Calculations of the activation energy of cyclization of α-sulfenyl-, α-sulfinyl-, and α-sulfonyl-5-hexenyl radicals and their respective 5-methyl-5-hexenyl counterparts at the G3MP2B3 level agree quite well with experimental data. The α-sulfinyl-5-hexenyl radical exhibits unexpected regioselectivity (93.99: 6.01) via the 5-exo mode, whereas the α-sulfenyl- and α-sulfonyl-5-hexenyl radicals show increasing branching ratios of the 6-endo product. In contrast, the cyclization of the α-sulfur-based 5-methyl-substituted counterparts yields essentially the 6-endo products in all cases; in particular, the α-SO2-5-CH3-5-hexenyl radical gives high regioselectivity (98.85:1.15) via the 6-endo mode. Several other 5-substituted moieties, including the electron-withdrawing (CN and NO 2) or electron-donating substituents (NH2), also proceed preferentially to 6-endo closure. The α-sulfonyl-5-amine-5-hexenyl radical is calculated to proceed to exclusively the 6-endo product, a demonstration of the high synthetic value of this reactant.
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