Aromatic compounds from benzylic halides

Nitrosobenzene, CjHsNO, which is obtained by the oxidation of phenylhydroxylamine, and p>nitrosodimethylaniline, p-(CH3)2NCjH4NO, which is easily prepared by the nitrosation of dimethylaniline, are fairly specific oxidizing agents for the preparation of aromatic aldehydes from benzyl halides or tosylates and of a-dicarbonyl compounds from from a-halo ketones [984, 985]. Also, a methylene group flanked by two carbonyls can be oxidized to a carbonyl group by nitrosodimethylaniline [986]. Pyridine is frequently used to form quaternary pyridinium salts from reactive halides prior to their oxidation to aromatic aldehydes, a-ketoaldehydes, or a-diketones [984] (equations 22 and 23). 

The net outcome of the Sommelet reaction is the oxidation of the benzyl methylene position into the corresponding carbonyl, and in that sense, the Krbhnke reaction may be considered a close variant. The KrOhnke reaction transforms benzyl halides to benzaldehydes using pyridine and an aromatic nitroso compound 16.16,17 As shown below, quaternary pyridinium salt (15) is formed from addition of pyridine to benzyl halide 1. Oxidation of 15 with p-nitrosodimethylaniline (16) affords nitrone 17, which is then hydrolyzed to benzaldehyde 3 and hydroxylamine 18. 

Several benzyl derivatives exhibit potentially hazardous properties arising from the activation by the adjacent phenyl group, either of the substituent or of a hydrogen atom. Halides, in particular, are prone to autocatalytic Friedel Crafts polymerisation if the aromatic nucleus is not deactivated by electron withdrawing substituents. Individually indexed compounds are ... 

Many di-Grignard reagents can be prepared routinely from the corresponding halo compounds, provided the halogens are sufficiently separated, for example, (42), X(CH2)4X, or m- and /t-dihalo aromatics. Precautions such as dilution, low temperature, and active magnesium (particularly Mg anthracene) may be important for a good yield, particularly where the halide is easily displaced, for example benzylic. 

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