Radicals from Sulfur Heterocycles

Gerson and co-workers °- have prepared anion-radicals from the macrocyclic heterocycles 113 and 114 and the sulfur analog of 5. Conformational and ion-pairing effects in the ESR spectra were investigated, and in the more recent work solution ENDOR spectroscopy was employed. 

The additions of thiyl radical to alkenes and alkynes can also occur intra-molecularly, thus leading to sulfur heterocycles. This subject has been explored by Surzur s group [32-33]. Based on steric and stereoelectronic factors, the Baldwin-Beckwith rules [34] predict the 5-exo ring closure as the favored pathway. Scheme 6 shows that the pent-4-enylthiyl radical, generated from the allyl sulfide, yields both five- and six-membered rings, in the ratio of 1 19. The predominance of the larger cycle, in contrast with the above-mentioned rules, is due to the reversibility of the reaction. The product distribution reflects a thermodynamic control. 

In any event, 02SbFg and 02AsFg act as very effective one-electron oxidants with regard to aromatic amines, nitrogen- and sulfur-containing heterocyclic compounds (Dinnocenzo and Banach 1986, 1988,1989) as well as to perfluorobenzene, benzotrifiuoride, and perfluoronaphthalene (Richardson et al. 1986). The oxidation proceeds in Freon (CHCIF2) at temperatures from -115 to -145°C, and the formation of the organic cation-radicals has been firmly established. 

Tandem radical additions have been used in the preparation of piperidines. Substituted nitrogen containing heterocycles with ring sizes ranging from 5-8 have been synthesized by radical methods [95JCS(P1)19]. Addition of sulfur radicals to 225 occurs selectively at the enamide portion followed by an intramolecular 6-exo cyclization to produce a diastereomeric mixture of 226 in good yield. 

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