Aromatic aldehydes, overoxidation

To avoid overoxidation, primary amines (e.g. 128, equation 89) can be converted into Schiff bases with an aromatic aldehyde. Subsequent oxidation of the resultant imines 129 with an excess of peracids produces oxaziridines 130 and/or nitrones 131. Both of them produce hydroxylamines 132 (equation 89) upon hydrolysis in moderate to good overall yields. Yields of hydroxylamines are considerably better if anisaldehyde instead of benzaldehyde is used for the protection . ... 

Sodium dichromate in aqueous sulfuric acid has been used since the turn of the century. It is a very strong oxidant the use of this system to oxidize primary alcohols is severely limited by overoxidation, via the aldehyde hydrate, to the corresponding acid. This problem can be p ally circumvented in the preparation of volatile aldehydes (in particular aromatic aldehydes ), by slow addition to excess alcohol and continuous removal of the aldehyde by distillation. Oxidation of secondary alcohols that are reasonably soluble is acceptable, but milder methods are now available, and are discussed in detail later. 

After the electrolysis the acetals can be hydrolyzed to their aldehydes and methanol is recovered. By this elegant way to avoid overoxidation to the acids, aromatic aldehydes are synthesized from toluene derivatives [11]. The electrosynthesis takes place in good yields for toluene derivatives with electron pushing para-substituents like the tert-bntyl group. It is carried out in an undivided cell developed by BASF the capillary gap cell which contains a stack of bipolar round graphite electrodes. The electrodes are separated by spacers and connected in series [12]. 

In fact, primary alcohols can be selectively oxidized to the corresponding aldehydes, without appreciable overoxidation, whereas poorer yields were obtained with secondary aliphatic aromatic alcohols [106c. Similarly, phenols can be converted to quinones [118]. The efficiency of the photocatalytic oxidation depends on the pretreatment and size of the photocatalyst [119], and analogous conversions are also obtained on heteropolyacids, which can be considered soluble analogs of Ti02 suspensions [120]. 

Deprotection of Silylated Ethers. TBBDS is also an efficient catalyst for the deprotection of alcoholic and phenolic silyl ethers. In the presence of an aprotic solvent such as dichloromethane and some drops of H2O, the alcohols are isolated in good to excellent yields (eq 10). The reaction is highly chemoselective bacause no halogenation of the aromatic ring occurs, and no overoxidation of the newly liberated alcohol to the corresponding aldehyde is observed. For this reaction again, the... 

Benzyl alcohol, p-nitrobenzyl alcohol and 2,4-dicholorobenzyl alcohol react very fast, whereas other benzylic alcohols required a longer reaction time. The nature of substituents on the aromatic ring affects the rate of the reaction. It was observed that the introduction of an electron withdrawing substituent into the aromatic ring increases the yield compared to electron donating substituent. Aliphatic alcohols were less reactive than aromatic alcohols and therefore, the reaction was observed to be slower. The oxidation of secondary alcohols was slower and the yield was also lower than with primary alcohols. It was interesting to observe that primary and secondary alcohols were oxidized to aldehydes and ketones without overoxidation to carboxyhc acids. 

---