Martín-Matute Belén, Aberg Jenny B, Edin Michaela, Bäckvall Jan-E
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden.
Chemistry. 2007;13(21):6063-72. doi: 10.1002/chem.200700373.
The racemization of sec-alcohols catalyzed by pentaphenylcyclopentadienyl-ruthenium complex 3a has been investigated. The mechanism involves ruthenium-alkoxide intermediates: reaction of tert-butoxide ruthenium complex 4 with a series of sec-alcohols with different electronic properties gave ruthenium complexes bearing a secondary alkoxide as a ligand. The characterization of these alkoxide complexes by NMR spectroscopy together with a study of the reaction using in situ IR spectroscopy is consistent with a mechanism in which the alkoxide substitution step and the beta-hydride elimination step occur without CO dissociation. The alkoxide substitution reaction is proposed to begin with hydrogen bonding of the incoming alcohol to the active ruthenium-alkoxide intermediate. Subsequent alkoxide exchange can occur via two pathways: i) an associative pathway involving a eta3-CpRu intermediate; or ii) a dissociative pathway within the solvent cage. Racemization at room temperature of a 1:1 mixture of (S)-1-phenylethanol and (S)-1-phenyl-[D4]-ethanol gave only rac-1-phenylethanol, and rac-1-phenyl-[D4]-ethanol, providing strong support for a mechanism in which the substrate stays coordinated to the metal center throughout the racemization, and does not leave the coordination sphere. Furthermore, racemization of a sec-alcohol bearing a ketone moiety within the same molecule does not result in any reduction of the original ketone, which rules out a mechanism where the intermediate ketone is trapped within the solvent cage. These results are consistent with a mechanism where eta3-Ph(5)C(5)-ruthenium intermediates are involved. Competitive racemization on nondeuterated and alpha-deuterated alpha-phenylethanols was used to determine the kinetic isotope effect kH/kD for the ruthenium-catalyzed racemization. The kinetic isotope effect kH/kD for p- X-C(6)H(4)CH(OH)CH(3) was 1.08, 1.27 and 1.45 for X=OMe, H, and CF3, respectively.
已对五苯基环戊二烯基钌配合物3a催化仲醇的外消旋化反应进行了研究。其反应机理涉及钌醇盐中间体：叔丁醇钌配合物4与一系列具有不同电子性质的仲醇反应，生成以仲醇盐为配体的钌配合物。通过核磁共振光谱对这些醇盐配合物进行表征，并结合原位红外光谱对反应进行研究，结果与一种反应机理相符，即醇盐取代步骤和β-氢消除步骤在没有CO解离的情况下发生。醇盐取代反应被认为始于进入的醇与活性钌醇盐中间体形成氢键。随后的醇盐交换可通过两条途径发生：i）涉及η3-CpRu中间体的缔合途径；或ii）溶剂笼内的解离途径。(S)-1-苯乙醇和(S)-1-苯基-[D4]-乙醇的1:1混合物在室温下外消旋化，只生成外消旋的1-苯乙醇和外消旋的1-苯基-[D4]-乙醇，这为底物在整个外消旋化过程中始终与金属中心配位且不离开配位球的反应机理提供了有力支持。此外，同一分子内带有酮基的仲醇外消旋化不会导致原酮的任何还原，这排除了中间体酮被困在溶剂笼内的反应机理。这些结果与涉及η3-Ph(5)C(5)-钌中间体的反应机理一致。利用非氘代和α-氘代α-苯乙醇的竞争性外消旋化反应来确定钌催化外消旋化反应的动力学同位素效应kH/kD。对于p-X-C6H4CH(OH)CH3，当X = OMe、H和CF3时，动力学同位素效应kH/kD分别为1.08、1.27和1.45。
