[7758-19-2] · ClNaO2 · Sodium Chlorite · (MW 90.44)
Physical Data: mp 180-200 °C (dec).
Solubility: 39 g/100 ml H2O (17 °C); slightly sol MeOH.
Form Supplied in: white solid; widely available as technical grade material (ca. 80%; the rest is mainly NaCl).
Analysis of Reagent Purity: iodometric titration at pH 2.
Purification: technical grade material can be used as received. Crystallization from H2O gives NaClO2.3H2O which dehydrates at >37 °C.
Handling, Storage, and Precautions: decomposition at >175 °C is highly exothermic. The reagent is not shock sensitive unless contaminated with organic materials. Causes severe irritation or burns to skin and eyes. Acute oral LD50 180 mg kg-1 (rat, 80% assay material).
First employed in cellulose chemistry,2 the reagent is now widely used for the oxidation of saturated,3-8 a,b-unsaturated,9-15 aromatic,8,13,16,17 and heteroaromatic13 aldehydes to the corresponding carboxylic acids (eq 1). The high yields, selectivity, mild conditions, and ease of workup make this the method of choice for most RCHO to RCO2H conversions. For example, with allylic primary alcohols13 it is advantageous to carry out a two-step MnO2/NaClO2 process instead of a direct oxidation using CrVI or AgI because of higher purity and yield. The hypochlorite formed (eq 1) may interfere with the substrate or the product, or react with the reagent to give ClO2 (eq 2), which also may have an adverse effect.
To eliminate the various unwanted positive Cl species present, the reaction is therefore usually conducted in the presence of a scavenger (Table 1). The use of resorcinol has the drawback that the byproduct formed (4-chlororesorcinol) must be removed from the carboxylic acid in a separate step.
The alkene double bond is retained in oxidations of unsaturated alcohols. In an enantiospecific synthesis19 of L-amino acids in >92% ee, intermediate chloroboronic esters (obtained from the alkyl Grignards) were directly oxidized to the acids, thus circumventing the problematic prior conversion to the aldehydes (eq 3).
Dithiocarbamates are oxidized to isothiocyanates,20 ArSMe to ArSOMe,21 RCH2NO2 to RCHO, and R2CH-NO2 to R2CO (eq 4).22
Tapio Hase & Kristiina Wähälä
University of Helsinki, Finland