Sulfonamidyl radicals

An improved synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide was reported by Togo and co-workers using photochemical conditions . Treatment of A-alkyl 2-(aryl)ethanesulfonamides 18 with (diacetoxyiodo)arenes under irradiation with a tungsten lamp at 20-30 °C afforded 2,1-benzothiazines 19 and 20. Chemical yields and selectivities were dependent upon the choice of solvents and the reactant s substituents 18 (Table 1). When THF and EtOH were used as solvents, the reactions failed to give the cyclized products, since their a-hydrogen was abstracted by the intermediate sulfonamidyl radical. Compound 20 was obtained as a major product when 1,2-dichloroethane was employed as a solvent. In contrast, in the case of EtOAc as solvent, compound 19 was obtained as the major product. 

Ames and Opalko have developed a palladium-catalysed cyclization of compound 24 to afford compound 25 in 56% yield (Scheme 5) <84T1919>. Glover and co-workers published a study involving the synthesis and photolysis of A-halogenobiphenyl-2-sulfonamides (26a or 26b) <86JCS(P2)645>. The photolysis of compound 26a or compound 26b in benzene formed an intermediate A-methylbiphenyl-2-sulfonamidyl radical, which resulted in compound 27a or 27b in 21% or 50% yield, respectively. In this procedure, the reaction only afforded the six-membered ring product 27(Scheme 6). 

Two very similar nitrogen radical cyclizations have been investigated this year. Liu et al. made use of a primary aminyl radical 161 to generate predominantly the 6-endo cyclization product 162 (93 7, 6-endo vs 5-exo) <07JOC10231>. Lu et al. utilized a sulfonamidyl radical 163 to exclusively generate the 6-endo cyclization product 164 <07JOC2564>. 

Oxidation of biphenyl-2-sulfonamide with persulfate gives a sulfonamidyl radical which cyclizes intramolecularly to yield, after further oxidation. 

The Su rez modification of the HLF reaction was the basis of the new synthetic method developed by H. Togo et al. The authors prepared A/-alkyl-1,2-benzisothiazoline-3-one-1,1-dioxides (A/-alkylsaccharins) from A/-alkyl(o-methyl)-arenesulfonamides using (diacetoxyiodo)arenes in the presence of iodine via sulfonamidyl radicals. The transformations did not work in the dark, indicating the radical nature of the reaction. The yields varied from moderate to excellent and the nature of the aromatic substituents on both the substrate and the (diacetoxyiodo)arenes were important. It should be noted that the oxygen atom at the C3 position most likely arises from the hydrolysis of a C3 diiodo intermediate (not isolated). 

The photolysis of A(-halo isothiazolidine produces the cyclic sulfonamidyl radical (see Section... 

The combination of an aryl- or alkylsulfonamide, 3 equiv. PhI(OAc)2 and substoichiometric I2 is able to promote C-H animation of simple hydrocarbons in the absence of any metal catalyst [120]. These reactions are best performed neat (10-fold excess of substrate relative to RS02NH2) at 50°C and appear to operate with reasonable selectivity to give benzylic sulfonamide products. Over-oxidation to the sulfonylimine is noted in some cases. A proposed reaction mechanism posits the intermediacy of a sulfonamidyl radical, formed from the AModosulfonamide. This pathway parallels in many ways the mechanism invoked for the Hofmann-Loffler-Freytag process. 

The electronic nature of a nitrogen centered radical, dictated by reaction conditions and/or the radical precursor employed, is crucial to the mode of reaction, to the ability to undergo efficient intramolecular cyclizations or intermolecular additions, and to the products isolated from the radical reaction. The types of radicals discussed in this review include neutral aminyl radicals, protonated aminyl radicals (aminium cation radicals), metal complexed aminyl radicals, and amidyl radicals. Sulfonamidyl and urethanyl radicals are known (71S1 78T3241), but they are not within the scope of this chapter. 

This reaction has been extended to amidyl radicalj sulfonamidyl radicalj and other nitrogenous radicals. 

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