Toluene radical anion

ABSTRACT. Toluene radical anion, generated by dissolving potasssium metal in toluene by the assistance of dicyclohexano-18-crown-6, has been proved to be especially effective for reductive removal of fluorine atom from unactivated alkyl fluorides that resist common reduction conditions. Stereochemical and mechanistic aspects of the present method is discussed. In connection with the preparation of substrates the effect of dipolar aprotic solvents on the nucleophilic fluorination with potassium fluoride/dicyclohexano-18-crown-6 system was also examined, and sulfolane or N,N-dimethylformamide was shown to be a solvent of choice. 

These curious chemical entities led us to study the reactivity of potassium anion (K ) generated from K/crown ether/diglyme (or Et20, THF) system and toluene radical anion from K/crown ether/toluene system. The effectiveness of the former system for reductive cleavage of sulfonamide was reported elsewhere- -. In the present paper, reductive defluorination with the latter system is reported- -. ... 

It can be reasonably stated from the result in Table II that i) toluene is a better solvent than diglyme, proving the effectiveness of toluene radical anion for this type of reduction, ii) the presence of protic solvent depressed the reduction and iii) slightly more than one molar equivalent amount of crown ether is required for the completion of the reduction. 

Conclusively, K/DC-18-C-6/toluene was confirmed to be a stronger reducing system for this type of reduction than Li/liq NH, K/HMPT (with or without H" " and/or THF) or Na naphthalenide in dimethoxyethane. The interesting point is that the rather stable radical anion system (Na naphthalenide) is weaker than the labile counterpart (toluene radical anion). 

K solution is black-blue and toluene radical anion solution is brown-red in color. 

Hitchcock PB, Lappert MF, Protchenko AV. The first crystalline alkali metal salt of a benzenoid radical anion without a stabilizing substituent and of a related dimer X-ray strucmres of the toluene radical anion and of the benzene radical anion dimer potassium-crown ether salts. Am Chem Soc. 2001 123 189-190. 

The sonochemistry of the other alkali metals is less explored. The use of ultrasound to produce colloidal Na has early origins and was found to greatly facilitate the production of the radical anion salt of 5,6-benzo-quinoline (225) and to give higher yields with greater control in the synthesis of phenylsodium (226). In addition, the use of an ultrasonic cleaning bath to promote the formation of other aromatic radical anions from chunk Na in undried solvents has been reported (227). Luche has recently studied the ultrasonic dispersion of potassium in toluene or xylene and its use for the cyclization of a, o-difunctionalized alkanes and for other reactions (228). 

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

As in the case of hexafluorobenzene solvent anion, EPR and ODMR spectroscopies suggests that no dimerization of monomer radical anions of benzene and toluene occur in liquid benzene and/or in alkane solutions of benzene (whereas the radical cation of benzene is known to dimerize rapidly). The conductivity studies also indicate that there is no volume change associated with the dimerization [45]. 

Photo-stimulated reactions of neopentyl iodide with several carbanionic nucleophiles have been studied in which inhibition experiments with the TEMPO radical trap suggest the reaction occurs via an SrnI mechanism.76 Comparison of 22 nucleophiles in then. Srn 1 reactions with iodobenzene by Fe(II)- and photo-induction has revealed that both are enhanced by high electron-donation ability of the nucleophile. The radical anion Phl is a key intermediate.77 The SET reactions of perfluoroalkyl iodides have been reviewed.78 Flash photolysis of H2O2 was used to generate HO and 0 radicals which were reacted with a, a. z-trifluorotolucnc (TFT) and 4-fluorotoluene (4FT) and the rate constants calculated.79 The diminished reactivity of TFT towards HO or O with respect to toluene or benzene was consistent with radical addition to the aromatic ring, whilst the reactivity of 4FT was of the same order as electron-deficient toluenes, which favour H abstraction from the aliphatic side-chain. 

The role of radical anions in the detonation of nitroaromatic explosives has been examined.215 The potassium salts of such radicals were formed by mono-, di-, and tri-nitrobenzenes and -toluenes in liquid ammonia solution and, on removal of the solvent, render the material highly susceptible to loss of the metal nitrite, which increases with nitro substitution. Cleavage of the C—N02 M+ bond follows the regioconserved or... 

