Cyclopentadiene, carbon acidity-carbanion

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 10 K using 254 nm light. Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylation, and this is reported to take place from an upper triplet excited state." A study of the transient photochemistry of 5-(p-toluyl)-l-methyl-2-pyrrolylacetic acid has been reported. Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed. This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (181) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), respectively. ... 

As a result of its aromaticity, the cyclopentadienyl anion is an unusually stable carbanion. This is why cyclopentadiene has an unusually low pA a. In other words, it is the stability conveyed by the aromaticity of the cyclopentadienyl anion that makes the hydrogen much more acidic than hydrogens bonded to other sp carbons. 

Direct reaction between sodium or potassium and alkyl and aryl halides is complicated by exchange and coupling reactions, which can lead to mixtures of products. These complications can be reduced by rapid stirring and the use of finely divided metal or amalgams. Phenylsodium can be made in this way PhCl -h Na->PhNa-h NaCl. Acidic hydrocarbons react with alkali metals in ether solvents. Cyclopentadiene, for example, affords sodium cyclopentadienide in tetrahydrofuran (p. 279). Triphenylmethylpotassium is obtained as a deep red solution from triphenylmethane and potassium in 1,2-dimethoxyethane. These carbanions, in which the negative charge is delocalized over several carbon atoms, do not attack ethers, in contrast to the simple alkyl or aryl carbanions present in methylsodium or phenylpotassium. 

Concerted reactions are commonly used to join carbons. For example, the Diels-Alder reaction is the formation of a cyclohexene from a diene and an alkene. Usually the alkene is rendered electrophilic by conjugation with a carbonyl group, and the diene may be rendered nucleophilic by electron-donating substituents. In the case shown in Equation 7.38 the alkene is further electron depleted by association with a Lewis acid [64], a common technique for accelerating Diels-Alder reactions. In some cases, the alkene is nucleophilic and the diene is electrophilic as in Equation 7.39 [65]. Examples of this sort are called reverse-electron-demand Diels-Alder reactions. It is important to point out here that the concerted reactions differ from the foregoing in that no carbanion or cation intermediate is involved, and in many cases, electrophilic and nucleophilic factors are not present, as in the very favorable dimerization of cyclopentadiene. These reactions are covered in more detail in Chapter 5. 

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