POLAR AND STERIC SUBSTITUENT EFFECTS IN 5-EXO-TRIG CYCLIZATIONS OF 1-ARYL-4-PENTEN-1-OXYL RADICALS

A series of 1-aryl-substituted 4-penten-1-oxyl radicals 2 were generat ed from parent N-alkoxypyridinethiones 6. The intermediates 2 undergo 5-exo-trig cyclizations which are sensitive to polar and steric substi tuent effects. Thus, para groups in the aromatic ring alter the reacti vity of radicals 2c-e by one order of magnitude without changing the s tereochemical course of the 5-exo-trig reaction (cis:trans = 50:50). S ubstitution of the ortho methyl groups for hydrogen atoms of the pheny l group affects the cis-ring closures more than the trans reactions wh ich lead to trans-selective alkoxyl radical reactions. Methyl groups a t the terminal olefinic double bond cause a significant acceleration o f k(5-exo) in general and of the trans reactions in particular. Thus, trans-2-isopropyl-5-phenyltetrahydrofuran (8i) was the major product i n the photoreaction of pyridinethione 6i in the presence of tri-n-buty l-tin hydride (98 % yield, cis:trans, 30:70).