2,2,6,6-Tetramethylpiperidine-based oxoammonium salts

Abstract

(A) In carbohydrate chemistry, oxoammonium salts, generated in situ, are selective oxidants. For instance, primary alcohols or hemiacetals can be oxidized in the presence of secondary alcohols, thus making them useful for the oxidation of many sugar derivatives. These oxidations are pH and temperature dependent. Under basic conditions, the selective oxidation of hemiacetals to the corresponding lactones/esters can be achieved in the presence of unprotected secondary alcohols. (B) As stoichiometric reagents, oxoammonium salts represent a family of non-heavy metal-based oxidation reagents. Since they are colored, many reactions can be interpreted colorimetrically. The oxidants tend to be highly functional group specific. The reactions show counter-ion-dependent rates and specificities. These oxidants do not induce isomerization of allylic alcohols, and phenolic benzyl alcohols can be oxidized without phenol protection. The reactions do not take place when the alcohol carries a β-oxygen or a β-electron-withdrawing group. Benzyl and allyl alcohols are rapidly oxidized, secondary aliphatic alcohols are less so, and primary aliphatic alcohols require silica gel catalysis. (C) Using a stoichiometric ammount of oxidant, oxidation of diols with 4 or 5 carbons generates cyclic lactones. 1,2-diols or 1,3-diols dimerize to generate cyclic acetals. However, under catalytic conditions, the oxidation of 1,2-diols leads to the synthesis of cyclic hemiacetals. (D) Selective oxyfunctionalization of enolizable ketones with oxoammonium salts can be achieved in high yields, thus making them reagents of choice for the synthesis of vicinal-diketones. (E) 1,2-Addition of oxoammonium salts to electron rich olefins such as vinyl ethers, enamines and trialkylalkenes takes place in high yields. (F) Oxoammonium salts will induce oxidative phenol couplings. The phenol or quinone coupling products are obtained in good yields.