Transition state geometry in hydroperoxidation of monoolefins with singlet oxygen

The reaction of singlet oxygen with olefins: the question of mechanism

“…Firstly, kinetic isotope effects appear to be small (less than 1.4).193'222-227 Secondly, it is clear from several systems that steric hindrance to C-0 bond formation in the photosensitized oxygenation reaction is more important than hindrance to C-H bond cleavage.225"230 Finally, it must be explained, according to the cyclic "ene" mechanism, why thermodynamic stability of the final double bond has little effect on the reaction,111 why conformational ring inversion (which sometimes must accompany a double bond shift) does not block oxygenation, and why the susceptibility of C-H to abstraction is not inherently related to whether it is primary, secondary, or tertiary. 222 To this concerted "ene" mechanism proponents222•230-232 respond that all the results can be explained if we assume a product-forming transition state in which oxygen, while partially bonded to olefinic carbon and to allylic C-H, has neither much perturbed the olefin geometry, nor extensively weakened the C-H bond. 233•234 To the suggestion235 236 that the low isotope effects might not be primary isotope effects but rather simply the result of cumulative secondary isotope effects in multideuterated systems, Nickon answered that for a transition state that resembles reactant (i.e., no hybridization change at CD3) there should be virtually no secondary (i.e., a) isotope effect.237 Furthermore, the slightly smaller steric size and slightly larger electron-donating effect238 of CD3 vs. CH3 could favor attack at an olefinic carbon carrying CD3 and thus produce inverse secondary effects.…”

Intermediate in the ene reaction of singlet oxygen with 1,4-diphenyl-cis-2-butene and 2-butene

“…(16) Hydroperoxide 9: NMR (MeOH-/, -70 °C) all peaks are broad, 5.4 (OH), 4.51 (1 H, C4-H), 4.37 (1 H, Cs-H), 4.21 (1 H, C6-H), 4.10 (1 H, C6-H), 3.58 (3 H, CH3). 2-methyl dehydroascorbic acid 10: 'H NMR (MeOH-/, at -20 °C) 5.4 (OH), 4.53 (1 H, C4-H), 4.37 (1 H, C5-H), 4.30 (1 H, C6-H), 4.17 (17) Reaction of 11A with diazomethane gave 11B, separated by column chromatography (65% yield): *H NMR (acetone-/) 8 (brd, OH, DzO exchangeable), 5.66 (d, 1 H, J = 8.1 Hz), 4.87 (q, 1 H, J = 8.0 Hz), 4.51 (m, 1 H), 4.22 (m, 1 H); MS, m/e 204 (M+).…”

Ring strain in bis(triethylphosphine)-3,3-dimethylplatinacyclobutane is small

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Metallacycloalkanes have been identified as intermediates in a variety of metal-catalyzed reactions,2"8 but little is known about tNIH Postdoctoral Fellow, 1979-1981 (5 f32 CA06462-02).

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