Thiohydroxamic esters, radical dimerization

Phenylselenyl radicals are also formed when benzeneselenol is used as a reducing agent. These radicals are usually unable to propagate chain reactions since they dimerize rapidly. Two important exceptions are known the propagation with acyl thiohydroxamates ( Barton esters ) and the use of polarity reversal catalysis. 

O-acyl thiohydroxamates was initially thwarted by competing ionic reactions but eventually culminated in the introduction of 3-bromo or 3-(trifluoromethyl)-3-phenyl diazirine as extremely effective reagents [27]. In contrast to almost all of the other reactions of Barton esters described in this chapter, however, the reaction sequence does not involve a chain process. Thus, as outlined in Scheme 25, capture of the alkyl radical by the diazirine is followed by dimerization and subsequent loss of nitrogen to give the product imine from which the desired amide or amine is easily liberated by mild hydrolysis. Some typical yields are shown in the Scheme 26. 

Free radicals generated under photochemical conditions from thiohydroxamates add readily to the NN double bond of diaziridines and in particular to the 3-bromo or 3-trifluoromethyl-3-phenyldiazirine to produce diaziridinyl radicals (Scheme 25) these intermediates undergo dimerization and fragmentation to yield the corresponding imines, which can be hydrolyzed to the corresponding amines. Barton esters can also be used for the direct preparation of nitroso compounds. From a synthetic standpoint, the yields are moderate and the method is limited to primary and secondary carboxylic acids, which form dimers as an end product. Tertiary nitroso compounds do not dimerize and further react with the radical present under the reaction conditions. 

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See also in sourсe #XX -- [ ]

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