-cyclopropenylium perchlorate in acetic acid gives the aza-triafulvalenium salts 33 and 34... 

Another convenient approach to the azapentatriafulvalene system is given by the in situ formation of cyclopropenylium salts from cyclo-propenones and dry HCl gas followed by their electrophilic attack on various indoles. Tire corresponding heterofulvalenium salts of type 35 were isolated as chlorides, which were somewhat photosensitive and thermally labile (68TL5537). 

Tris(t r/-butylsulfanyl)cyclopropenylium perchlorate (17) behaves as a bi-electrophile towards benzene-1,2-diamines, yielding 3, 4-bis(tm-butylsulfanyl)-17/-l,5-benzodiazepines 18.267... 

Cyclopropenylium-Salze werden durch Lithiumalanat10,11 und Natriumboranat11 12 zu Cyclopropenen umgesetzt. 

Lithiumalanat reduziert in Diathylather auf ahnliche Weise 1,2,3-Triphenyl-cyclopro-penylium-bromid mit 60-70%iger Ausbeute zu l,2,3-Triphenyl-cyclopropen2. 1,2-Di-propyl-cyclopropenylium-perchlorat ergibt dagegen mit Lithiumalanat in Diathylather bei 0° neben 27% d.Th. Octin-(4) 3% d.Th. 1,2-Dipropyl-cyclopropen3 ... 

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). 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

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). 

Cation 130 is of interest as a representative of the trimethine cyclopropenylium cyanine system 131. 

A variety of functionalized [3]radialenes have been prepared starting from the appropriately substituted cyclopropanes or cyclopropenes. West and Zecher have pioneered the chemistry of [3]radialenes with quinoid substituents. The general strategy of this synthesis is outlined in Scheme 513. A tris(4-hydroxyphenyl)cyclopropenylium... 

Carbene 2 was identified by comparison with the calculated IR spectrum.2411 At that time we were not yet able to carry out own calculations and therefore we gratefully accepted Prof. Schaefer s offer to help us in this respect. Cyclopro-penylidene (2) has, as predicted24 a singlet electronic ground state. The molecule can best be described as a cyclopropenylium ylide with the structure S-2. ... 

TRI- ferf-BUTYLCYCLOPROPENYL FLUOROBORATE (Cyclopropenylium, tri- er/-butyl tetrafluoroborate)... 

The pKR+ value for the parent tri(l-azulenyl)methyl cation (2a+) is 11.3. Hydrocarbon-based carbocations, which comprise only of carbon and hydrogen, are generally very reactive species. Some extremely stable hydrocarbon carbocations, which exist even under basic conditions, were reported in the literature (5). However, most of these examples are cyclic cations, such as cyclopropenylium or tropylium ions (Figure 8). The tropylium ion 8+ annelated to three bicyclo[2.2.2]octane units is one of the most stable hydrocarbon-based carbocation ever reported (9). 

The phosphirene cation (5) (with two resonance structures 5a and 5b) has been predicted to possess a Zji aromatic system for quite some time." The aromaticity of the corresponding cyclopropenylium cation (C3H3+) is well-known.Substituted derivatives of 5, however, have only recently been observed in solution and in complexed form. 5 is the global... 

B) mit 2,3-Diphenyl-l-ethoxy-cyclopropenylium-tetrafluoroborat gelingt die Herstellung von 4,5-Di-cyan-2-(cis-l,2-diphenyl-ethenyl)-imidazol (s.Bd. V/2c, S. 30)... 

Amino-anilino)-l, 2-diphenyl-cyclopropenylium-tetrafluoroborat [mesomer mit (2-Amino-phenyl)-(l,2-diphenyl-cyclopropenyliden)-ammonium-tetrafluoroborat] cyclisiert beim Ver-such der Deprotonierung mit molaren Mengen an Diisopropyl-ethyl-amin (Hunig-Base) unter Ringspaltung zu 2-[(Z)-l,2-Diphenyl-ethenyl -benzimidazol (89% Schmp. 273-2740)59 (vgl. 

Die Wahl des Losungsmittels bestimmt, welches Produkt bei der Cyclisierung von 1,2-Diamino-benzol mit Tris-[tert.-butylthio]-cyclopropenylium-perchlorat gebildet wird. In Methanol ent-steht 2-(1,2-Bis-[tert.-butylthio]-ethenyl)-benzimidazol (77%, tiber ein Allyl-Kation), im apro-tischen Dimethylformamid in einer inneren Michael-Addition 3,4-Bis-[tert.-butylthid -IH-l, 5-benzodiazepin (63% roh 100%)189. 

The structure of 2,3-dihydroxycyclopropen-l-one, deltic acid (184), which has been determined by XD at 135 K, reflects significant contributions from cyclopropenylium or dipolar resonance forms286. The difference between formally double bonds and single... 

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