Tropylium chloride

Absolute ethanol (400 ml.) is vigorously stirred in a 1-1. widenecked Erlenmeyer flask immersed in an ice bath (Note 4). The tropylium hexachlorophosphate-tropylium chloride double salt4 is separated from the reaction mixture by suction filtration, washed briefly with fresh carbon tetrachloride, and transferred as rapidly as possible into the cold, well-stirred ethanol (Note 5). The salt dissolves rapidly and exothermally to give a reddish solution. Fifty milliliters (0.39 mole) of 50% aqueous fluoboric acid is added rapidly to the cold stirred solution (Note 6). The dense white precipitate of tropylium fluoborate that forms is separated by suction filtration, washed with a little cold ethanol and with ether, and air-dried at room temperature (Note 7) weight 34-38 g. (80-89%) decomposition point about 200° A j, HC1 218 mja (log e 4.70), 274 m/i (log e 3.61). The product is 98-100% pure (Notes 8 and 9). 

In another paper Abraham provided a thermodynamic analysis that placed f-Bu Cl and the separated ions at about the same free energy, 14.5 kcal/mol above the reactant. This implies a AG of about 5 kcal/mol for collapse of the contact ion pair back to reactants, although the uncertainties in Abraham s analysis are at least 5 kcal/mol. Few related data are available, except some dynamic NMR results for the collapse of trityl and tropylium chloride ion pairs. " In both the NMR work and Abraham s analysis, solvent-separated ion pairs are not included as distinct entities. However, the interconversion of contact and solvent-separated ion pairs has been observed in ultraviolet studies of delocalized carbanions in THF. ... 

Joshi el al.15 have now been able to isolate the adduct (9) of phenanthrene with dichlorocarbene, generated in the presence of cetyltrimethylammonium chloride. The adduct rearranges at its melting point (140°) to 6-chlorodibenzo[a,<-]tropylium chloride... 

In an improved process for the synthesis of tropilidene (5) by E. Muller, a solution of diazomethane in benzene is added gradually to refluxing benzene containing cuprous bromide as catalyst. Benzene is used in large excess, and the product is isolated most easily by filtering the solution from the catalyst and adding it to a solution of phosphorus pentachloride in carbon tetrachloride. The tropylium chloride which separates is dissolved in water and treated with perchloric acid to afford tropylium perchlorate in 85% yield. The success of the method is attributed to formation of the intermediate (3), a deactivated electrophilic carbon metal complex. Tropilidene... 

Problem 8.20 Give plausible explanation for the following facts. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization of monomers such as isobutene and styrene, but t-butyl and cumyl chlorides are effective. On the other hand, triphenylmethyl chloride and cyclo-heptatrienyl (tropylium) chloride are not very efBcient in polymerizing isobutylene and styrene but produces rapid polymerization of p-methoxystyrene, vinyl ethers and N-vinylcarbazole. 

The result is calculated taking into account several interfering reactions and some impurity in the sample. It is calculated on the basis of hydrogen absorbed. The reactant is unstable in air. Both this and the following reaction amount to hydrogenation of the tropylium ion and a solvated proton. The heat of formation of dissolved tropylium chloride is 19.8 kcal mol 1. 

HuckeFs rule leads one to expect resonance stabilization in the cyclopentadienate and cycloheptatrienyl (tropyllum) ions. With a view to determining the resonance energy of the latter, Turner has measured the heat of hydrogenation of tropylium chloride to cycloheptaiie and hj drogen chloride, in acetic acid solution, ag — 86 23 rt 0 08 kcal/mole. An energetic comparison of the relative stabilities of tropylium chloride and the isomeric benzyl chloride... 

Heat of isomerization of tropylium chloride to benzyl chloride (kcal/mole) (taken from Turner, ref. 21). 

