Arylmethyl anion

Solvent can play a very significant role in the photoreactions of carbanions, affecting both the covalency of the carbon-metal bond (249) and ion pairing. Some solvents may, furthermore, be active participants, for example, in Tolbert s photo-methylation of arylmethyl anions, eq. 83 (250-253) ... 

By correlating the formation of an arylmethyl anion with the difference between its tt energy and that of the fragments, we obtain equation 16 for the anionic case This equation is formally identical to the result given for the cationic situation in equation 13. 

Figure 3. Correlation of the relative rates of formation of arylmethyl anions with calculated SCF energy differences...

An additional stabilization of the negative charge provided by the adjacent aryl group in aryllithiomethyl intermediates 400 makes l-(arylmethyl)benzotriazoles 399 attractive starting materials for many syntheses. Thus, reaction of anions 400 with esters of carboxylic acids leads to a-(benzotriazole-l-yl) ketones 401, which can be easily reduced to carbinols 402... 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

Mesitylfhjorenyl anion (9MsF ) is unreactive towards Mel at temperatures below —78 °C.100 Above —60 °C the absorption spectrum of 9MsF in the presence of Mel is replaced by that of the corresponding 9-mesitylfluorenyl radical (9MsF), and 9-methyl-9-mesitylfluorene is formed in low yield, hi a study of the electron-transfer photochemistry of chrysanthemol, an intramolecular S 2 reaction of a vinylcycloprop-ane radical cation has been observed.101 hi a long series of studies of the reactivity of the acids of trivalent phosphorus and their derivatives, the behaviour of P—O nucleophiles towards arylmethyl bromide systems has been examined.102 Further evidence for an X-philic substitution/SET tandem mechanism has been obtained. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C, Ph2CH- and PhCH2 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of diphenylcarbene are typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals.Ketyl radicals containing the enthrone moiety and the 4-(methyl sulphonate) benzophenone ketyl radical anion are related studies in this field. 

The X-ray analysis of the salt displays the dimeric nature of the cation bridged by the fluorine atoms of the tetrafiuroborate anion. Recently, Power eta], synthesized a one-coordinated lead cation by abstracting methyl group from an arylmethyl plumbylene with tris(pentafluorophenyl) boron in toluene at room temperature, equation (11). 

The photoinduced anti-Markovnikov addition of methanol to 1,1-diphenylethene reported by Arnold and co-workers in 1973 provides the first example of the addition of a nucleophile to an arylalkene radical cation. There are now a number of studies that demonstrate the generality of nucleophilic addition of alcohols, amines, and anions such as cyanide to aryl- and diaryl-alkene radical cations. Product studies and mechanistic work have established that addition occurs at the 3-position of I-aryl or 1,1 -diarylalkene radical cations to give arylmethyl or diaryl-methyl radical-derived products as shown in Scheme I for the addition of methanol to 1,1-diphenylethene. For neutral nucleophiles, such as alcohols and amines, radical formation requires prior deprotonation of the 1,3-distonic radical cation formed in the initial addition reaction. The final product usually results from reduction of the radical by the sensitizer radical anion to give an anion that is then protonated, although other radical... 

In addition to the examples discussed above, arynes have also been found to insert into C-C single bonds. The reaction of either 2- or 3-bromophenol with arylacetonitriles in the presence of a strong base, such as lithium 2,2,6,6-tetramethylpiperidide (LiTMP), led to the same major products, 2-arylmethyl-6-hydroxybenzenecarbonitriles 23 (Scheme 12.9) [26]. In both cases, lithium 2,3-didehydrophenoxide (21) was generated. The anion of nitrile 20 then adds solely at the 3-position of 21 to afford adduct 22, which subsequently leads to the final product 23 after intramolecular rearrangement. When 2-bromo-l-naphthol was used instead of 2-bromophenol, a similar reachvity was observed [27]. 

In these reactions, the key step is loss of a proton from the initially formed radical cation to form an arylmethyl radical. The driving force is the production of a highly stabilized radical. In other cases, the radical cation can combine with an anion to form a neutral radical which then undergoes further oxidation. Thus the first step in the oxidation of anthracene involves a process of this kind. 

Tosylhydrazones of benzaldehyde have been reacted with a benzotriazole-stabilized benzyl anion, formed from an (arylmethyl)benzotriazole and butyl lithium, to provide a stereoselective, one-step synthesis of stilbenes, with lithium chelation probably setting up the geometric preference for -product. ... 

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