Department of Applied Chemistry, Graduate Course of Engineering, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji, Tokyo 192-0397, Japan.
Photochem Photobiol Sci. 2010 Jul 30;9(7):931-6. doi: 10.1039/c0pp00052c. Epub 2010 May 12.
Two key reaction intermediates in the photochemical oxygenation of alkene sensitized by carbonyl-coordinated ruthenium(ii)-porphyrin complex, with water acting both as an electron and oxygen atom donor, are postulated. Under the low concentration of hydroxide ion (<2 x 10(-3) M) added to the reaction mixture of tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(ii) (Ru(II)TMP(CO)), K(4)PtCl(6) as a sacrificial electron acceptor, and cyclohexene as a substrate in aqueous acetonitrile, the major reaction product was cyclohexaneoxide ("Epoxide"), while it drastically decreased along with an increase of 2-cyclohexenol ("Alcohol") by increasing the amount of hydroxide ion (>2 x 10(-3) M). The tendency was more obvious in the case of tetrasodium tetra(4-sulfonate)phenylporphyrinatoruthenium(ii) (Ru(II)TSPP(CO)) in aqueous solution. The "Alcohol" was exclusively formed in the higher concentration region of OH(-), strongly suggesting the presence of acid-base equilibrium among two reaction intermediates. Theoretical DFT calculation indicates that the hydroxyl-coordinated one-electron oxidized Ru-porphyrin (Intermediate (I)), which is formed by the axial ligation of hydroxide ion to the cation radical of Ru-porphyrin generated through electron transfer from the excited triplet state of the sensitizer porphyrins, suffers deprotonation of its axial hydroxide group to lead to an oxo-type complex (Intermediate (II)) formation. The DFT calculation also indicates that the electron spin on the Intermediate (I) is shared by the axial oxygen atom and the central Ru metal, while it is mostly localized on the axial oxygen atom to behave as an oxygen radical in the case of the Intermediate (II). These are very strong indications towards understanding how OH(-) (water molecule) is oxidatively activated on the Ru center: the water molecule is serving as an electron donor ion in the redox cycles. Theoretical calculation predicts that Intermediate (I) allows the epoxidation of alkene and Intermediate (II) can proceed through hydrogen abstraction from the substrate and is rebound to form hydroxylated compound, "Alcohol."
两个关键的反应中间体在烯烃的光氧化羰基配合钌（ii）-卟啉配合物敏化，与水作为一个电子和氧原子供体，被假定。在低浓度的氢氧根离子（<2×10（-3）M）添加到反应混合物的四（2,4,6-三甲苯基）卟啉钌（ii）（Ru（II）TMP（CO）），K（4）PtCl（6）作为一个牺牲电子受体，和环己烯作为底物在水-乙腈，主要的反应产物是环己烷氧化物（“环氧化物”），而它急剧下降随着增加氢氧根离子的量（>2×10（-3）M）。这种趋势在水溶液中的四（4-磺酸基）苯基卟啉钌（ii）（Ru（II）TSPP（CO））更为明显。“醇”完全形成在较高浓度的OH（-）的区域，强烈表明存在两种反应中间体之间的酸碱平衡。理论密度泛函计算表明，羟基配合的单电子氧化钌卟啉（中间体（I）），这是由轴向配体的氢氧根离子对生成的通过从敏化剂卟啉的激发三重态电子转移阳离子自由基的钌卟啉，遭受其轴向羟基的去质子化导致形成氧型配合物（中间体（II））。密度泛函计算还表明，在中间体（I）上的电子自旋是由轴向氧原子和中心 Ru 金属共享的，而在中间体（II）的情况下，它主要定位于轴向氧原子上，表现为氧自由基。这些是理解 OH（-）（水分子）如何在 Ru 中心上被氧化活化的非常有力的迹象：水分子在氧化还原循环中充当电子供体离子。理论计算预测，中间体（I）允许烯烃的环氧化，中间体（II）可以通过从底物中提取氢原子并进行重排形成羟基化化合物“醇”来进行。
