Isolation of hydrocarbon salts

A few observations of the isolation of carbocation-carbanion salt containing heteroatoms, such as oxygen and nitrogen, have been reported. Over 30 years ago LeGoff and LaCount (1963) reported the formation of a 

Theoretical calculations by Mixon and Cioslowski (1991) suggested that a push-pull substituted ethane (H2N)3C—C(N02)3 exists as an ion pair (H2N)3C C(N02)3. Recently, a similar hexamethylguanidinium- 

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). 

Four years after Doering s discovery of the hygroscopic bromide of the tropylium ion [3 ] (Doering and Knox, 1954), Vol pin et al. (1957) reported that the tropylium ion (pXr+ 3.88) gives a covalent compound with acetate ion (pXb 9.24) but not with isocyanate ion (pK 10.1). This suggests that an anion with pXb greater than 10.1 would give a salt with the tropylium ion. 

When a comparable examination was made with 1,3,5-tricyclo-propylcyclopropenylium ion [24+] (pKr+ 10.0), the transition from covalent compound formation to no reaction in water was observed between the nucleophiles with the pXb values 4.79 (CN ) and 6.85 (p-nitrophenoxide ion) (Komatsu et al., 1980). As a corollary, it is expected that Kuhn s anion [2 ], whose pXb value (8.1) is greater than that of the p-nitrophenoxide ion, would give a stable salt when combined with the carboeations with a pKr+ value 9 or greater. 

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The synthetic procedures for isolation of the salt appear to be rather simple. First, one prepares a solution in which the carbocation and carbanion coexist free from any combination reactions. Then, the hydrocarbon cation-anion salt is isolated after separation of the concomitant inorganic salt and evaporation of the solvent. For the purification of the crude salt recrystallization or reprecipitation with proper solvents is used. 

An example of planar anions (Knhn s anion) Okamoto, K. Kitagawa, T. Takeuchi, K. Komatsn, K. Kinoshita, T. Aonnma, S. Nagai, M. Miyabo, A. Isolation and properties of hydrocarbon salts. J. Org. Chem. 1990, 55, 996-1002. 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

The Stability of hydrocarbon ions is discovered intuitively by observing whether the hydrocarbon ion can be isolated as a salt, for example, a sodium salt of the carbanion or a tetrafluoroborate salt of the carbocation. Conversely, a single hydrocarbon ion produced in the gas phase is obviously an unstable and short-lived species. Thus, many of the aliphatic carbocations in the gas phase are merely observable species but are not usable for synthesis. 

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). 

The solution of Agranat s cation [1 ] instantly gave black precipitates of mixed KCIO4 and [l 2 ]. The crude hydrocarbon salt was isolated by recrystallization from DMSO as greenish black needles. 

The distinguishing feature of the salt [28 2 ] is most evident in THF. As shown in Fig. 7, the anion [2 ] reacts rapidly with the cation [28 ]. However, the concomitant formation of the radical [2-] in smaller amount than the consumed anion is observed spectrophotometrically. The difference between the amounts of consumed [2 ] and of produced [2-] is attributed to the formation of the covalent hydrocarbon [28-2] in THF solution. Furthermore, the dimer [28-28] of 1,3,5-tricyclopropyltropyl radical [28-] is isolated in 20% yield from the reaction mixture after 48 h at room temperature. 

These effects are particularly pertinent in type (I) -> (II) reactions, where the high anion activity is required to overcome the lattice energy of the non-molecular precursor (I). Thus many non-molecular metal halides, sulfides and thiolates which are insoluble and unreactive in aqueous media will dissolve with anionic ligands in aprotic solvents, and clusters can be isolated from the resulting solutions. Anion activation by use of cation complexands to solubilize salts in hydrocarbon solvents is advantageous, as is the use of complexands in the isolation of polymetallate anions.342... 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

The use of ethyl a-bromopropionate simplifies the preparation of a-(isopropylideneaminooxy)propionic acid. Resolution in ethyl acetate solution has proved less erratic than in the hydrocarbon solvents previously recommended, and the isolation of both diastereoisomeric salts formed is facilitated. TAPA has found use in the resolution of polycyclic aromatic compounds that do not possess functional groups that would permit resolution by other methods. ... 

Mechanistic information on the photolytic Bamford-Stevens reaction is provided by the successful isolation of a diazo hydrocarbon as a reaction intermediate from the direct photolysis of a tosylhydrazone sodium salt (90 Scheme 10). This study also clarifies that the carbene derived thermally and that derived photolytically behave differently with respect to the stereoselectivity of the 1,2-hydrogen shift that produces the alkene. 

Azulenes, which are intensely blue, can be extracted from petroleum ether with 62% sulfuric acid as the colorless conjugate acid sulfates. Corresponding salts with picric acid, which are intensely colored, are useful for isolation of the hydrocarbons. Thus the guaiazulene present in an oily mixture obtained by dehydrogenation of technical guaiene with sulfur can be isolated by treating the oil with a hot saturated... 

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