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NAD(P)+-NAD(P)H Models. 88. Stereoselection without Steric Effect but Controlled by Electronic Effect of a Carbonyl Group:  Syn/Anti Reactivity Ratio, Kinetic Isotope Effect, and an Electron-Transfer Complex as a Reaction Intermediate

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posted on 30.01.1998, 00:00 by Atsuyoshi Ohno, Yoshiteru Ishikawa, Norimasa Yamazaki, Mutsuo Okamura, Yasushi Kawai
1,4,6,7-Tetrahydro-1,6,11-trimethyl-5-oxo-5H-benzo[c]pyrido[2,3-e]azepin (11Me-MMPAH) and its 4-mono- and 4,4-dideuterated analogues have been oxidized with a series of p-benzoquinones. The compounds have axial chirality with respect to the orientation of the carbonyl dipole. The hydrogen (deuterium) at the 4-position has different reactivity toward oxidation:  the anti (with respect to the carbonyl dipole) hydrogen is from 3 to 32 times more reactive than the corresponding syn hydrogen (deuterium). The syn/anti reactivity ratio depends on the reactivity of the quinone (stereoselection without steric effect). The initial electron-transfer process, which is associated with the weakening of the C4−H(D) bond, results in the formation of an electron-transfer complex prior to the chemical reaction. The isotope effect affecting this equilibrium constant is the origin of the “product isotope effect”, which has long been a subject of controversy. Kinetic primary and secondary isotope effects in the oxidation are also calculated.