Mechanism of Ene Reactions of Singlet Oxygen. A Two-Step No-Intermediate Mechanism

• Published in: Journal of the American Chemical Society Vol. 125; no. 5; pp. 1319 - 1328
• Main Authors: Singleton, Daniel A, Hang, Chao, Szymanski, Michael J, Meyer, Matthew P, Leach, Andrew G, Kuwata, Keith T, Chen, Jenny S, Greer, Alexander, Foote, Christopher S, Houk, K. N
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 05.02.2003; Amer Chemical Soc
• Subjects: Alkenes - chemistry; Carbon Isotopes; Chemistry; Chemistry, Multidisciplinary; Exact sciences and technology; Kinetics; Kinetics and mechanisms; Models, Chemical; Organic chemistry; Physical Sciences; Reactivity and mechanisms; Science & Technology; Singlet Oxygen - chemistry; MOLLER-PLESSET; WAVE-FUNCTIONS; DIELS-ALDER; DIRECT DYNAMICS; MOLECULE; AB-INITIO; TRANSITION-STATE; REARRANGEMENTS; HARTREE-FOCK; EXCITED OXYGEN; Kinetic isotope effect; CAS SCF method; Oxygen molecules; Ene reaction; Reaction path; Transition state; Theoretical study; Stepwise mechanism; Hartree Fock method; Coupled cluster calculations; Experimental study; Energy barrier
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja027225p

The mechanism of the ene reaction of singlet (1Δg) oxygen with simple alkenes is investigated by a combination of experimental isotope effects and several levels of theoretical calculations. For the reaction of 2,4-dimethyl-3-isopropyl-2-pentene, the olefinic carbons exhibit small and nearly equal 13C isotope effects of 1.005−1.007, while the reacting methyl groups exhibit 13C isotope effects near unity. In a novel experiment, the 13C composition of the product is analyzed to determine the intramolecular 13C isotope effects in the ene reaction of tetramethylethylene. The new 13C and literature 2H isotope effects are then used to evaluate the accuracy of theoretical calculations. RHF, CASSCF(10e, 8o), and restricted and unrestricted B3LYP calculations are each applied to the ene reaction with tetramethylethylene. Each predicts a different mechanism, but none leads to reasonable predictions of the experimental isotope effects. It is concluded that none of these calculations accurately describe the reaction. A more successful approach was to use high-level, up to CCSD(T), single-point energy calculations on a grid of B3LYP geometries. The resulting energy surface is supported by its accurate predictions of the intermolecular 13C and 2H isotope effects and a very good prediction of the reaction barrier. This CCSD(T)//B3LYP surface features two adjacent transition states without an intervening intermediate. This is the first experimentally supported example of such a surface and the first example of a valley−ridge inflection with significant chemical consequences.