Carboxyl radicals functional group compatability

GENERATION OF CARBOXYL RADICALS FUNCTIONAL GROUP COMPATABILITY... 

Below are shown a few examples of the types of complex structures that can be assembled by intramolecular free-radical cyclization. Note the presence of a great many polar functional groups present in the cyclization substrates which are compatible with the process. While the examples shown do not need protecting groups, a great number of other free-radical cyclizations are known which have unprotected alcohols, carbonyl groups, and carboxylic acids in the cyclization precursor. 

Because C-H bonds are usually less reactive towards dioxirane oxidation than heteroatoms and C-C multiple bonds, it is instructive to give a few general guidelines on the compatibility of functional groups within the substrate to be submitted to oxidative C-H insertion Substances with low-valent heteroatoms (N, P, S, Se, I, etc.), C-C multiple bonds, and C=X groups (where X is a N or S heteroatom) are normally not suitable for C-H insertions, because these functionalities react preferably. Even heteroarenes are more susceptible to dioxirane oxidation than C-H bonds, whereas electron-rich and polycyclic arenes are only moderately tolerant, but electron-poor arenes usually resist oxidation by dioxiranes. N-oxides and N-oxyl radicals are not compatible because they catalyze the decomposition of the dioxirane. Oxygen insertion into Si-H bonds by dioxirane is more facile than into C-H bonds and, therefore, silanes are not compatible. Substance classes normally resistant towards dioxirane oxidation include the carboxylic acids and their derivatives (anhydrides, esters, amides, and nitriles), sulfonic acids and their de-... 

Acyl hypohalites are usually prepared in situ by reaction of a metal salt of die carboxylic acid widi a halogen (equation 3). Classically the silver salt is used, but problems associated widi die preparation of dry silver carboxylates, as well as the more obvious economic factor, have led to the development of methods using mercury and thallium salts. Evidently, those functional groups which react readily widi halogens are not compatible with this approach. A major limitation of the acyl hypohalites is the readiness with which they transfer halogen atoms to alkyl radicals this property essend ly limits their use to decarboxylative halogenation reactions. 

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