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

In all examples discussed up to now the radical cation of Qo is involved in the reaction mechanism. However, due to the electronic features reduction of the fullerenes leading to radical anions should be much easier performed. For example, a useful method to synthesize 1-substituted l,2-dihydro-[60]fullerenes is the irradiation of Q0 with ketene silyl acetals (KAs) first reported by Nakamura et al. [216], Interestingly, when unstrained KAs are used, this reaction did not yield the expected [2 + 2]-cycloaddition product either by the thermal, as observed by the use of highly strained ketene silyl acetals [217], or by the photochemical pathway. In a typical reaction Q0 was irradiated for 10 h at 5°C with a high pressure mercury lamp (Pyrex filter) in a degassed toluene solution with an excess amount of the KA in the presence of water (Scheme 11). Some examples of the addition of KAs are summarized in Table 11. 

Surprisingly, in separate experiments, the removal of benzene and toluene from groundwater were relatively unaffected by solution pH [54], despite the fact that the groundwater contained carbonate/bicarbonate alkalinity. It was postulated that the carbonate radical anion [product of Eqs. (89) and (90)] may also degrade these solutes at high solution pHs. 

Phthalimides have often been used in the double role of electron acceptor sensitizers and radical traps, and are in fact benzylated by irradiation in the presence of toluene and several other precursors. 4,5-Dicyanophthalimide undergoes substitution of a benzyl for a cyano group when irradiated with toluene, but mainly attack at the imide carbon occurs with diphenylmethane. With various donors the competition between the two modes of reaction has been found to depend on the cage vs. out of cage radical cation cleavage, in the first case with the assistance of the radical anion [223] (Sch. 18). 

Toluene, durene, hexamethylbenzene, 1- and 2-methylnaphthalenes are oxidized to the corresponding benzaldehydes by irradiation in oxygen-equilibrated acetonitrile sensitized by 1,4-dicyanonaphthalene, 9-cyano-, 9,10-dicyano-, and 3,7,9,10-tetracyanoanthracene. The reaction involves proton transfer from the radical cation of the donor to the sensitizer radical anion or the superoxide anion, to yield the benzyl radical which is trapped by oxygen. In the case of durene, some tetramethylphthalide is also formed with this hydrocarbon it is noteworthy that the same photosensitization, when carried out in an nonpolar medium, yields the well-known singlet oxygen adduct, not the aldehyde [227,228] (Sch. 20). 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

It has been suggested that the ketone enolate ions 27 initiate the photostimulated cycle. The coupling reaction of the phenyl radical with 31 is faster than with 27, forming a radical anion that fragments to give benzyl radicals, which are reduced to toluene or react with 31 to afford a new radical anion (161) , responsible for forming the substitution product 161 (equation 105). 

Consistently, the PIA spectra of toluene solutions containing MP-Ceo and OPVn (n = 2, 3 or 4) in a 1 1 molar ratio, recorded using selective photoexcitation of MP C60 at 528 nm (Fig. 1.28b), invariably exhibit an absorption at 1.78 eV with an associated shoulder at 1.54 eV, characteristic of MP-C6o(7i) [103]. The monomolecular decay (—AT oc Ip, p = 0.89-0.96) with lifetime 150-260 ps associated with these PIA bands supports this assignment. Furthermore, weak fullerene fluorescence at 1.73 eV (715 nm) is observed under these conditions for all three mixtures. No characteristic PIA bands of OP Vw+ radical cations or MP-Cg0 radical anions are discernible under these conditions. From these observations we conclude that electron transfer from the ground state of the OPVn molecules to the singlet or triplet excited state of MP-Cgo does not occur in toluene solution. 

Tellurium, potassium hydroxide, water, phenylacetylene, a trialkylmethylammonium chloride as phase-transfer catalyst, either tin(IJ) chloride or hydrazine hydrate as reducing agent and toluene as the organic solvent upon heating at approximately 100 for six hours produced Z,Z-bis[2-phenylvinyl ditellurium in low yields. Polytelluride radical anions were postulated as intermediates2 3. 

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