Calculated effect of substitutions on the regioselectivity of cyclization of alpha-sulfenyl-, alpha-sulfinyl-, and alpha-sulfonyl-(5R)-5-hexenyl radicals.
Calculations of the activation energy of cyclization of alpha-sulfenyl-, alpha-sulfinyl-, and alpha-sulfonyl-5-hexenyl radicals and their respective 5-methyl-5-hexenyl counterparts at the G3MP2B3 level agree quite well with experimental data. The alpha-sulfinyl-5-hexenyl radical exhibits unexpected regioselectivity (93.99:6.01) via the 5-exo mode, whereas the alpha-sulfenyl- and alpha-sulfonyl-5-hexenyl radicals show increasing branching ratios of the 6-endo product. In contrast, the cyclization of the alpha-sulfur-based 5-methyl-substituted counterparts yields essentially the 6-endo products in all cases; in particular, the alpha-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 NO2) or electron-donating substituents (NH2), also proceed preferentially to 6-endo closure. The alpha-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.
Wu
Ching-Wen
CW
Department of Chemistry, National Taiwan Normal University, 88, Taipei, Taiwan 116.
Ho
Jia-Jen
JJ
D003500:Cyclization; D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D015394:Molecular Structure; D013237:Stereoisomerism; D013457:Sulfur Compounds / Q000138:chemical synthesis* / Q000737:chemistry
D005609:Free Radicals; D013457:Sulfur Compounds