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DOI: 10.1055/sos-SD-008-00826

Jonczyk, A.; Kowalkowska, A.Science of Synthesis, (200681052.

The base-mediated (usually aqueous sodium or potassium hydroxide) disproportionation of aldehydes into alcohols and carboxylic acids is named the Cannizzaro reaction. A useful modification, consisting of the reaction of an aldehyde with formaldehyde, leads to the formation of the alcohol and the alkali salt formate (crossed Cannizzaro reaction).[‌590‌] A single-electron-transfer process is possibly involved in the formation of Cannizzaro products.[‌591‌] The starting aldehydes should not possess hydrogen atoms on the α-carbon, otherwise aldol products are formed which in turn may enter into the Cannizzaro process. Benzyltriethylammonium chloride[‌219‌] or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts[‌420‌] suppress the Cannizzaro reaction of benzaldehyde when carried out in the presence of concentrated sodium hydroxide,[‌219‌] but in the case of the crossed process, both selectivity and rate are improved by hydrotropes [poly(ethylene glycols)];[‌592‌] the rate is also dramatically enhanced by microwave irradiation.[‌593‌] The rate of Cannizzaro product formation is significantly increased when the reaction is carried out in a solvent-free system in the presence of a sodium hydroxidebasic alumina mixture and aided by microwave irradiation.[‌594‌] Pyridine-4-carbaldehyde undergoes the Cannizzaro reaction when stirred in chloroform under phase-transfer catalysis conditions, while the isomeric 2- and 3-substituted aldehydes give the corresponding (trichloromethyl)pyridylmethanols. Such reactivity of the former aldehyde is ascribed to the facile formation of a tetrahedral intermediate, e.g. hydration of the formyl group.[‌535‌] The Cannizzaro reaction is sometimes observed to accompany other processes, e.g. the Wittig reaction of alkyltriphenylphosphonium salts with benzaldehyde carried out in the presence of slightly hydrated solid sodium hydroxide in dioxane.[‌466‌]