Cyclopropenium cation

WIU1AM L JORGENSEN AND LIONEL SALEM 37. Cyclopropenium Cation Symmetry D3h... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

Finally, the alkylation could be extended to vinyl halogen compounds38 and in this way, bis(trichlorovinyl)cyclopropenone 30) was prepared from cation 24 and trichloroethylene. A recently reported variation of the West procedure40) makes use of the difluoro chloro cyclopropenium cation 33. 

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

Li, Na, or Grignard compounds of cyclopentadiene, indene, or fluorene add to di-substituted cyclopropenium cations forming cyclopentadienyl cyclopropenes, which can be transformed to calicenes by subsequent hydride abstraction and deprotonation, as shown by the following examples 9S91 and 9992 (cf. p. 13) ... 

The first stable derivative of methylene cyclopropene was the quinocyclopropene 108 reported in 1964 by Kende98 it was prepared from the cyclopropenium cation 106 which underwent pyrolysis and bromination with NBS to the p-hydroxy-phenyl substituted cation 107, which gave quinocyclopropene 108 by deprotonation ... 

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 ... 

The cyclopropenium system m y combine with a large number of donor groups, mainly heterocyclic and carbocyclic systems to give cyclopropenium cyanines. En-amines or ketene acetals of appropriate basicity, e.g. Fischer Base (135) or 134, can be easily cyclopropenylated by the ethoxy cyclopropenium cation 75 as well as 2- or 4-alkylsubstituted heterocyclic quartemary salts of pyridine, quinoline, and benzothiazol in the presence of a tertiary base76,10s ... 

The cyanosubstituted diphenylcyclopropenyl heptafulvenylium cation 144 (R = CN) was recently synthesized by Kitahara110 from 8-cyano heptafulvene and ethoxy cyclopropenium cation 75 and its structure has been proven by X-ray analysis111. ... 

A final type of cyclopropenium cyanines is found in the azatriapentafulvalenium ions 145,146, and 147/148, which have been prepared93,112,113 from diphenyl and di-n-propyl cyclopropenone or ethoxy cation 75 and indole derivatives as well as from diphenyl methylthio cyclopropenium cation and phenyl-substituted pyr-roles114> ... 

The highly stable cyclopropenium immonium cations 153 are formed from the ethoxy cyclopropenium cation 75 with primary and secondary aliphatic or aromatic amines42-119> ... 

This effect might be due to the better stabilization of the protonated species 179 by electron-donating groups, which is parallelled in the increase of pKr+ of cyclopropenium cations when going from triphenyl to trialkyl substitution101. ... 

This lack of a major diamagnetism may be attributed to two factors. First even in the completely delocalized cyclopropenium cation the diamagnetic ring current effects are small and in the range of only 25% of those in benzene as concluded from NMR data146. ... 

In addition to the determined molecular geometry this is supported by 13C—NMR signals at -158.3 ppm (CVC2) and —155.1 ppm (C3) relative to TMS for comparison, cyclopropenium cation has its 13C-NMR signal at -174 ppm147 ... 

In diaryl cyclopropenones, the UV data are not very characteristic and resemble those of diaryl cyclopropenones and stilbenes as well as diaryl cyclopropenium cations, as shown in Table 8. Recently a new long-wave absorption band was found164) in diphenyl cyclopropenone [362 nm, log e = 3.06 (cyclohexane)] which was assigned tentatively to an intramolecular charge-transfer. 

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 ... 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

Selective reduction of the cyclopropenone carbonyl group to a CH2 group has been described for diphenyl cyclopropenone utilizing its protonation product 294 or the diphenyl chloro cyclopropenium cation 292, which yielded 1,2-diphenyl-A1,2-cyclopropene (293) on treatment with trimethylamine borane210 ... 

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). 

Alkylation of cyclopropenones — effected by means of trialkyloxonium tetra-fluoroborates42,119) — leads to the easily hydrolyzable O-alkyl cyclopropenium cations 295, which are potential sources for triafulvene synthesis ... 

Though O-acyl cyclopropenium cations have not yet been isolated, several cyclopro-penone reactions need to be interpreted via intermediary O-acylation. 

Reactivity of triafulvenes toward electrophiles and nucleophiles is determined by their clearly established (see p. 35) electron distribution. Thus, protonation of the intensely colored triafulvenes 480119 and 11575 — readily occurring when dissolved in CF3COOH — leads to colorless cyclopropenium cations 481/482 by attack of the electrophile at the exocyclic site of the triafulvene system. 

The 3H, 6Li and 13C NMR chemical shifts of the cyclopropenium cation and its lithium derivatives 115 118 were calculated both at GIAO-DFT and GIAO-HF level using B3LYP optimized geometries.143... 

It was shown that lithium is even more effective than the amino group in stabilizing the ethyl, vinyl, allenyl and cyclopropenium cations.144... 

Further information on the electron delocalization in 41 was provided by Katz and Gold. These workers pointed out that the UV spectra of alkyl-substituted derivatives of 41 were intermediary in position (/.max ca 250 nm) between that of corresponding allyl or cyclopropenium cations (Amax ca 300 and 185 nm, respectively)123. 

Notably, early attempts to similarly prepare cyclopropenyl complexes of group 6 molybdenum and tungsten, using [CpM(CO),] anions (M = Mo, W) and [C,(Bu-/),]BF4, resulted in the electrophilic attack of the cyclopropenium cation on the peripheral cyclopentadienyl ligand, to give hydride complexes (equation 196)270. These air-sensitive hydride complexes readily react with CC14, to afford the corresponding air-stable chloro complexes. 

In this section we will discuss charged silaaromatic systems which formally possess An +2 7r-electrons, such as 45-49. These species are isoelectronic with the cyclopropenium cation, tropylium cation and cyclopentadienyl anions, respectively, which are all well... 

The cyclopropenium cation (45, M = C) is a well established aromatic system with a high aromatic stabilization energy of 58.7 kcalmol-1 [equation 18, M = C, at B3LYP/6-311++G(2d,2p)]70. What happens when one carbon atom is substituted by silicon At HF/STO-2G the monosilacyclopropenium cation, 46, has a rather delocalized structure with r(C-C) = 1.393 A, r(C-Si) = 1.722 A and with CSiC and SiCC bond angles of 46.3° and 66.9°, respectively413. However, according to equation 19 which measures the effect of delocalization, 46 is destabilized by 11 kcalmol-1 (HF/3-21G//HF/STO-2G) while 45, M = C is stabilized by 36 kcalmol-1. Unfortunately, this study used a very low computational level and should be repeated using more reliable methods. 

---