Singlet‐Oxygen Ene Reactions of (E)‐4‐Propyl[1,1,‐2H3]oct‐4‐ene. Short Communication

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Abstract
Rose bengal‐sensitized photooxygenation of 4‐propyl‐4‐octene (1) in MeOH/Me2CHOH 1:1 (v/v) and MeOH/H2O 95:5 followed by reduction gave (E)‐4‐propyl‐5‐octen‐4‐ol (4), its (Z)‐isomer 5, (E)‐5‐propyl‐5‐octen‐4‐ol (6), and its (Z)‐isomer 7. Analogously, (E)‐4‐propyl[1,1,1‐2H3]oct‐4‐ene (2) gave (E)‐4‐propyl[1,1,1‐2H3]oct‐5‐en‐4‐ol (14), its (Z)‐isomer 15, (E)‐5‐[3′,3′,3′‐2H3]propyl‐5‐octen‐4‐ol (16), its (Z)‐isomer 17, and the corresponding [8,8,8‐2H3]‐isomers 18 and 19 (see Scheme 1). The proportions of 4–7 were carefully determined by GC between 10% and 85% conversion of 1 and were constant within this range. The labeled substrate 2 was photooxygenated in two high‐conversion experiments, and after reduction, the ratios 16/18 and 17/19 were determined by NMR. Isotope effects in 2 were neglected and the proportions of corresponding products from 1 and 2 assumed to be similar (% 4 ≈︁ % 14; % 5 ≈︁ % 15; % 6 ≈︁ % (16 + 18): % 7 ≈︁ % (17 + 19)). Combination of these proportions with the ratios 16/18 and 17/19 led to an estimate of the proportions of hydroperoxides formed from 2. Accordingly, singlet oxygen ene additions at the disubstituted side of 2 are preferred (ca. 90%). The previously studied trisubstituted olefins 20–25 exhibited the same preference, but had both CH3 and higher alkyl substituents on the double bond. In these substrates, CH3 groups syn to the lone alkyl or CH3 group appear to be more reactive than CH2 groups at that site beyond a statistical bias.

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