Nitrobenzyl halides

Following an explosion dining drying of 2-nitrobenzyl bromide, the thermal stability of all six isomeric nitrobenzyl halides was investigated by TGA, DSC and ARC techniques. All the halides decomposed exothermally with abundant gas evolution, and the 2-nitro chloride is less thermally stable than the 3- or 4-isomers. See 2-Nitrobenzyl bromide... 

The fact that the anion radical is an intermediate in this case falls in line with the observation that it is also an intermediate in the reduction of the same substrates by homogeneous or heterogeneous outer sphere electron donors and also that nitrobenzyl halides are quite easy to reduce (see Section 2, p. 66). In the other cases, the generation of the R radical has been assumed to proceed by halogen-atom transfer (158). It should, however, be noted that an outer sphere, dissociative electron-transfer reaction (163) would also... 

Other examples of the use of electron acceptors whose ion radicals are unstable with respect to fragmentation to an anion and a radical capable of initiation of polymerization were provided by Eaton (63,101,102). It was shown that -nitrobenzyl halides could be used in dye-sensitized compositions of semiconductor pigments such as Ti02 and CdS to induce polymerization of vinyl monomers using visible light. The sequence of events is outlined in eqs. 46-49 and Scheme 6 ... 

A few papers on phosphonate chemistry have appeared.235-237 The synthesis of die novel nucleoside bicyclic trisanhydrides (256 A = adenosin-r -yl) has been reported.235 A kinetic sriidy of the alkaline hydrolysis of 4-substiriited phenyl ethyl benzyl phosphonates (257) supports an associative A-E mechanism for the hydrolysis.236 The direct preparation of the esters of /Miitrobenzylphosphonic acid from /r-nitrobenzyl halides has been reported.237... 

If the arenes are very electron-deficient direct attack of the nucleophile at the arene might also compete with displacement of the benzylic leaving group, to yield complex structures such as that shown in Scheme 4.28. 2-Nitrobenzyl halides can also react with amines to yield, instead of simple products of nucleophilic substitution, 2fi-indazoles [122] (Scheme4.28). 4-Nitrobenzyl halides, however, yield the expected benzyl amines on treatment with amines [123]. 

Scheme4.28. Reactivity of nitrobenzyl halides toward amines [122,124],...

Halomalonic acid derivatives, 1-halo-l-nitroalkanes [186], and related electrophiles can, upon treatment with a base, also dimerize to yield substituted ethylenes, in the same way as nitrobenzyl halides (see above). The reaction conditions required for this dimerization do not differ much from those required for successful nucleophilic substitution (Scheme 4.46), and if substitution is desired a low concentration of the electrophile should be maintained during the reaction to minimize dimerization. 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

Although the SRNl reaction is shown to be prone to steric limitations, most of the C-alkylated products obtained in these studies are sterically congested molecules, whose preparation would be difficult by other methods. Indeed, the reaction of sterically hindered /7-nitrobenzyl halides with nucleophiles by an S l mechanism affords a synthetic route to extremely sterically crowded molecules72. 

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

To gain additional information on these fragmentation reactions, the reactivity of a wide variety of structurally modified nitrobenzyl halide radical anions has been investipted (Table 13) [283, 290, 294],... 

Reduction of o- and / -nitrobenzyl halides at mercury or platinum gives mainly the bibenzyl derivative, with nitrotoluene as a minor product [121-126]. However, m-nitro-toluene is the predominant product of the electrolysis of / -nitrobenzyl halides [123]. 

Reduction of a substituted nitrobenzene follows generally one of the reaction paths described, as the nitro group usually is the more easily reducible group. Nitrobenzyl halides [145,146], however, are reduced in the side chain rather than in the nitro group, and dinitrobibenzyl and nitrotoluene are prominent products (see Chapter 8). 

The use of 2-nitrobenzyl halides in the Reissert alkylation, followed eventually by the Pschorr cyclization, has provided an attractive route to various aporphines (31). Thus this procedure had led to the synthesis of apomor-phine, ° apomorphine derivatives, atheroline, imenine, N-methyl-ovigerine, oconovine, 8-hydroxyaporphines, 10-hydroxyaporphines, ... 

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