Tropone allowed to react with phosgene or oxalyl chloride, the resulting chloro-tropylium chloride dissolved in acetic anhydride, ice-cooled, and treated dropwise with the equimolar amount of 70%-HC104 dilorotropylium perchlorate (Y 80-90%) stirred 2 hrs. at room temp, with excess dry isopropanol -> isopropoxy-tropylium perchlorate (Y 81%). F. e., also substitution with mercaptans and amines, s. E. Haug and B. Fohlisch, Z. Naturf. 24b, 1353 (1969). 

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Phosphorus pentachloride, for conversion of pentaacetylgluconic add to add chloride, 41, 80 for oxidation of cycloheptatriene to tropylium fluoborate, 43, 101 with cyanoacetic acid, 41, 5 Phosphorus tribromide, reaction with 1.5-hexadien-3-ol, 41, 50 Phthalic anhydride, reaction with L-phenylalanine to yield N-phthalyl-L-phenylalanine, 40, 82 Phthalic monoperacid, 42, 77 N-Phthalyl-i.-alanine, 40, 84 N-Phthalyl-/3-alanine, 40, 84 N-Phthalylglycine, 40, 84 N-Phthalyl-l-/5-phenylalanine, 40, 82... 

In the presence of aluminum chloride, tricyclic tropones 234a-c (Scheme 57) are easily reduced by complex hydrides to exceed the tropol stage and to afford tropilidenes 235a-c. Hydride abstraction (Section II,C,l,a) leads to tropylium salts 236a-c, but 236a could not be obtained in pure form [83CS(22)53]. 

Other alkyl groups are more difficult to remove. Begtrup has recorded examples of dealkylation of 1,2- and 1,3-dialkyltriazolium salts when they are heated, or treated with benzoyl chloride. 1-Glycosyltriazoles can be cleaved by heating with hydrochloric acid. The cleavage of 1-cycloheptatrienyltriazoles (Scheme 52) is also brought about by acid and is probably facilitated by the stability of the tropylium ion. ... 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

There is direct evidence, from ir and nmr spectra, that the f-butyl cation is quantitatively formed when f-butyl chloride reacts with A1CI3 in anhydrous liquid HCI.246 In the case of olefins, Markovnikov s rule (p. 750) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride247 and 1-chloroadamantane248 alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole but not benzene.249 It was noted on p. 337 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates.250... 

Aryl tellurium trihalides accept halide ions from tropylium bromide, ammonium halides, phosphonium halides, arsonium halides, sulfonium chlorides, telluronium chlorides, and diphenyl iodonium chloride to form aryltetrahalotellurates(lV)1-5. 

Apart from the di- and oligoolefm iron tricarbonyl complexes, which nowadays are frequently used in organic synthesis [71, 72], the chemistry of the readily accessible cyclohepatriene chromium and molybdenum tricarbonyls 2 and 3 was the focus of intense research efforts as well. Only a few months after the synthesis of 2 and 3 was published [58,59], both Hyp Dauben and Peter Pauson reported that these compounds react with triphenylmethyl tetrafluoro-borate in methylene chloride to give the tropylium complexes 4 and 5 in excellent yield (Scheme 7.1) [73, 74]. Later this method of hydride abstraction was also used for the preparation of the tropylium cation itself and subsequently led to the generation of several cationic rc-complexes of iron, manganese and cobalt [71, 72], The reactions of the cations of 4 and 5 with nucleophilic... 

The importance of the bulkiness of the two addend moieties in the preferred formation of either the 1,2- or the 1,4-adduct (cf. Figure 1.12a) was nicely evidenced by work of Komatsu and co-workers who reacted the oct-1 -ynyl[60]fullerenide ion with different electrophiles.306 Whereas treatment with methyl iodide afforded only the 1,2-adduct, the addition of tropylium tetrafluo-roborate gave a 55 45 mixture of 1,2- and 1,4-adduct (( )-137, Figure 1.35), and the 1,4-addition only (product ( )-138) was observed in the reaction with benzoyl chloride. 

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